Abstract
Context
Growth hormone (GH) is known to be nutritionally regulated, but the effect of dietary composition on detailed GH secretion parameters has not previously been comprehensively evaluated.
Objective
The objective of the study was to determine whether specific macro- and micronutrients are associated with discrete parameters of GH secretion among subjects with wide ranges of body mass index.
Design
Detailed macro- and micronutrient intake was assessed by four-day food records while GH secretion was assessed by standard stimulation testing in 108 men and women in one study (Study 1), and by overnight frequent blood sampling in 12 men in another study (Study 2).
Results
Peak stimulated GH was positively associated with vitamin C (r=+0.29; P=0.003), dietary fiber (r=+0.27; P=0.004), arachidic acid (r=+0.25; P=0.008), and behenic acid (r=+0.30; P=0.002) intake in univariate analysis. Controlling for age, gender, race/ethnicity, visceral fat, HOMA-IR, total caloric intake and these four dietary factors in step-wise multivariate modeling, peak GH remained significantly associated with vitamin C and visceral fat (both P<0.05). In addition, vitamin C intake was associated with various parameters of endogenous GH secretion including basal GH secretion (r=+0.95; P<0.0001), GH half-life (r=+0.75; P=0.005), total GH production (r=+0.76; P=0.004), GH area-under-the-curve (r=+0.89; P=0.0001), mean log10 GH pulse area (r=+0.67; P=0.02), and overnight maximum (r=+0.62; P=0.03), nadir (r=+0.97; P<0.0001), and mean GH secretion (r=+0.89; P=0.0001).
Conclusions
These results suggest that certain micronutrients such as vitamin C intake are strongly and uniquely associated with stimulated and endogenous spontaneous GH secretion.
Keywords: diet, vitamin C, ascorbic acid, frequent sampling
Introduction
Growth hormone (GH) secretion is nutritionally regulated. Metabolic stimuli including insulin, glucose, and free fatty acids are known to affect GH secretion, and may act through growth hormone releasing hormone (GHRH), somatostatin, ghrelin, and other pathways. Endogenous and stimulated GH secretion are reduced in the context of generalized and excess visceral adiposity1–3 and are known to be enhanced by fasting4. Furthermore, protein intake, in particular gelatin, can acutely increase GH secretion in the short term5,6. However, the long-term contribution of various macro- and micronutrients on GH secretion has not been comprehensively investigated. While Merimee et al. previously demonstrated decreased summated GH secretion assessed by obtaining hourly blood samples over 24 hours, after a high-carbohydrate diet7, this was a small study, involving only eight normal weight men, that did not control for covariates and was limited in its evaluation of GH secretion. Moreover, to our knowledge, the effect of micronutrient intake on GH secretion has not previously been studied in detail.
One micronutrient of interest is vitamin C. Plasma concentration of vitamin C is reduced in obesity8, 9 and is inversely associated with waist-to-hip ratio, a measure of central adiposity, independent of BMI10. In addition, several studies have demonstrated an association between circulating vitamin C concentration and cardiovascular disease risk11,12, paralleling the association of reduced GH secretion with abdominal adiposity and increased cardiovascular disease risk3,13,14. Furthermore, Tran CD et al. previously demonstrated a positive dose-response association between dietary vitamin C intake and IGF-1 concentrations15, indicating a possible relationship between vitamin C intake and GH secretion.
We therefore investigated the role of dietary composition, including vitamin C intake, on GH secretion by analyzing four-day food and supplementation records of subjects from two previously completed clinical research studies in which detailed parameters of stimulated and endogenous spontaneous GH secretion were investigated. We hypothesized that intake of specific macro- and micronutrients would be associated with discrete parameters of both stimulated and endogenous spontaneous GH secretion.
Methods and Procedures
Study subjects
Study 1
One-hundred-and-eight generally healthy men and women from the Boston area were studied. Data from a subset of these subjects were previously published in a study evaluating the relationship between GH secretion and cardiovascular indices14, however, detailed macro- and micronutrient intake of these subjects have not been reported. These subjects were adults, age 18 to 55 years, with a wide range of BMI. Subjects were without known pituitary disease. Exclusion criteria included a history of diabetes mellitus, thyroid disorders, chronic medical conditions such as HIV infection, or any medical condition known to affect the GH axis. Subjects taking medications known to affect GH secretion were excluded. Subjects with serum creatinine >1.5 mg/dl, hemoglobin <11 g/dl, or aspartate aminotransferase >2.5 times the upper limit of the normal range were also excluded.
