Dairy Food Consumption and Meal-Induced Cortisol Response Interacted to Influence Weight Loss in Overweight Women Undergoing a 12-Week, Meal-Controlled, Weight Loss Intervention^,,

Abstract

Dairy food enhances weight loss in animal models, possibly by modifying the metabolic effects of cortisol. This study determined in overweight women (ages 20.0–45.9 y; n = 51) whether including dairy food in an energy-restricted diet affects cortisol concentrations and whether differences in provoked cortisol explain the magnitude of weight loss. Women received either an adequate amount of dairy food (AD), the equivalent of ≥711 mL/d milk, or a low amount of dairy food (LD), the equivalent to ≤238 mL/d milk, in a 12-wk, energy-restricted dietary intervention. Participants were tested in a 12-h laboratory visit, which included 2 standard meals and a dinner buffet that was consumed ad libitum. Salivary cortisol was measured from waking to bedtime. Energy restriction increased (P ≤ 0.04) the minimum and decreased (P ≤ 0.02) the diurnal amplitude in the salivary cortisol concentration from baseline to postintervention. Energy restriction enhanced the dinner meal–stimulated salivary cortisol response (DMR) (P ≤ 0.02) but only in the LD group. Compared with the LD treatment, the AD treatment induced (P ≤ 0.04) greater reductions in body weight and fat, but only in women characterized as having a baseline DMR (responders) (n = 26); weight and fat lost in the AD and LD groups were similar in nonresponders (n = 25). Overall, energy restriction dampened diurnal salivary cortisol fluctuations [symptomatic of hypothalamic-pituitary-adrenal (HPA) axis dysfunction] and enhanced dinner meal–stimulated salivary cortisol concentrations. The AD treatment prevented the latter. Furthermore, certain phenotypic markers of HPA axis function may help to expose the weight-reducing effects of consuming dairy food.

Introduction

Obesity is a worldwide health problem (1). Moderate energy restriction is considered to be a safe and effective means to lose weight. Inclusion of dairy food in a moderate, energy-restricted diet may improve weight loss (2). Whereas animal data suggest a beneficial effect of calcium and dairy proteins on weight loss, human studies have yielded mixed results (3, 4). In fact, we recently reported that dairy food provided no additional effect to diet-induced weight loss in overweight and obese adults (5). Aside from differences in study design, the discrepancy in findings may result from phenotypic (e.g., physiological) variability that inherently exists within a study population. Given the link among hypothalamic-pituitary-adrenal (HPA)⁷ axis regulation, cortisol, and energy balance, individual differences in cortisol regulation may help explain why some persons are more successful than others at losing, and keeping off, body weight (6, 7).

Circulating cortisol is primarily determined by activity in the HPA axis. A healthy HPA axis is characterized in part by a discernable diurnal rhythm in circulating cortisol and brisk cortisol increases in response to psychological and nutritional stimuli. For example, circulating concentrations of cortisol typically increase in response to waking, meals (e.g., lunch), and stress (8–10), but there does exist a high degree of person-to-person variability in the responsiveness of cortisol to these stimuli (11, 12). Abnormally high or low cortisol concentrations and/or its responsiveness to stimuli have been linked to dysregulation in the HPA axis, chronic stress, upper body fat, and insulin resistance (13–16).

Chronic stress, stress-related visceral obesity, and HPA dysregulation are frequently affiliated with a dampened or flat diurnal cortisol rhythm (17). A relatively flat cortisol diurnal rhythm and a blunted meal-induced cortisol response are indicative of HPA dysregulation. In individuals with this type of HPA profile, upper body obesity and other cardiovascular risk factors positively correlate with stress- and meal-associated cortisol responses. On the other hand, in people with a normal cortisol rhythm, meal-induced cortisol inversely correlates with BMI and positively with other indices of optimum metabolic health (18). Overall, reduced variability in cortisol concentrations across the day is indicative of dysregulation in the HPA axis, which typifies a chronically overactive HPA axis as might occur in response to persistent stress exposure (19).

