Abstract
Context
Incidence of insulin resistance (IR) in Americans is steadily rising. IR may be ameliorated with ≥ 5% loss in body weight.
Objective
To examine effects of 2 weight-loss diets on body weight and composition in overweight adults with IR.
Design
Participants randomly assigned to a high-protein, low-fat (HPLF) or a high-carbohydrate, low-fat (HCLF) diet for 8 weeks.
Setting
All meals prepared and weighed in the metabolic kitchen at Arizona State University. Lunch consumed on-site; all other meals packaged for home consumption.
Patients
Twenty overweight, healthy participants with elevated fasting serum insulin (≥ 15 µU/L) were recruited.
Interventions
Both diets were low-fat (27% kcal from fat; < 7% saturated, ≤ 10% monounsaturated, and ≤ 10% polyunsaturated) and energy-restricted (energy levels were 1200, 1500, 1700 or 2000 kcal); HPLF: 32% protein, 41% carbohydrate; HCLF: 59% carbohydrate, 14% protein. Energy levels were assigned on the basis of participant's resting metabolic rate.
Main Outcome Measures
Body composition, metabolic indices, fasting plasma glucose, and insulin.
Results
No significant differences were found in the main outcome measures between the diets. Body weight (HPLF: −4.9 kg; HCLF: −4.0 kg) and total percent body fat (HPLF: −1.5%; HCLF: −0.4%) significantly reduced from baseline to week 8 (P = .005 and P = .035, respectively).
Conclusion
Both diets promoted ≥ 5% loss in body weight and significantly reduced percent body fat.
Introduction
An estimated 50% of women and 33% of men in the United States consider themselves to be on a diet at any given time.[1] Americans spend more than $30 billion annually on weight-loss products and programs.[2] This interest in dieting and weight-loss products and programs is no coincidence, as nearly 65% of all American adults are overweight or obese.[3]
Many health guidelines and organizations[4,5] tout the weight-loss benefits of a high-carbohydrate, low-fat (HCLF) diet. However, the recent rebirth of high-protein diets (eg, the Atkins diet) encourages Americans to consume meats and cheeses which may be high in saturated fat. More beneficial may be a high-protein diet low in fat, which promotes healthy weight loss without risking the potential adverse effects on lipoproteins.[6,7] A high-protein, low-fat (HPLF) diet has also been shown to reduce total and truncal adiposity,[6,7] spare lean body mass,[8] and increase satiety[9] and thermogenesis.[10] Conversely, some researchers speculate that high-carbohydrate diets may negatively affect insulin sensitivity (IS)[8, 11–13] and, therefore, would not be the most appropriate diet for individuals with diminished IS or insulin resistance (IR).
It is estimated that 25% to 50% of overweight adults have IR.[14] A 5% to 10% loss in body weight has been shown to improve insulin levels and thereby reduce the risk of developing certain chronic diseases, such as type 2 diabetes mellitus[15] and heart disease.[14] However, many adults have difficulty in achieving – let alone maintaining – any amount of weight loss. And in light of the rapidly expanding waistlines of American adults and the increasing number of adults with type 2 diabetes, there exists a need for an effective and tailored diet to help promote weight loss in overweight adults with IR.
While several studies have shown the beneficial effects of weight loss on IR,[6, 15–21] research investigating the most effective weight-loss strategy for individuals with IR is limited. The primary aim of this study was to investigate the effects of a HPLF diet as compared with a HCLF diet on weight loss in healthy, overweight adults with IR and, secondarily, to examine the effects of weight loss and diet on other biomarkers for disease.
Materials and Methods
Participants
Healthy women (n = 14) and men (n = 6) who desired to lose weight were recruited by public advertisement in the greater metropolitan Phoenix area. All participants were nonsmokers and free of renal or hepatic disease, type 2 diabetes, heart disease, alcohol or drug dependence, hypertension, and thyroid disease. Baseline characteristics of the participants are given in Table 1. Participants were overweight (body mass index [BMI] ≥ 25 kg/m2), with elevated fasting serum insulin concentrations ≥ 15 µU/L (used as a marker of IR). Exclusion criteria included the inability to adhere to study protocols for the duration of the study, tobacco smoking, the inability to abstain from alcohol use for the 8-week study, use of any metabolism-affecting medication, food allergies or extreme food preferences, or presence of a serious medical ailment. Other medications were allowed as long as metabolism was not affected, treatment at the current dosage had been for longer than 6 months, and the medication dosage was unlikely to change during the 8-week study. All participants were asked to maintain any physical activity and exercise programs at levels in which they had engaged prior to the study.
Table 1.