Study 2
Twelve generally healthy men from the Boston area were studied. Data from these subjects were previously published in a study evaluating the effect of a short-term strategy to augment endogenous GH pulsatility using growth hormone releasing hormone16. Detailed micronutrient intake of these subjects has yet to be reported and we now compare macro- and micronutrient intake to parameters of endogenous GH secretion, obtained at baseline, prior to any intervention in the study. These subjects were adult men, age 18 to 60 years, with BMI between 20–35 kg/m2. Subjects were without known pituitary disease, history of cranial radiation, or severe renal disease, liver disease, or chronic illness. Additional exclusion criteria included use of corticosteroids, gonadal steroids, or anti-diabetic agents.
All subjects underwent written informed consent in compliance with the guidelines of the Subcommittee on Human Studies at the Massachusetts General Hospital prior to the administration of any study procedures.
Assessment of GH secretion
All subjects in Study 1 underwent standard GH stimulation testing with GHRH and arginine as previously reported3,14. Briefly, after an overnight fast, subjects were administered GHRH 1–29 [1 ug/kg] (Sermorelin acetate, Geref, Serono Laboratories, Inc., Norwell, Ma) intravenously followed by arginine hydrochloride [30 g/300 ml (max 30 g)]. GH levels were measured at 0, 30, 45, 60, 90, and 120 minutes. All subjects in Study 2 underwent overnight frequent blood sampling for GH at a frequency of every 10 minutes from 20:00 hours to 07:40 hours with the exception of one subject who underwent frequent sampling every 20 minutes. Parameters describing endogenous GH secretion including basal secretion, physiologic half-life, pulse frequency, and pulse area were determined by the automated deconvolution analysis program Auto-Decon17. For two GH profiles in which three consecutive samples were missing, the last GH value was carried forward × 1 to allow for analysis.
Laboratory methods
Serum GH was determined by the Beckman Access Ultrasensitive human GH assay, a paramagnetic particle, chemiluminescent immunoassay (Beckman Coulter, Chasta, MN) with an effective analytical sensitivity of 0.01 µg/liter. The intra-assay CV is 1.90–2.78% and the inter-assay CV is 1.77–2.65%. Fasting glucose was determined using standard methodology at our clinical laboratory. Insulin was measured using the paramagnetic particle, chemiluminescent Access immunoassay system (Beckman Coulter, Chasta, MN), with an analytical sensitivity of 0.03 IU/ml, and a precision of 3–5.6%. Serum IGF-1 was measured by EIA (Alpco Diagnostics, Inc., Salem, NH), with a detection limit of 2.3 µg/liter, an intra-assay CV of 6.6 to 9.7%, and an inter-assay CV of 11.3 to 13.7% and available for 85 subjects.
Anthropometric measurements
Height and body weight were obtained after an overnight fast. Total body fat percentage was determined by dual X-ray absorptiometry (DXA) testing using a Hologic-4500 densitometer (Hologic, Inc., Waltham, MA). In addition, 1-cm cross-sectional abdominal CT scans were performed at the level of L4 to assess the distribution of abdominal subcutaneous adipose tissue (SAT) and abdominal visceral adipose tissue (VAT) as previously described18.
Assessment of nutritional intake
The absolute intake (grams/day) of macro- and micronutrients, including specific carbohydrates, amino acids, fatty acids, and vitamins, was determined using 4-day food records facilitated by a trained registered dietician during direct interview. Data were analyzed using Nutrition Data Systems for Research (NDSR) software with the NDSR 2008 data (Version 2, University of Minnesota, Minneapolis, MN). Dietary records were available for all 108 subjects in Study 1 and all 12 subjects in Study 2.