Dieting and fat loss can lead to changes in the regulation of the HPA axis (20). Energy restriction stimulates the HPA axis in obese rats (21). In humans, evidence suggests that dieting can disrupt HPA function, circulating cortisol concentrations, and tissue-specific cortisol metabolism (20, 22, 23). These diet-induced shifts in HPA regulation may result in part from the psychological stress of chronic energy restriction (20).

Recent evidence suggests that the beneficial weight loss effect observed from dairy food supplementation results in part from reduced appetite and desire to eat (24). Given the role of cortisol in motivating food intake, the effect of dairy products on food intake may in part result from their effects on cortisol. Previous studies have shown the effects of consuming dairy foods or their constituents on HPA regulation and concentrations of both tissue and circulating cortisol (25). Diets low in calcium stimulate cortisol synthesis in visceral adipose tissue and consumption of whey protein or compounds present in the whey fraction of milk decreases both basal and stress-related circulating salivary cortisol and improves mood and cognitive function (26, 27). Therefore, the inclusion of dairy food in an energy-restricted diet may prevent the dieting-induced disruption of HPA axis regulation.

Identifying predisposing phenotypic differences in HPA regulation as well as determining the consequences of dairy food consumption during extended periods of dieting and weight loss on HPA regulation and circulating cortisol may highlight subpopulations that respond to the consumption of dairy food with improved weight loss. However, the data to support this theory are controversial. The aims of the current study were to determine if 1) weight loss induced by energy restriction disrupts normal patterns of circulating salivary cortisol concentrations in obese women; 2) the inclusion of dairy food in an energy-restricted diet mitigates the effects of energy restriction on circulating salivary cortisol and salivary cortisol responsiveness; and 3) baseline differences in salivary cortisol responsiveness explain the magnitude of weight loss in response to energy restriction and dairy food consumption.

Materials and Methods

The study was conducted in accordance with ethical standards set by the University of California, Davis Office of Research Institutional Review Board.

Study design.

A detailed description of the study design was reported by Van Loan et. al (5). Women aged 20.0–45.9 y with a BMI between 28.0 and 37.0 kg/m² were recruited to participate in a 15-wk clinical weight loss study at the USDA Western Human Nutrition Research Center (WHNRC). For eligibility, participants’ habitual dairy food consumption had to equal ≤238 mL/d milk, ≤43.0 g/d cheese, or ≤227.0 g/d yogurt and total calcium intake had to be ≤0.6 g/d. The typical daily intake of dairy food from all sources is 247.0 g for U.S. adults (28). A 3-wk baseline period preceded the dietary intervention and was used to establish individual maintenance energy needs. This was followed by a 12-wk, energy-restricted diet (−2.09 MJ/d). Participants were randomly assigned to 1 of 2 treatment groups: a low amount of dairy food (LD), the equivalent to ≤238 mL/d milk, ≤43.0 g/d cheese, or ≤227.0 g/d yogurt (n = 27); or an adequate amount of dairy food (AD), the equivalent to ≥711 mL/d milk, ≥129.0 g/d cheese, ≥681.0 g/d yogurt, or any combination thereof (n = 24). Total fat mass, android or upper body fat mass, and gynoid or lower body fat mass were determined by DXA. The android region spans the area between the pelvis cut and 20% of the distance between the pelvis and neck cuts. The gynoid region begins at the upper part of the greater trochanters and spans the distance equal to twice the height of the android region. The current assessment of circulating cortisol concentrations was focused on the women from the larger study, which included men and women. Previous research suggests that gender influences both basal and exogenously induced cortisol secretion and metabolism (29, 30). The present study enrolled too few men to observe gender differences, so only data from women were considered in order to eliminate potential confounding effects of gender.

Diets.