Descriptive Characteristics of Study Participants at Baseline1
| HCLF Diet group2 (n = 7) | HPLF Diet group2 (n = 9) | |
|---|---|---|
| Characteristics on which study participants were matched at baseline3 | ||
| Gender | Females: 5, Males: 2 | Females: 7, Males: 2 |
| Age (y) | 34.6 ± 3.6 (23–53) | 35.2 ± 4.9 (20–57) |
| Body mass index (kg/m2) | 35.9 ± 1.4 (30.5–40.5) | 34.6 ± 2.2 (25.7–43.3) |
| Fasting insulin (µU/L) | 19.7 ± 2.6 (15.2–28.6) | 29.8 ± 11.7 (15.4–42.3) |
| Additional characteristics of study participants at baseline | ||
| Anthropometric characteristics | ||
| Weight (kg) | 107.1 ± 7.8 (57.7–122.5) | 94.5 ± 3.6 (97.2–120.8) |
| Height (cm) | 173.5 ± 2.5 (167.6–187.9) | 164.8 ± 3.2 (149.9–175.3) |
| Body fat (%) | 44.3 ± 2.8 (30.3–51.4) | 44.3 ± 2.8 (28.0–53.7) |
| Fat mass (kg) | 47.7 ± 3.2 (32.6–58.8) | 39.2 ± 5.3 (18.5–66.8) |
| Fat free mass (kg) | 132.0 ± 7.9 (121.2–165.2) | 121.8 ± 7.8 (87.2–157.8) |
| Leg fat (per region) (%) | 44.3 ± 3.0 (30.5–56.0) | 46.5 ± 3.8 (27.9–56.3) |
| Truncal fat (per region) (%) | 45.0 ± 2.3 (38.2–52.5) | 44.6 ± 1.5 (37.9–49.1) |
| Waist circumference (cm) | 108.7 ± 3.8 (89.5–118.1) | 105.4 ± 6.0 (86.4–132.2) |
| Physiologic characteristics4 | ||
| Fasting glucose (mg/dL) | 110.9 ± 5.3 (95.9–128.7) | 98.6 ± 4.0 (84.4–115.6) |
| HOMA insulin sensitivity4 | 5.3 ± 0.7 (2.6–7.8) | 3.1 ± 0.8 (2.9–7.2) |
| Total cholesterol (mg/dL) | 188.9 ± 12.9 (148.0–243.0) | 225 ± 8.9 (196.0–266.0) |
| HDL cholesterol (mg/dL)5 | 49.7 ± 5.4 (30.0–66.0) | 54.8 ± 2.7 (39.0–66.0) |
| LDL cholesterol (mg/dL)5 | 119.7 ± 10.9 (93.0–170.0) | 146.0 ± 7.8 (121.0–174.0) |
| VLDL cholesterol (mg/dL)5 | 20.0 ± 4.6 (10.0–44.0) | 24.3 ± 4.7 (15.0–54.0) |
| Triacylglycerides (mg/dL) | 118.7 ± 27.7 (59.0–261.0) | 146.0 ± 27.5 (93.0–321.0) |
| Blood pressure (mm Hg) | ||
| Systolic | 116.9 ± 2.3 (112–128) | 113.0 ± 3.1 (100–122) |
| Diastolic | 70.9 ± 3.9 (60–90) | 71.8 ± 2.0 (68–80) |
All values are mean ± SEM; range in parentheses. Groups did not differ significantly at baseline.
HCLF, high-carbohydrate, low-fat; HPLF, high-protein, low-fat.
Data presented are for the 16 participants who finished the study.
Desirable values for physiologic characteristics (40): fasting plasma glucose (70–110 mg/dL); total cholesterol (< 200 mg/dL); HDL (> 45 mg/dL M, > 55 mg/dL F); LDL (60–180 mg/dL); VLDL (25–50 mg/dL); triacylglycerides (40–160 mg/dL M, 35–135 mg/dL F); systolic blood pressure (≤ 120 mm Hg); diastolic blood pressure (≤ 80 mm Hg).
HOMA, Homeostasis Model of Assessment; HDL, high-density lipoprotein; LDL, low-density lipoprotein; VLDL, very-low-density lipoprotein.
A total of 20 participants were selected to enroll in this study. Four participants were dropped or voluntarily withdrew due to an inability to comply with study protocol. The remaining 16 participants (HPLF: n = 9; HCLF: n =7) completed the study in its 8-week entirety. Written and informed consent was obtained from all participants prior to screening for study enrollment. The study protocol was approved by, and conducted in accordance with, the Biosciences Committee of the Institutional Review Board (IRB) at Arizona State University (ASU).
Study Design
This study was designed as a 2-arm parallel feeding trial for 8 weeks. Participants were matched by age, gender, BMI, and fasting serum insulin concentrations at screening and then were assigned to 1 of 2 groups. Once all participants had been paired, the groups were randomly assigned to a HPLF diet or a HCLF diet. Energy levels (1200 kcal, 1500 kcal, 1700 kcal, or 2000 kcal) were assigned to participants on the basis of resting metabolic rate (RMR) – which was approximately 70% to 75% of total daily energy expenditure – to produce a weight loss of 1–1.5 pound (lb) per week.
All meals ran on a 2-week cyclic menu (1-week menu in Table 2) and were prepared using scales and liquid measures in the metabolic kitchen of the Department of Nutrition at ASU. Hot lunches were served to participants every weekday in the dining room adjacent to the metabolic kitchen. All other meals, including weekend meals, were prepared for home consumption. Participants were asked to consume only foods and beverages provided to them. All foods were analyzed for nutrient and energy composition using a nutrient analysis database (Food Processor SQL software, version 7.12; ESHA Research; Salem, Oregon).
Table 2.