Statistical analysis
In Study 1, peak stimulated GH was related to demographic and anthropometric parameters, including age and BMI, as well as the intake of various macro- and micronutrients using univariate regression analysis with Pearson correlations. Univariate regression analysis was also performed to assess the relationship between IGF-1 and various macro- and micronutrients. Nutrient indices that showed a significant association with peak stimulated GH upon univariate analysis were examined further using multivariate regression models to evaluate the relationship between individual nutrients and peak stimulated GH, while adjusting for the effect of potential covariates whose role in the regulation of GH secretion has previously been established. Covariates that were controlled for in each model included age, gender, race, ethnicity, anthropometric measurements, HOMA-IR, and total caloric intake. Although total caloric intake has not previously been demonstrated to have a relationship with GH secretion, it was included in all multivariate regression modeling to discern effects of individual nutrients from overall caloric intake. Least squares regression modeling was performed using BMI and repeated using visceral fat as representative anthropometric measurements. In a final model, step-wise regression analysis was performed including all the individual nutrient indices shown to be independently related to peak stimulated GH in the prior multivariate models (fiber, vitamin C, behenic acid and arachidic acid), to avoid any potential inter-relatedness between dietary indices and determine which indices are most strongly related to peak GH stimulation. In Study 2, Pearson univariate regression analysis was used to relate parameters of endogenous spontaneous GH secretion including basal secretion, physiologic half-life, number of pulsatile secretion events, total production, percent pulsatile secretion, area-under-the-curve, mean log10 pulse area, and overnight maximum, nadir, and mean GH to nutrient variables that were found to be significantly related to peak stimulated GH in Study 1. Sensitivity analyses were performed excluding 4 subjects with vitamin C intake of >1,000 mg/day in Study 1. No sensitivity analyses was performed in Study 2 as all subjects had vitamin C intake of <1,000 mg/day. Statistical analysis was performed using JMP 9.0.0 (SAS Institute, Cary, North Carolina, USA). Statistical significance was defined as P <0.05.
Results
Clinical characteristics and nutrient intake values of Study 1 subjects
Subjects in Study 1 ranged in age from 18 to 55 years with a median age of 43 years (IQ range: 33–48.75 years). Subjects were 53.7% male and 58.3% Caucasian. BMI of subjects ranged from 19.3 to 62.8 kg/m2 with median of 32.7 kg/m2 (IQ range: 23.6–38.9 kg/m2). Anthropometric, metabolic and nutritional intake values of subjects in Study 1 are presented in Table 1. The current US Recommended Dietary Allowance (RDA) for vitamin C is 75 mg/day for women and 90 mg/day for men for the ages of 19 to 60 years old19. Forty-eight subjects (44%) did not meet the recommended US RDA in our sample.
Table 1.
Characteristics and nutrient intake values of Study 1 (n=108). Values are presented as median (IQ range).
| All Subjects | |
|---|---|
| N | 108 |
| Age (yr) | 43 (33–48.75) |
| Gender, no. of males (%) | 58 (53.7%) |
| Race, n (%) | |
| Caucasian | 63 (58.3%) |
| Not Caucasian | 45 (41.7%) |
| Body composition | |
| BMI (kg/m2) | 32.7 (23.6–38.9) |
| Waist circumference (cm) | 106.2 (84.7–121.7) |
| SAT by Abd CT (cm2) | 332 (166–539) |
| VAT by Abd CT (cm2) | 117 (57–207) |
| % Total Fat by DEXA | 31.0 (23.7–39.9) |
| Nutrient intake | |
| Calories (kcal) | 2053 (1610–2563) |
| Carbohydrate (g) | 259 (182–309) |
| Protein (g) | 87 (72–113) |
| Fat (g) | 79 (58–107) |
| Cholesterol (mg) | 261 (179–411) |
| Saturated Fatty Acids (g) | 25 (17–34) |
| Trans Fatty Acids (g) | 3.9 (2.4–5.8) |
| Fiber (g) | 16 (12–22) |
| Vitamin C (mg) | 94 (52–156) |
| SFA 20:0 (arachidic acid) (g) | 0.1 (0.07–0.15) |
| SFA 22:0 (behenic acid) (g) | 0.05 (0.03–0.11) |
| TRANS 18:1 (elaidic acid) (g) | 3.3 (1.9–4.8) |
| TRANS 18:2 (trans-linolelaidic acid) (g) | 0.5 (0.3–0.7) |
| Growth hormone parameters | |
| Fasting GH (µg/L) | 0.12 (0.03–0.31) |
| Peak Stimulated GH (µg/L) | 10.6 (5.2–28.3) |
| IGF-1 (µg/L) | 79.1 (64.5–101.3) |
Univariate associations with peak stimulated GH in Study 1
Peak stimulated GH was negatively associated with age (r=−0.28; P =0.004), BMI (r=−0.62; P<0.0001), waist circumference (r=−0.72; P<0.0001), SAT (r=−0.53; P<0.0001), VAT (r=−0.66; P<0.0001) and percent body fat (r=−0.41; P<0.0001) as well as fasting glucose (r=−0.27; P=0.005), fasting insulin (r=−0.44; P<0.0001) and HOMA-IR (r= −0.37; P=0.0002).