The diets for both the baseline period and the treatment arms were created to resemble usual consumption of the U.S. population for macronutrient and fiber intake (fat, ∼35% of total energy; carbohydrates, ∼49% of total energy; protein, ∼16% of total energy; fiber, 8–10 g/4.18 MJ). Participants were provided all of their food for the duration of the study. During the first 5 wk and last 3 wk of the study, participants ate 48% of their meals at the WHNRC. Between wk 6 and 12, participants were permitted to consume all of their meals at home, but they were asked to return to the WHNRC twice weekly to get weighed and pick up their packed food.the

Test day protocol.

The purpose of the test day was to measure circulating cortisol concentrations, cortisol responsiveness, and eating behaviors in a controlled laboratory setting. During the last week of the baseline period and again during the final 2 wk of the intervention, participants came to the WHNRC at ∼0810 h to consume a standardized breakfast (1.87 MJ, 17.5 g fat, 55.2 g carbohydrate, 18.1 g protein, and 1.9 g total fiber). They were required to complete the meal, which consisted of a bagel, scrambled eggs, butter, water, and orange juice, within 10 min. Upon completion, participants were allowed to leave the center but were asked to return 4 h later by 1220 h for a 15-min lunch (2.24 MJ, 21.1 g fat, 67.2 g carbohydrate, 22.2 g protein, and 7.6 g total fiber). Lunch consisted of a turkey sandwich on wheat bread (with tomato, lettuce, and mayonnaise), potato chips, sliced apples, a green salad (with tomatoes, carrots, and Italian dressing), and water. All test meals contained 25 and 30% of the participants’ prescribed restricted daily energy needs for breakfast and lunch, respectively.

After lunch, the participants stayed at the WHNRC for the remainder of the afternoon. At ∼1600 h, they were sequestered in private rooms and were told to inform staff when they were ready to eat dinner. Upon request, a cart containing a variety of fresh food items in excess (totaling ∼35.6 MJ) of that which could normally be eaten was delivered to the participant’s room. Food was presented with packaging removed. Participants had an unlimited amount of time to eat and they were allowed to consume as much as they wanted. Participants were asked to inform a staff member when they were finished, at which point the cart was removed from the room. All remaining food on the cart was weighed back. At 1945 h, the participants were permitted to leave the WHNRC.

Saliva collection and cortisol analysis.

Circulating cortisol concentrations were measured by collecting and assaying saliva. Saliva samples were collected in salivettes (Salimetrics) for the measurement of free cortisol at 10 specific time points during the test day. These samples were taken immediately after waking; 15 min post waking; upon arrival at the WHNRC and prior to lunch; 30, 45, and 60 min post lunch; upon request of the dinner buffet; 30 min after the completion of the dinner buffet; upon leaving the center at ∼1945 h; and before bed. For measurement of salivary cortisol, thawed saliva samples were centrifuged and assayed per the manufacturer’s instructions in duplicate using high sensitivity enzyme immunoassay (Salimetrics). The salivary cortisol awakening response (CAR) was calculated by subtracting the sample taken 15 min post waking from the waking sample. The lunch meal-stimulated salivary cortisol response (LMR) and dinner meal–stimulated salivary cortisol response (DMR) were calculated by subtracting the sample taken 30 min after the meal from the premeal sample. The salivary cortisol daily mean, minimum, maximum, and amplitude were calculated from all basal samples taken from the test day, which included the samples at waking, 15 min post waking, prior to lunch, upon request of the dinner buffet, upon leaving the center at ∼1945 h, and before bed. As previously described, the total variability in the salivary cortisol concentrations during the day was estimated by calculating the total daily variance for all 10 salivary cortisol samples (18). The mean, minimum, maximum, and amplitude were calculated only if all 10 salivary cortisol samples were available for analysis. The change from baseline for all variables was calculated by subtracting the baseline value prior to the intervention from the end of intervention value after 12 wk of the intervention.