Menus for High-Carbohydrate, Low-Fat (HCLF) and High-Protein, Low-Fat (HPLF) Diets for 1 Week at 1500-kcal Level
| Diet | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday |
|---|---|---|---|---|---|---|---|
| HCLF Breakfast | Pink Grapefruit 1/2–3 3/4" Cinnamon-Raisin Bagel 3–1/2" Low-fat Cream Cheese 1 oz Skim Milk 1 cup |
Whole Wheat Toast 1 slice Canola Margarine 2 tsp Strawberries 6 each Low-fat Granola 1/2 cup Skim Milk 1 cup |
Orange Juice 1 cup Total Raisin Bran 1 cup Whole Wheat English Muffin 1 each Canola Margarine 2 tsp Skim Milk 1 cup |
Frozen Waffles 2 each – 4" Canola Margarine 1 Tbsp LoCal Syrup 4 Tbsp Nectarine 1 each Skim Milk 1 cup |
Orange Juice 1/2 cup Total Corn Flakes 1 cup Whole Wheat English Muffin 1 each Canola Margarine 1 Tsp Skim Milk 1 cup |
Non-fat Yogurt 6 oz Whole Wheat Toast 2 slices Peanut Butter 2 Tbsp Banana 1 each Herbal Tea 1 cup |
Frozen Pancakes 2 each – 4" Light Syrup 4 Tbsp Peach 1 each Skim Milk 1 cup |
| HPLF Breakfast | Pink Grapefruit 1/2 – 3 3/4" Egg Beaters 1 cup Canola Margarine 1 Tbsp Canadian Bacon 3 oz Skim Milk 1 1/2 cup |
Whole Wheat Toast 1 slice Strawberries 6 each Fat-free Cottage Cheese 1 cup Skim Milk 1 cup |
Ricotta Cheese Roll-ups 1 corn tortilla (6") 3/4 cup low-fat ricotta 2tsp strawberry. jam Skim Milk 1 1/2 cup |
Ground Turkey Sausage 2 oz Nonfat Cottage Cheese 1 cup Nectarine 1 each Skim Milk 1 cup |
Nonfat Yogurt 1 cup Whole Wheat English Muffin 1/2 muffin Peanut Butter 1 tsp Skim Milk 1 1/2 cup |
Nonfat Yogurt 1 cup Whole Wheat Toast 1 slice Peanut Butter 1 Tbsp Banana 1 each Skim Milk 1 cup |
Swanson Ham & Cheese Omelette with Hash Browns 1 each Sliced Peaches 1/2 cup Skim Milk 1 cup |
| HCLF Lunch | Subway ColdCut Trio Sandwich 6" - Wheat 1/2 meat, cheese Nonfat Mayo 2 Tbsp Dill Pickle 1 spear Red Grapes 1 cup Crystal Light 1 cup Skittles Candy 20 pieces |
Garden Veggie Burger Patty on Multigrain Bun with Light Cucumber Ranch Dressing 2 Tbsp Lettuce Leaf & Tomato Slice 1 each Green Grapes 1 cup Crystal Light 1 cup Reese's Pieces 20 pieces |
Lean Pockets Chicken Fajita Sandwich 1 each Red & Green Pepper Strips 1/4 Red, 1/2 Green Pear 1 each Crystal Light 1 cup Snickers Miniatures Candy 3/4 oz |
Egg Salad Sandwich on Whole Wheat Bread with Lettuce Leaf & Tomato Slice Baby Carrots 6 each Celery Stalks 2 each Skim Milk 1 cup Golden Delicious Apple 1 each |
Totino's Cheese Party Pizza 1/4 pizza Mixed Salad Greens Salad with Tomato Wedges 2 cups w/ 1/2 Tomato Light Ranch Dressing 2 Tbsp Skim Milk 1 cup Banana 1 each |
Budget Gourmet Ziti Parmesan Dinner 1 each Romaine Lettuce with Cucumber & Sweet Red Bell Pepper 2 cups w/ 1/2 cup each Light Ranch Dressing 2 Tbsp Crystal Light 1 cup Candy Kisses 5 each |
Grilled Cheese on Whole Wheat Bread 1 oz Low-fat cheese 2 tsp margarine Mixed Salad Greens with Green, Red, Yellow Bell Peppers 2c 1/2 cup each Light Ranch Dressing 2 Tbsp Apple 1 each Crystal Light 1 cup |
| HPLF Lunch | Subway ColdCut Trio Sandwich 6" - Wheat Nonfat Mayo 2 Tbsp Dill Pickle 1 spear Red Grapes 1/2 cup Crystal Light 1 cup Skittles Candy 20 pieces |
Hamburger Patty 4 oz Light Ranch Dressing 1 Tbsp Lettuce Leaf & Tomato Slice 1 each Green Grapes 1/2 cup Crystal Light 1 cup Butterscotch Candies 3 each |
Salmon Fillet with Dill Weed & Lemon 4 oz salmon Glazed Asparagus 1 cup sautéed with balsamic vinegar Pear 1 each Crystal Light 1 cup M&Ms Plain Candy 10 pieces |
Egg Salad Sandwich on Whole Wheat Bread with Lettuce Leaf & Tomato Slice Baby Carrots 6 each Celery Stalks 2 each Crystal Light 1 cup Candy Kisses 5 each |
Totino's Cheese Party Pizza with Beef & Extra Cheese 1/4 pizza/3 oz beef & 1 oz skim mozzarella Mixed Salad Greens with Tomato Wedges 1 cup Tomato 1/4 of whole Nonfat Ranch Dressing 1 Tbsp Crystal Light 1 cup |
Meatloaf 3 oz Romaine Lettuce with Cucumber & Sweet Red Bell Peppers 2 cup/ 1/2 cup each Cucumber Ranch Dressing 1 Tbsp Crystal Light 1 cup |
Low-fat Tuna Salad on Whole Wheat Bread 3 oz tuna salad, 2 slices bread Mixed Salad Greens with Red & Yellow Bell Peppers 2 cups w/ 1/2 cup each Light Italian Dressing 2 Tbsp Crystal Light 1 cup |
| HCLF Dinner | Lean Cuisine Shrimp & Angel Hair Pasta Steamed Broccoli 1 cup Sliced Roma Tomato 1 each Skim Milk 1 cup Apple 1 each |
Iceberg & Romaine Lettuce, Chopped Broccoli 1 cup each Louis Rich Roasted Turkey & Low Sodium Deli Ham 1 oz each Tomato Slices 2 each Carrots 1/2 cup Croutons 1/2 cup Light Italian Dressing 2 Tbsp Whole Wheat Roll Skim Milk 1 cup Bartlett Pear 1 each |
Whole Wheat Fettuccini Alfredo Dinner Romaine Lettuce 2 cups Parmesan Cheese 1/2 oz Croutons 1/4 cup Nonfat Caesar Dressing 2 Tbsp Crystal Light 1 cup |
Frozen Beef Burrito Salsa 1/2 cup Mixed Salad Greens 2 cups Grated Carrots 1/2 cup Red Grapes 1/2 cup Crystal Light 1 cup Butterscotch Candies 4 each |
Healthy Choice Chicken Teriyaki Dinner Frozen Pea/Carrot Mix 1 cup Apple 1 each Crystal Light 1 cup Starburst Fruit Chews 1 oz |
Vegetable Stir Fry 1 cup veggies 1 oz chicken Brown Rice 2/3 cup Navel Orange 1 each Skim Milk 1 cup |
Healthy Choice BBQ Beef & South-western Rice Dinner Steamed Broccoli 1 cup Pineapple in own Juice 1 cup Skim Milk 1 cup Raisinets 1 oz |
| HPLF Dinner | Chicken Breast 4 oz Sliced Roma Tomato 1 each Steamed Broccoli 1 cup raw Light Italian Salad Dressing 1 Tbsp Skim Milk 1 cup |
Salad with Iceberg & Romaine Lettuce 1 cup each Chopped Broccoli 1 cup Roasted Turkey 2 oz Tomato Slices 2 each Carrots 1/2 cup Light Italian Dressing 2 Tbsp Whole Wheat Roll 1 each Pear 1 each Skim Milk 1 cup |
Lean Cuisine Chicken Fettuccini Dinner Mock Caesar Salad 2 cups Romaine 2 Tbsp Parmesan 1 Tbsp Fat-free Caesar Dressing Crystal Light 1 cup |
Frozen Beef Burrito Salsa 1/4 cup Mixed Salad Greens with Tofu Cubes 2 cup greens/1 cup tofu Grated Carrots 1/2 cup Crystal Light 1 cup |
Healthy Choice Chicken Teriyaki Dinner Peas 1/2 cup frozen Tofu Cubes 3/4 cup Firm Fat-free Italian Dressing 1 Tbsp Skim Milk 1 cup Butter Mints 6 each |
Vegetable Stir Fry with Chicken 1 cup vegetables 4 oz chicken Brown Rice 1/3 cup Skim Milk 1 cup M&M Plain Candies 20 pieces |
Lean Cuisine Chicken in Peanut Sauce Dinner Steamed Broccoli 1 cup Skim Milk 1 cup Butterfinger BB's 8 pieces |
Diets
Both energy-restricted diets (based on individual RMR values) were low in total fat, deriving approximately 27% of total energy intake from fat (<7% saturated fat, ≤ 10% monounsaturated fat, and ≤ 10% polyunsaturated fat). The composition of the HPLF diet (32% protein, 41% carbohydrate) was based on protein amounts typically used in other high-protein, moderate-carbohydrate diets, specifically the Zone diet.[22] The macronutrient composition of the HCLF diet (59% carbohydrate, 14% protein) was modeled after the 1996 US Food Guide Pyramid,[5] which included a variety of foods from each of the food groups. The micronutrient composition of the diets was similar. All meals on both diets were composed primarily of complex carbohydrates (eg, whole grains, fruits, and vegetables), lean protein sources (eg, poultry, fish, beans, egg whites, and extra-lean beef), and low-fat dairy products (eg, skim milk, nonfat yogurt, low-fat cheese, and nonfat cottage cheese). Dietary fiber intake was approximately 25 g/d and 17 g/d for the HCLF and HPLF diets, respectively. All foods served to participants were commercially available items that represented a well-rounded diet.
Dietary Intake and Satiety
Participants' satiety and consumption of nonstudy foods were recorded using a weekly satiety questionnaire. Using a 7-point Likert scale, all participants were asked to rate overall weekly levels of satiety or hunger by placing an “X” on a line indicating one of the following: “extremely hungry”; “hungry”; “semi-hungry”; “no particular feeling”; “semi-satisfied”; “satisfied”; or “extremely full.” In an effort to evaluate participant noncompliance, participants were also asked to indicate how many times per week nonstudy foods were consumed using a 12-point Likert scale (choices ranged from “never” to “every meal”) and how many times per week provided foods were not consumed using a 12-point Likert scale (choices ranged from “never” to “every meal”). Space was provided on the form for the participants to voluntarily record the nonstudy foods consumed and the study foods that were not consumed. However, the investigators did not require the participants to record the nonstudy foods consumed in order to increase the accuracy of the participants' reporting on the Likert scale.
Anthropometric Variables
Weight was measured using a digital scale (Tanita; TBF-300A; Arlington Heights, Illinois) at baseline and every Monday. Height was measured at the start of the trial using a standard stadiometer (a nonstretch tape attached to a vertical board with a moveable horizontal headboard). BMI was assessed at baseline using the following standard equation: weight (kg) / height (m2). Waist circumference was measured over clothing using a nonstretch measuring tape (Gulick II; Country Technology, Inc.; Gay Mills, Wisconsin) at baseline. Body composition was measured using dual x-ray absorptiometry (DEXA) (Prodigy Pro; GE Lunar; Waukesha, Wisconsin) at the Department of Nutrition at ASU at baseline and week 8 to assess changes in total, truncal, and leg body fat and body composition.
Laboratory Analyses
Fasting blood draws to measure plasma glucose and serum insulin were conducted at baseline and week 8. Approximately 10 mL of blood was drawn from the nondominant arm by the same trained technician. Participants were instructed to consume only water for 12 hours prior to the blood draw. All blood samples were centrifuged and stored at −80° C. Fasting plasma glucose values were measured at ASU using glucose oxidase methodology (Sigma Aldrich; St. Louis, Missouri). Fasting plasma insulin concentrations were measured (as a marker of IS) using a radioimmunoassay (MP Biomedicals; Irvine, California). For purposes of this study, IR was classified as fasting serum insulin ≥ 15 µU/L. IR was also assessed using the Homeostasis Model Assessment (HOMA-IR) formula: [fasting plasma insulin (in µU/L) X fasting plasma glucose (in mM)] / 22.5.
Metabolic Analyses
RMR and thermic effect of a meal (TEM) were determined using a metabolic cart (Sensormedics; Viasys Healthcare; Conshohocken, Pennsylvania) in the Department of Exercise and Wellness at ASU. Metabolic measurements were determined using a respiratory mask with a 2-way non-rebreathing valve (Oro-Nasal Mask; Hans-Rudolph, Inc.; Kansas City, Missouri) interfaced with an open-circuit spirometry metabolic analysis apparatus (MAX-II; AEI Technologies; Naperville, Illinois). All RMR and TEM measurements were taken by one trained technician at baseline and week 8 to determine an appropriate energy level and to assess changes in RMR with regard to diet or weight loss. Prior to RMR and TEM measurements, participants were positioned in a reclining chair and habituated to the metabolic analysis apparatus for 20 minutes in a temperature-controlled (25–27° C) quiet room. Participants were instructed to remain awake and not move, fidget, or talk once the mask was fitted. TEM values were determined using the energy expenditure of participants 2.5 hours after consuming a test meal representative of their respective trial diet. For both RMR and TEM, data was collected every 30 seconds until a minimum of 30 minutes of steady-state data (defined as a 10-minute period in which oxygen consumption volume, ventilation, and respiratory quotient did not vary by > 10%) was collected. Participants were asked to consume only water for 12 hours prior to each test meal.