Statistical analysis.

All statistical analyses were done using PASW Grad Pack 18 software. The normal distribution of each variable was tested using the Kolmogorov-Smirnov test and variables that were not normally distributed were log transformed.

Change in daily salivary cortisol concentration.

The change in salivary cortisol concentration from baseline to end of intervention was calculated for each of the 10 sampling times. A mixed model, repeated-measures design with a first-order autoregressive covariance structure was applied to examine these salivary cortisol Δ for all 10 sampling times. Treatment group, sample time, and their interaction were included as fixed effects in the repeated-measures model.

Change in salivary cortisol rhythm parameters.

Based on the output of our repeated-measures analysis (above) and on commonly examined parameters (CAR, LMR, DMR, mean, minimum, maximum, amplitude, and total variance) (10, 31) to characterize the salivary cortisol rhythm, a GLM ANCOVA was also employed to test whether energy restriction and, in separate models, dairy food consumption induced changes in these individual cortisol parameters. The Δ from baseline to end of intervention was calculated for each parameter and separately tested. For each model that tested the effects of dairy food consumption, the salivary cortisol parameter tested (e.g., Δ CAR) was the dependent variable. Independent variables included the corresponding baseline value (e.g., baseline CAR), treatment (dairy food), and treatment × baseline. In each case, the treatment × baseline interaction was not significant and this interaction term was therefore removed from the model. If no dairy food effects were observed, the Δ salivary cortisol parameter (e.g., Δ minimum) was examined to determine whether this Δ represented a significant change (different from 0 change) over the period of energy restriction. For this analysis, the baseline salivary cortisol parameter (e.g., baseline minimum) was standardized to a mean of 0 and SD of 1 and then entered as an independent variable in the model.

Salivary cortisol responsiveness and weight loss.

Salivary cortisol responsiveness at baseline was determined by simple median split of baseline CAR, LMR, and DMR salivary cortisol values using all participants. A repeated-measures mixed model was performed to test whether weight and fat loss differed between treatment conditions (LD group vs. AD group) in salivary cortisol responders and nonresponders. Treatment was included as the between-participant factor and time (baseline vs. end of intervention) as the within-participant factor. Treatment × time was included as an interaction term in the model. Dependent variables assessed were body weight, total fat, upper body fat mass, and lower body fat mass. Results are expressed textually and in tables as means ± SD. Significance was set at P ≤ 0.05.

Results

Fifty-one women (LD group, n = 27; AD group, n = 24) completed all phases of the intervention, including baseline and end of intervention testing, and were included in the final analyses. However, for the salivary cortisol variables, CAR (n = 46), minimum (n = 30), maximum (n = 30), mean (n = 30), and variance (n = 25), only a subset of the data were available for analysis.

Participants from both the LD and AD groups lost body weight and fat mass during the intervention period. BMI and waist circumference also decreased for these groups. However, baseline and end of intervention body weight, fat mass, BMI, and waist circumference did not significantly differ between the LD and AD groups (Table 1).

TABLE 1.

Baseline (preintervention) and end of intervention characteristics of women exposed to a 12-wk, energy-restricted diet and given either an LD (≤238 mL milk) or AD (≥711 mL milk)¹

	LD (n = 27)		AD (n = 24)
Treatment	Baseline2	End of intervention	Baseline	End of intervention
Body weight, kg	86.9 ± 10.5	81.4 ± 11.0*	90.7 ± 10.1	84.2 ± 9.8*
Fat mass, kg	40.1 ± 6.9	35.4 ± 8.1*	41.7 ± 5.5	36.7 ± 6.1*
Fat mass, % of body weight	46.5 ± 4.6	43.5 ± 6.5*	46.5 ± 3.5	43.9 ± 4.9*
BMI, kg/m2	32.2 ± 2.6	30.1 ± 2.9*	33.4 ± 2.5	31.0 ± 2.5*
Waist, cm	90.5 ± 6.7	86.5 ± 7.0*	93.6 ± 6.0	89.2 ± 6.1*
IAAT, cm2	32.9 ± 13.0	26.5 ± 10.7*	35.6 ± 13.0	29.2 ± 12.1*
Upper body fat, kg	3.8 ± 0.9	3.3 ± 1.0*	3.9 ± 0.8	3.4 ± 0.8*
Lower body fat, kg	7.7 ± 1.9	6.7 ± 1.4*	7.7 ± 1.4	6.8 ± 1.4*
Age, y	32.6 ± 6.0		31.0 ± 8.7