Statistical Analyses
According to power analysis calculations, a minimum of 8 participants (4 per group) were needed to observe a statistically significant difference in weight loss between groups of 5% ± 0.5% of their initial body weight; 32 participants (16 per group) were needed to observe a statistically significant difference in serum insulin values between groups of 5.1 ± 1.1 μU/L, with a confidence interval of 95% and 80% power.[9,23] Due to limited funding, 20 participants were selected for participation.
All data are reported as mean ± standard error of the mean (mean ± SEM) only for participants who completed the study in its entirety (n = 16). All datasets were tested for normal distribution and outliers. A multivariate general linear model for repeated measures analysis of variance (MANOVA) was used to determine significant time and time X diet interactions for all variables. RMR and TEM were also analyzed using analysis of covariance (MANCOVA). When a time-effect was demonstrated by the multivariate test, paired t-tests (with Tukey correction) were used to make post-hoc comparisons within groups. Statistical significance was set at P < .05 with a 95% confidence interval. Statistical analysis was performed using SPSS for Windows software, version 12.0 (SPSS, Inc.; Chicago, Illinois).
Results
Of the 20 men and women randomly assigned to a HPLF or a HCLF diet, 2 male participants (1 from each diet) were dropped during the first week of the study due to an inability to comply with study protocol. An additional 2 participants on the HCLF diet (both females) voluntarily withdrew in the middle of the study. One withdrew due to an inability to comply with study protocols; the other withdrew due to a preexisting medical condition not revealed to researchers. Thus, 16 participants completed the study in its entirety (HPLF: n = 9; HCLF: n = 7). Baseline characteristics of the 16 participants who completed the study are shown in Table 1. The majority of participants were non-Hispanic whites (n = 10; HPLF: 6; HCLF: 4); 2 were African American (HPLF: 1; HCLF: 1); 1 was Asian (HPLF: 1); 1 was Hispanic (HPLF: 1); 1 was Native American (HCLF: 1); and 1 was a Pacific Islander (HCLF: 1). There were no significant differences in age, body weight, body composition, BMI, RMR, TEM, fasting glucose, or insulin concentrations between diet groups at baseline.
Reported Dietary Intake and Satiety
Participants in both diet groups consumed nonstudy foods approximately ≤ 1.5 times per week during the study. Differences in reported noncompliance and satiety did not differ significantly over time or between the 2 groups at any time point in the study (P > .05).
Body Weight and Composition
The effects of diet on body weight, percent body fat, and fat mass are shown in Table 3. Changes in body weight, fat mass, and percent body fat did not differ significantly between diet groups at any time point during the study, although a significant time effect on these parameters was noted. From baseline to week 8, body weight changed by −4.1 ± 0.6 kg and −4.9 ± 0.7 kg in the HPLF and HCLF diet groups, respectively. Total percent body fat (as measured using DEXA) differed significantly from baseline to week 8 in participants of both diet groups (HPLF: −1.5 ± 0.4%; HCLF: −0.4 ± 0.0%).
Table 3.
Changes in Body Weight and Fat Mass Between Groups1
| Baseline | Week 8 | P Value2 | |
|---|---|---|---|
| Weight (kg)3 | .005 | ||
| HPLF diet3 | 107.1 ± 7.8 | 102.2 ± 3.9 | |
| HCLF diet3 | 94.5 ± 3.6 | 90.5 ± 7.8 | |
| Fat mass (kg)5 | .001 | ||
| HPLF diet | 39.8 ± 3.5 | 36.8 ± 3.8 | |
| HCLF diet | 46.0 ± 4.0 | 43.6 ± 4.3 | |
| Body fat, total (%)5 | .035 | ||
| HPLF diet | 43.7 ± 2.2 | 42.2 ± 2.5 | |
| HCLF diet | 45.1 ± 2.5 | 44.7 ± 2.8 | |
| Leg fat (%)5 | .001 | ||
| HPLF diet | 44.3 ± 3.0 | 42.3 ± 3.2 | |
| HCLF diet | 46.5 ± 3.4 | 45.6 ± 3.9 | |
| Truncal fat (%)5 | .523 | ||
| HPLF diet | 44.6 ± 1.5 | 43.4 ± 2.0 | |
| HCLF diet | 45.0 ± 2.3 | 45.4 ± 2.0 |
All values are mean ± SEM. Groups did not differ significantly at baseline.
Data were analyzed by multivariate general model for repeated measures from baseline to week 8. Significance was set at P < .05. Significance was found for time interactions only; significance was not found for interactions between diets.
Values as measured by electronic scale.
HPLF, high-protein, low-fat (n = 9); HCLF, high-carbohydrate, low-fat (n = 7).
Values as measured by dual x-ray absorptiometry (DEXA).
Metabolic Indices
After adjusting for gender, age, fat mass, and fat-free mass (FFM), RMR (kcal/kg FFM) and TEM (kcal/kg FFM) did not differ significantly between diet groups at any time point during the study. RMR and TEM both decreased significantly from baseline to week 8 in participants of both diet groups (Table 4).
Table 4.
Metabolic Changes Between Groups1
| Baseline (kcal/kg FFM) | Week 8 (kcal/kg FFM) | P Value2 | |
|---|---|---|---|
| Resting Metabolic Rate | .001 | ||
| HPLF3 | 27.9 ± 2.2 (19.7–34.0) | 23.7 ± 1.5 (19.0–30.8) | |
| HCLF | 30.4 ± 1.5 (18.2–34.6) | 25.3 ± 1.7 (16.0–29.4) | |
| Thermic Effect of Meal | .001 | ||
| HPLF | 34. 9 ± 3.1 (24.3–47.1) | 26.3 ± 1.5 (20.2–33.3) | |
| HCLF | 35.3 ± 1.9 (25.5–39.8) | 26.3 ± 2.2 (14.7–33.3) | |
All values are mean ± SEM; range in parentheses. Groups did not differ significantly at baseline.
Data were analyzed by analysis of covariance (covariates: mass, fat-free mass, age, and % body fat). Significance was set at P < .05. No significance was found for interactions between diets.
HPLF, high-protein, low-fat (n = 9); HCLF, high-carbohydrate, low-fat (n = 7).
Fasting Glucose and Insulin
Neither diet significantly changed fasting plasma glucose or fasting serum insulin at any point during the study (as shown in Table 5). IS, as determined by the HOMA formula, did not differ significantly between groups from baseline to week 8.
Table 5.
Physiologic Changes Between Groups1
| Baseline | Week 8 | P Value2 | |
|---|---|---|---|
| Primary Physiologic Measurements | |||
| Fasting insulin (µU/L) | .955 | ||
| HPLF3 | 29.8 ± 11.7 | 30.0 ± 11.7 | |
| HCLF3 | 19.7 ± 2.6 | 19.6 ± 6.2 | |
| HOMA insulin sensitivity | .493 | ||
| HPLF | 3.1 ± 0.8 | 4.6 ± 0.4 | |
| HCLF | 5.3 ± 0.7 | 4.4 ± 0.7 | |
| Fasting glucose (mg/dL) | .137 | ||
| HPLF | 98.6 ± 4.0 | 110.8 ± 12.3 | |
| HCLF | 111.0 ± 5.3 | 106.4 ± 6.2 | |
| Secondary Physiologic Measurements | |||
| Total cholesterol (mg/dL) | .5614 | ||
| HPLF | 225.0 ± 9.0 | 207.4 ± 8.0 | |
| HCLF | 188.9 ± 12.9 | 178.6 ± 9.6 | |
| LDL cholesterol (mg/dL) | .880 | ||
| HPLF | 146.0 ± 7.8 | 135.5 ± 7.2 | |
| HCLF | 119.7 ± 11.0 | 111.1 ± 15.3 | |
| HDL cholesterol (mg/dL) | .909 | ||
| HPLF | 54.8 ± 2.7 | 48.6 ± 3.7 | |
| HCLF | 49.7 ± 5.4 | 43.1 ± 3.2 | |
| VLDL cholesterol (mg/dL) | .216 | ||
| HPLF | 24.3 ± 4.7 | 19.9 ± 2.6 | |
| HCLF | 20.0 ± 4.6 | 23.0 ± 5.1 | |
| Triacylglyceride (mg/dL) | .574 | ||
| HPLF | 146.0 ± 27.5 | 153.3 ± 40.8 | |
| HCLF | 118.7 ± 27.7 | 138.3 ± 30.2 | |
| Systolic blood pressure (mm Hg) | .821 | ||
| HPLF | 113 ± 3 | 114 ± 2 | |
| HCLF | 116 ± 2 | 119 ± 4 | |
| Diastolic blood pressure (mm Hg) | .914 | ||
| HPLF | 72 ± 2 | 69 ± 3 | |
| HCLF | 71 ± 4 | 69 ± 3 |
All values are mean ± SEM. Groups did not differ significantly at baseline.
Data were analyzed by multivariate general model for repeated measures. Significance was set at P < .05 (with Tukey correction).
HPLF, high-protein, low-fat (n = 9); HCLF, high-carbohydrate, low-fat (n = 7).
Differences in total cholesterol (P = .039) and HDL cholesterol (P = .001) were significant for time interaction between baseline and week 8, but no significance was found between diet groups at any time point in the study.
Discussion
The results of this study demonstrate that both energy-restricted diets were equally effective at producing a ≥ 5% loss of body weight in overweight adults with IR. Fat mass and total percent body fat were similarly and significantly reduced by both diets.
Differences in satiety and noncompliance did not differ significantly between groups. Previous studies have found that high-protein diets – as compared with high-carbohydrate diets – may be more satiating and more effective for weight loss because high-protein diets are purported to increase postprandial thermogenesis after a high-protein meal.[10,24] The results of this study, however, found that both RMR and TEM were significantly reduced in participants of both diet groups. Additionally, the higher fiber content of the HCLF diet (25 g/d vs 17 g/d for the HPLF diet) may have contributed to the satiating effect of the HCLF diet. Because fiber-rich meals are processed more slowly, the satiating effect of the higher-fiber meal is enhanced.[25]
Both diets were significantly effective for reducing body weight, fat mass, and total percent body fat during the 8-week study. A previous study by Skov and colleagues[7] reported that participants consuming a HPLF diet lost more body weight than participants consuming a HCLF diet (30% fat). However, their study was conducted over a 6-month period and included a greater number of participants (high-carbohydrate diet: n = 25; high-protein diet: n = 25). The shorter duration of the current study may have masked any weight loss benefit of the HPLF diet.
Although participants were asked to maintain any physical activity and exercise programs at levels in which they had engaged prior to the study, activity levels were not monitored during this study. This is a potential confounder and a limitation to this study because participants may have altered their activity and exercise programs to enhance their weight loss.
Currently, a diet high in carbohydrate and low in fat is recommended for weight loss by various health agencies and organizations.[2,4] Previous studies have shown that a HCLF diet can promote weight loss,[26,27] although some recent studies have shown that a high-carbohydrate diet may increase serum insulin levels and contribute to IR.[8, 11–13] However, other studies that investigated the effects of a high-carbohydrate diet on IS have reported improvements in IS compared with diets lower in carbohydrate content.[28,29] On the other hand, diets high in protein and moderate or low in carbohydrate have also been shown to reduce serum insulin, thereby significantly improving IS.[8,10,12,17]
Fasting insulin did not differ significantly during this study. Previous studies have reported both increases and decreases in serum insulin while participants were consuming a high-carbohydrate diet.[8–10, 12,13,28,29] The most likely explanation for the lack of statistically significant changes in fasting insulin is that the study was underpowered with regard to changes in fasting insulin. Other possible contributing factors were that participants did not lose enough body weight to precipitate an improvement in IS[6,7,9,12,15–17, 19,21, 30–32] or that the higher fiber content of the HCLF diet countered any potential increase in serum insulin for the participants on the HCLF diet. Additionally, the decrease in weight on the HCLF diet may have countered any diet-related increase in insulin levels for participants on that diet. Recent research claims that a moderate loss of just 5% of body weight effectively reduces serum insulin.[15,26] However, participants in both diet groups lost approximately 5% of body weight during this study, yet fasting insulin did not significantly improve in any participants.