¹Values are mean ± SD. *Different from baseline, ≤ 0.01. AD, adequate amount of dairy food; IAAT, intra-abdominal adipose tissue; LD, low amount of dairy food.

²The treatment groups did not differ at baseline (preintervention) or at end of intervention.

Cortisol changes associated with energy restriction.

The mean energy restriction (percent reduced from maintenance) was 21.3 ± 2.3 for the LD group and 20.7 ± 2.4 for the AD group. The change in minimum salivary cortisol (β = 0.15; P ≤ 0.02) and the change in amplitude (β = −0.17; P ≤ 0.05) differed from zero, whereby the minimum increased and the amplitude decreased from baseline (Table 2). The CAR, LMR, DMR, maximum, mean, and variance did not change.

TABLE 2.

The effects of energy restriction (with treatment group collapsed) on salivary cortisol parameters for women exposed to a 12-wk, energy-restricted diet and given either an LD (≤238 mL milk) or AD (≥711 mL milk)¹²

Salivary cortisol variable	n	nmol/(L · 12 wk)
Δ CAR3	46	1.12 ± 0.76
Δ LMR4	50	−0.93 ± 1.01
Δ DMR4	50	1.18 ± 1.37
Δ Mean5	30	1.24 ± 0.36
Δ Minimum5	30	1.45 ± 1.21*
Δ Maximum5	30	1.21 ± 0.46
Δ Amplitude5	30	−0.64 ± 0.91**
Δ Variance5	25	−0.94 ± 0.23

¹Values are mean ± SD. Different from zero, *≤ 0.05, **P ≤ 0.01. AD, adequate amount of dairy food; CAR, salivary cortisol awakening response; DMR, dinner meal–stimulated salivary cortisol response; LD, low amount of dairy food; LMR, lunch meal–stimulated salivary cortisol response.

²Δ was calculated by subtracting the end of intervention value from the baseline value (preintervention).

³The change in salivary cortisol concentration from waking to 15 min post waking.

⁴The change in salivary cortisol concentration from immediately prior to the meal to 30 min after the completion of the meal.

⁵Value is calculated from all basal salivary cortisol concentrations from the test day (samples at waking, 15 min post waking, prior to lunch, upon request of the dinner buffet, upon leaving the center at ∼1945 h, and before bed).

Dairy food consumption and salivary cortisol changes.

As expected, the change in salivary cortisol concentrations varied throughout the test day from waking to bedtime (P ≤ 0.01). Additionally, the analysis revealed a treatment × sample time interaction (P ≤ 0.01). Based on this interaction and inspection of the curve, we tested the effect of the dairy food treatment on the DMR change from baseline. The change in the DMR varied with treatment, such that the LD treatment (P ≤ 0.05) elevated DMR compared with baseline and the AD treatment. DMR did not change from baseline to the end of the intervention in the AD group (Supplemental Table 1).

Baseline salivary cortisol responsiveness, dairy food consumption, and weight loss.

The LD group and the AD group did not differ with respect to changes in body weight and body composition. However, a median split of baseline dinner meal–stimulated salivary cortisol responsiveness revealed 2 very distinctive nonresponsive (n = 25) and responsive (n = 26) groups (Table 3). The mean baseline (preintervention) salivary cortisol concentration taken from all basal samples from the test day (taken at waking, 15 min post waking, prior to lunch, upon request of the dinner buffet, upon leaving the center at ∼1945 h, and before bed) was lower in the responsive group compared with the nonresponsive group (P ≤ 0.05) (Supplemental Table 2). In the dinner meal–stimulated salivary cortisol responders, the AD treatment led to a greater loss of body weight and total and upper body fat than in the LD treatment (P ≤ 0.04) (Table 4). A similar split of CAR and LMR did not yield significant effects of dairy food consumption on changes in weight and fat. Therefore, the AD treatment enhanced weight and upper body fat loss in the subset of women characterized at baseline as having a positive DMR.

TABLE 3.

The baseline (preintervention) DMR of women exposed to a 12-wk, energy-restricted diet and given either an LD (≤238 mL milk) or AD (≥711 mL milk) and grouped by a median split of their dinner meal salivary cortisol response¹²

Dinner meal–stimulated salivary cortisol nonresponder		Dinner meal–stimulated salivary cortisol responder
LD (n = 12)	AD (n = 13)	LD (n = 15)	AD (n = 11)
nmol/L		nmol/L
0.72 ± 0.46	0.61 ± 0.37	4.39 ± 3.97*	3.50 ± 2.73*

¹Values are mean ± SD. *Within the dairy food treatment groups, different from nonresponder group, ≤ 0.01. Within responder and nonresponder groups, dairy food treatment groups did not differ. AD, adequate amount of dairy food; DMR, dinner meal–stimulated salivary cortisol response; LD, low amount of dairy food.

²DMR was calculated as the change in salivary cortisol concentration from immediately prior to meal to 30 min after the completion of the dinner meal.

TABLE 4.

Loss of body weight and fat mass in women grouped by their baseline (preintervention) DMR and exposed to a 12-wk, energy-restricted diet and given either an LD (≤238 mL milk) or AD (≥711 mL milk)¹²

	Dinner meal–stimulated salivary cortisol nonresponder		Dinner meal–stimulated salivary cortisol responder
Variable3	LD (n = 12)	AD (n = 13)	LD (n = 15)	AD (n = 11)
	kg/12 wk		kg/12 wk
Δ Body weight	−5.49 ± 3.20	−5.06 ± 2.57	−5.38 ± 2.84	−8.11 ± 2.09#
Δ Fat mass	−4.73 ± 2.74	−3.68 ± 2.88	−4.71 ± 2.71	−6.90 ± 2.23#
Δ Upper body fat	−0.5 ± 0.3	−0.4 ± 0.3	−0.5 ± 0.3	−0.7 ± 0.3#
Δ Lower body fat	−0.8 ± 0.6	−0.7 ± 0.3	−1.2 ± 1.9	−1.3 ± 0.3

¹Values are mean ± SD. *Within the responder group, different from the LD, ≤ 0.05. ^#Between the responder group, different from the AD and LD groups, P ≤ 0.05. AD, adequate amount of dairy food; DMR, dinner meal–stimulated salivary cortisol response; LD, low amount of dairy food.

²Salivary cortisol response was determined by performing a median split of the baseline DMR.

³Δ was calculated by subtracting the end of intervention value from the baseline value (preintervention).

Discussion

In our study, energy restriction increased the salivary cortisol minimum without affecting the maximum concentration, which led to a flattening or dampening of the salivary cortisol diurnal amplitude. A dampened diurnal salivary cortisol amplitude has been linked to HPA dysfunction, chronic stress, depression, anxiety, chronic fatigue syndrome, and glucose dysregulation (32–35). Whereas the DMR was elevated by 12 wk of energy restriction, consumption of an AD prevented this effect. Finally, in women characterized at baseline as having a DMR (responders), dairy food consumption enhanced diet-induced body weight and upper fat loss.