The results of this study demonstrate that both low-fat diets were equally effective for weight loss in overweight individuals with IR. Because both diets significantly reduced body weight by the study's end, researchers postulate that weight loss appeared to be more affected by low dietary fat intake and energy restriction than by the macronutrient composition of the 2 diets. Although fasting serum insulin was not significantly improved at weight loss ≥ 5% contrary to existing research,[15,22] this is most likely due to the low number of participants that were entered into and completed the study. Additionally, because many additional factors – independent of weight loss – influence IS (eg, fat mass, abdominal adiposity, hormones, age, gender, and exercise), it is difficult to assess the role that weight loss specifically played in serum insulin changes over a relatively short time period (8 weeks). The findings from this study suggested that both of the low-fat diets, regardless of macronutrient distribution, resulted in an overall weight loss of approximately 1 lb per week or a 5% loss in body weight over the course of the 8-week study.
Funding Information
The research in this article is supported by a grant from the Gustavus & Louise Pfeiffer Research Foundation.
Footnotes
Readers are encouraged to respond to George Lundberg, MD, Editor of MedGenMed, for the editor's eye only or for possible publication via email: glundberg@medscape.net
Contributor Information
Rima E. Kleiner, Department of Nutrition, Arizona State University, Mesa, Arizona.
Andrea M. Hutchins, Department of Health Sciences, Beth-El College of Nursing and Health Sciences, University of Colorado at Colorado Springs.
Carol S. Johnston, Department of Nutrition, Arizona State University, Mesa, Arizona.
Pamela D. Swan, Department of Exercise and Wellness, Arizona State University, Mesa, Arizona.
References
- 1.Serdula MK, Mokdad AH, Williamson DF, et al. Prevalence of attempting weight loss and strategies for controlling weight. JAMA. 1999;282:1353–1358. doi: 10.1001/jama.282.14.1353. [DOI] [PubMed] [Google Scholar]
- 2.Department of Health and Human Services. Publication No. (FDA) 92-1189: The facts about weight loss products and programs. December 3, 2004. Available at: http://www.cfsan.fda.gov/~dms/wgtloss.html Accessed November 3, 2006.
- 3.Centers for Disease Control and Prevention. Fast stats A to Z: Overweight prevalence. February 28, 2006. Available at: http://www.cdc.gov/nchs/fastats/overwt.htm Accessed November 3, 2006.
- 4.American Dietetic Association. Position paper: Weight management. December 3, 2005. Available at: http://www.eatright.org/cps/rde/xchg/ada/hs.xsl/advocacy_adar0802_ENU_HTML.htm Accessed November 3, 2006.
- 5.United States Department of Agriculture. U.S. Food Guide Pyramid. March 15, 2006. Available at: http://www.cnpp.usda.gov/Publications/MyPyramid/OriginalFoodGuidePyramids/FGP/FGPPamphlet.pdf#xml=http://209.248.219.257/texis/search/pdfhi.txt?query=USDA+Food+Guide+Pyramid&pr=CNPP&prox=page&rorder=500&rprox=500&rdfreq=500&rwfreq=500&rlead=500&sufs=202&order=r&mode=&opts=&cq=&sr=&id=244baf16612 Accessed November 3, 2006.
- 6.Parker B, Noakes N, Luscombe N, Clifton P. Effect of a high protein, high monounsaturated fat weight loss diet on glycemic control and lipid levels in type 2 diabetes. Diabetes Care. 2002;25:425–430. doi: 10.2337/diacare.25.3.425. [DOI] [PubMed] [Google Scholar]
- 7.Skov A, Toubro S, Ronn B, et al. Randomized trial on protein versus carbohydrate in ad libitum fat reduced diet for the treatment of obesity. Int J Obes (Lond) 1999;23:528–536. doi: 10.1038/sj.ijo.0800867. [DOI] [PubMed] [Google Scholar]
- 8.Piatti PM, Monti LD, Fermo I, et al. Hypocaloric high protein diet improves glucose oxidation and spares lean body mass: comparison to hypocaloric high carbohydrate diet. Metabolism. 1994;43:1481–1487. doi: 10.1016/0026-0495(94)90005-1. [DOI] [PubMed] [Google Scholar]
- 9.Johnston CS, Tjonn SL, Swan PD. High-protein, low-fat diets are effective for weight loss and favorably alter biomarkers in healthy adults. J Nutr. 2004;134:586–591. doi: 10.1093/jn/134.3.586. [DOI] [PubMed] [Google Scholar]
- 10.Johnston CS, Day CS, Swan PD. Postprandial thermogenesis is increased 100% on a high-protein, low-fat diet versus a high-carbohydrate, low-fat diet in healthy, young women. J Am Coll Nutr. 2002;21:55–61. doi: 10.1080/07315724.2002.10719194. [DOI] [PubMed] [Google Scholar]
- 11.Borkman M, Campbell LV, Chisholm DJ, Storlien LH. Comparison of the effects on insulin sensitivity of high carbohydrate and high fat diets in normal subjects. J Clin Endocrinol Metab. 1991;72:432–437. doi: 10.1210/jcem-72-2-432. [DOI] [PubMed] [Google Scholar]
- 12.Meckling KA, O'Sullivan C, Saari D. Comparison of a low fat diet to a low carbohydrate diet on weight loss, body composition and risk factors for diabetes and cardiovascular disease in free-living, overweight men and women. J Clin Endocrinol Metab. 2004;89:2717–2723. doi: 10.1210/jc.2003-031606. [DOI] [PubMed] [Google Scholar]
- 13.Schulze MB, Hu FB. Dietary approaches to prevent the metabolic syndrome. Diabetes Care. 2004;27:613–614. doi: 10.2337/diacare.27.2.613. [DOI] [PubMed] [Google Scholar]
- 14.National Institutes of Health. Press release: The increasing number of adults with high blood pressure. August 23, 2004. Available at: http://www.nhlbi.nih.gov/new/press/04-08-23.htm Accessed October 30, 2006.