Although starvation predictably stimulates cortisol (36), few studies have examined the effects of moderate energy restriction on the daily pattern of salivary cortisol. Whereas some reports suggest that energy restriction normalizes activity in the HPA axis (22, 37), others have clearly shown that energy restriction leads to perturbations in the HPA axis. Dieting induces both metabolic and behavioral changes that may, in vulnerable persons, increase psychological stress and stimulate the HPA axis. Dieting in some women, particularly those who monitor their energy intake, may induce chronic psychological stress and elevated activity in the HPA axis, which in turn contribute to a higher rate of weight regain (38, 39). Mice exposed to a 3-wk episode of energy restriction had higher plasma corticosterone concentrations both before and in response to a laboratory stressor (40). In that study, relative to controls that consumed diets ad libitum, energy-restricted mice consumed greater amounts of fat following psychological stress even after complete refeeding and weight regain (40).

Increases in minimum cortisol concentrations have been linked to stress and major depression and are considered a hallmark of Cushing’s Syndrome (41, 42), which is associated with upper body obesity. Whereas moderate, acute, diet-induced weight loss in women was shown to increase evening (typically lower at this time of day) plasma cortisol concentrations, the fasting concentrations were unaffected (43). In the context of energy restriction and weight loss, the variability in the diurnal minimum concentrations of cortisol may be more indicative of HPA axis health than fasted or morning (typically maximum) values. Our results provide further evidence that, similar to chronic stress, energy restriction and weight loss may disrupt the normal diurnal patterns of circulating cortisol concentrations by elevating cortisol at the time of day when cortisol concentrations are typically low.

Energy restriction did not affect either the awakening salivary cortisol response or the LMR. CAR has become recognized as a primary characteristic of the daily cortisol rhythm. It has been hypothesized to facilitate arousal (44) and its magnitude positively correlates with BMI and waist circumference (45, 46). The CAR was reported to be greater in women who had recently lost a measurable amount of body weight relative to lean or obese women (45). However, CAR is highly variable within and between individuals. It is also influenced by emotional state and short-term changes in perceived stress (47). Therefore, it is possible that this inherent variability masked our ability to detect an effect of energy restriction and weight loss on CAR.

To our knowledge, effects of energy restriction and weight loss on cortisol LMR have not been reported. Similar to waking, standardized midday mixed meals reliably stimulate predictable spikes in circulating cortisol concentrations (10). The amplitude of the LMR has been shown to associate with BMI and body fat distribution, but the direction of these associations is ambiguous (48, 49). Similar to the CAR, the LMR varies from person to person. The type and significance of the relationship between LMR and energy balance may depend on factors such as nutrient composition and underlying basal activity in the HPA axis (18, 50), which may help to explain why we did not find an effect of energy restriction on lunch meal-stimulated salivary cortisol concentrations.

A second aim of this study was to determine whether dietary inclusion of dairy food in participants might buffer the effects of energy restriction on patterns of salivary cortisol concentrations. In a recent study, drinking milk did not alter fasting serum cortisol in response to a dietary weight loss program, which is a finding consistent with our results (24). However, we did find that the dairy food intervention attenuated the inductive effects of energy restriction on the DMR. Whereas energy restriction increased the salivary cortisol response to consumption of a self-selected meal, consuming AD during the 12-wk prevented this enhanced meal-stimulated cortisol response. The magnitude of the DMR was not associated with energy or macronutrient intake from the dinner buffet, suggesting that some component other than the amount or type of energy consumed from the buffet mediated the effect of the AD treatment on the DMR. Furthermore, we cannot definitively say whether the postintervention increase in the DMR has negative or beneficial implications. However, we propose that the free-choice buffet meal may have stimulated an exaggerated cortisol response. The lunch, which was a structured meal and similar in composition to the participants’ restricted diet, did not generate a differential treatment-dependent response. Conversely, the meal structure and energy offered at the buffet were different from the typical intervention diet. It consisted of a range of palatable foods, which were provided in excess quantity and only upon request from the participant. Under the context of chronic energy restriction and weight loss, a psychological stress-associated anticipation of deviating from the controlled diet may have altered the central regulation of HPA responsiveness and the cortisol response to this unrestricted eating opportunity (51). A component unique to the dairy food supplemented diet may have mitigated this possible stressor.