- 15.Lara-Castro C, Garvey WT. Diet, insulin resistance and obesity: zoning in on data for Atkins dieters living in South Beach. J Clin Endocrinol Metab. 2004;89:4197–4205. doi: 10.1210/jc.2004-0683. [DOI] [PubMed] [Google Scholar]
- 16.Cox KL, Burke V, Morton AR, et al. Independent and additive effects of energy restriction and exercise on glucose and insulin levels in sedentary overweight men. Am J Clin Nutr. 2004;80:308–316. doi: 10.1093/ajcn/80.2.308. [DOI] [PubMed] [Google Scholar]
- 17.Farnsworth E, Luscombe ND, Noakes M, et al. Effect of a high-protein, energy-restricted diet on body composition, glycemic control and lipid concentrations in overweight and obese hyperinsulinemic men and women. Am J Clin Nutr. 2003;78:31–39. doi: 10.1093/ajcn/78.1.31. [DOI] [PubMed] [Google Scholar]
- 18.Ferrannini E, Natali A, Bell P, et al. Insulin resistance and hypersecretion in obesity. J Clin Invest. 1997;100:1166–1173. doi: 10.1172/JCI119628. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Foster GD, Wyatt HR, Hill JO, et al. A randomized trial of a low-carbohydrate diet for obesity. N Engl J Med. 2003;348:2082–2090. doi: 10.1056/NEJMoa022207. [DOI] [PubMed] [Google Scholar]
- 20.Jeppesen J, Schaaf P, Jones C, et al. Effects of low fat, high carbohydrate diets on risk factors for ischemic heart disease in postmenopausal women. Am J Clin Nutr. 1997;65:1027–1033. doi: 10.1093/ajcn/65.4.1027. [DOI] [PubMed] [Google Scholar]
- 21.Weinstock RS, Huiliang D, Wadden TA. Diet and exercise in the treatment of obesity: effects of three interventions on insulin resistance. Arch Intern Med. 1998;158:2477–2483. doi: 10.1001/archinte.158.22.2477. [DOI] [PubMed] [Google Scholar]
- 22.Klein S, Sheard NF, Pi-Sunyer X, et al. Weight management through lifestyle modification for the prevention and management of type 2 diabetes: rationale and strategies. Am J Clin Nutr. 2004;80:257–263. doi: 10.1093/ajcn/80.2.257. [DOI] [PubMed] [Google Scholar]
- 23.Wolever TMS, Mehling C. Long-term effect of varying the source or amount of dietary carbohydrate on postprandial plasma glucose, insulin, triacylglycerol, and free fatty acid concentrations in subjects with impaired glucose tolerance. Am J Clin Nutr. 2003;77:612–621. doi: 10.1093/ajcn/77.3.612. [DOI] [PubMed] [Google Scholar]
- 24.Eisenstein J, Roberts SB, Dallal G, Saltzman E. High-protein weight-loss diets: are they safe and do they work? a review of the experimental and epidemiologic data. Nutr Rev. 2002;60:189–199. doi: 10.1301/00296640260184264. [DOI] [PubMed] [Google Scholar]
- 25.Marlett JA, McBurney MI, Slavin JL. Position of the American Dietetic Association: health implications of dietary fiber. J Am Diet Assoc. 2002;102:993–1000. doi: 10.1016/s0002-8223(02)90228-2. [DOI] [PubMed] [Google Scholar]
- 26.Davy BM, Melby CL. The effect of fiber-rich carbohydrates on features of syndrome X. J Am Diet Assoc. 2003;103:86–89. doi: 10.1053/jada.2003.50005. [DOI] [PubMed] [Google Scholar]
- 27.Popitt SD, Keogh GF, Prentice AM, et al. Long term effects of ad libitum low fat, high carbohydrate diets on body weight and serum lipids in overweight subjects with metabolic syndrome. Am J Clin Nutr. 2002;75:11–20. doi: 10.1093/ajcn/75.1.11. [DOI] [PubMed] [Google Scholar]
- 28.Cornier M-A, Donahoo WT, Pereira R, et al. Insulin sensitivity determines the effectiveness of dietary macronutrient composition on weight loss in obese women. Obes Res. 2005;13:703–709. doi: 10.1038/oby.2005.79. [DOI] [PubMed] [Google Scholar]
- 29.McAuley KA, Hopkins CM, Smith KJ, et al. Comparison of high-fat and high-protein diets with a high-carbohydrate diet in insulin-resistant obese women. Diabetologia. 2005;48:8–16. doi: 10.1007/s00125-004-1603-4. [DOI] [PubMed] [Google Scholar]
- 30.Jenkins DJA, Axelson M, Kendall CWC, et al. Dietary fiber, lente carbohydrates and the insulin resistant diseases. Br J Nutr. 2000;83:157S–163S. doi: 10.1017/s0007114500001100. [DOI] [PubMed] [Google Scholar]
- 31.Park SH, Lee WY, Lee YS, et al. The relative effects of obesity and insulin resistance on cardiovascular risk factors in nondiabetic and normotensive men. Korean J Intern Med. 2004;19:75–80. doi: 10.3904/kjim.2004.19.2.75. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 32.Shepherd PR, Kahn BB. Glucose transporters and insulin action: implications for insulin resistance and diabetes mellitus. N Engl J Med. 1999;341:248–257. doi: 10.1056/NEJM199907223410406. [DOI] [PubMed] [Google Scholar]