There is some indication that dairy products have stress-reducing properties. Low-fat cheese intake in women was positively associated with lower perceived stress and social impairment (52). Consumption of whey protein and one of its constituents, α-lactalbumin, increased cognitive performance, improved mood, and reduced plasma cortisol in stress-prone individuals (26, 27). Consumption of whey and α-lactalbumin, which are high in tryptophan, increase the plasma tryptophan:large neutral amino acid ratio. Manipulation of this plasma ratio can modulate the synthesis and neural uptake of serotonin. Although speculative, it is possible that several weeks of dairy food consumption buffered some alterations in HPA regulation and cortisol responsiveness. Further, these possible effects of dairy food intake may have occurred through the stress-reducing actions of whey protein or α-lactalbumin.

For reasons still relatively unknown, many studies report a large variability in weight loss within their sample population despite tightly controlling food intake (53). Given the link between HPA axis regulation, cortisol, and energy balance, individual differences in cortisol regulation may also help to explain why some persons are more successful than others at losing and keeping off body weight. It may also explain our inability to detect a dairy food treatment effect that was reported in our primary paper (5) and which contradicts earlier cell culture, animal, and human data. Therefore, we tested whether baseline differences in salivary cortisol influenced fat and weight loss in response to dairy food consumption. We found that, in women characterized at baseline as having a typical brisk DMR, loss of weight and upper body fat were enhanced when dairy products were added to the energy-restricted diet. This finding suggests that cortisol responsiveness to a meal may be linked to the capacity for diet-induced weight loss under certain conditions and shows that baseline phenotypic differences in the regulation of salivary cortisol responsiveness help to reveal the effects of dairy food consumption on diet-induced weight and upper body fat loss. Although we assessed the impact of other baseline indices of cortisol status (e.g., CAR, LMR, mean salivary cortisol) on intervention effects, only baseline differences in the buffet-stimulated salivary cortisol response significantly influenced the magnitude of weight loss in response to dairy food consumption. This novel finding highlights the importance of understanding the influence of phenotypic differences (e.g., patterns of cortisol secretion) in the body weight response to dietary interventions.

A previous report demonstrated that dairy food consumption in humans enhanced diet-induced upper body fat loss (2). Calcium-rich diets have been suggested to enhance fat loss peripherally, by decreasing visceral adipose cortisol production through the actions of cholecalciferol on 11-β hydroxysteroid dehydrogenase-1 (25), and centrally, through the inhibitory effects of dairy protein on stress-induced cortisol (27). It is possible that dairy food consumption influences upper body fat loss in part by modulating central HPA regulation and/or local cortisol activity but only in individuals who exhibit robust cortisol meal-induced responsiveness, a pattern that appears to be indicative of a healthy HPA axis.

In conclusion, moderate weight loss induced by energy restriction in overweight and obese women dampened the diurnal salivary cortisol rhythm, a pattern indicative of HPA axis dysregulation. Furthermore, inclusion of low-fat dairy food (≥711 mL milk, ≥129.0 g cheese, or ≥681.0 g yogurt) in the diet appears to have prevented the facilitatory effects of energy restriction on DMR. Whether this effect has clinical importance or is physiologically beneficial or detrimental cannot be determined from the current study, but our findings do warrant further investigation. Finally, our results suggest that some underlying factors related to cortisol regulation influence the weight- and fat-reducing effects of dairy food consumption. Future studies are needed to examine whether this interaction is mechanistically linked. Our findings also further highlight the general importance of phenotypic variation and its potential impact on obesity intervention responsiveness.