Peak stimulated GH was positively associated with vitamin C (r=+0.29; P=0.003), dietary fiber (r=+0.27; P=0.004), arachidic acid (r=+0.25; P=0.008), and behenic acid (r=+0.30; P=0.002) intake in univariate analysis (Table 2). Peak stimulated GH was negatively associated with dietary cholesterol (r=−0.26; P=0.006), total trans fatty acids (r=−0.24; P=0.01), elaidic acid (r=−0.24; P=0.01), and trans-linolelaidic acid (r=−0.24; P=0.01) intake in univariate analysis (Table 2).
Table 2.
Univariate analyses of peak stimulated GH to nutrient intake variables in all Study 1 subjects
| Nutrient intake variable | R | P |
|---|---|---|
| Calories | −0.03 | 0.76 |
| Carbohydrate | −0.002 | 0.99 |
| Protein | −0.07 | 0.45 |
| Fat | −0.05 | 0.61 |
| Cholesterol | −0.26 | 0.006 |
| Trans Fatty Acids | −0.24 | 0.01 |
| Fiber | +0.27 | 0.004 |
| Vitamin C | +0.29 | 0.003 |
| SFA 20:0 (arachidic acid) | +0.25 | 0.008 |
| SFA 22:0 (behenic acid) | +0.30 | 0.002 |
| TRANS 18:1 (elaidic acid) | −0.24 | 0.01 |
| TRANS 18:2 (trans-linolelaidic acid) | −0.24 | 0.01 |
Vitamin D, E and omega 3 fatty acids were not associated with peak stimulated GH. All other specific carbohydrates, amino acids, and fatty acids that were examined were not significantly associated with peak stimulated GH.
Univariate associations with IGF-1 in Study 1
IGF-1 was negatively associated with age (r=−0.41; P=0.0001) and BMI (r=−0.26; P=0.02). IGF-1 was positively associated with intake of dietary fibers (r=+0.24; P=0.03) but was not associated with macronutrients, or vitamin C, behenic or arachidic acid intake (all P>0.05).
Vitamin C and measures of central adiposity in Study 1
Dietary vitamin C intake was inversely associated with VAT as measured by cross-sectional CT scan (r=−0.20; P=0.04) and trended to an inverse association with waist circumference (r=−0.03; P=0.08) and waist-to-hip ratio (r=−0.18; P=0.08).
Multivariate analyses for peak stimulated GH in Study 1
Separate multivariate regression models assessing the relationship between individual dietary indices that were significantly associated with peak stimulated GH on univariate analyses, controlling for age, gender, race, ethnicity, VAT, HOMA-IR, and total caloric intake. In these models, peak GH remained significantly associated with vitamin C (β=0.03, P=0.007, R2 for model=0.50, P for model<0.0001) (Table 3A), fiber (β=0.62, P=0.009, R2 for model=0.50, P for model<0.0001) (Table 3B), arachidic acid (β=43.79, P=0.05, R2 for model=0.48, P for model< 0.0001) (Table 3C), and behenic acid (β=33.92, P=0.04, R2 for model=0.48, P for model< 0.0001) (Table 3D) intake. When controlling for BMI in lieu of VAT in the multivariate models, Vitamin C (β=0.02, P=0.03, R2 for model=0.62, P for model<0.0001) and behenic acid (β=31.01, P=0.03, R2 for model=0.62, P for model<0.0001) remained significant, while fiber (β=0.40, P=0.06, R2 for model=0.61, P for model<0.0001) and arachidic acid (β=34.95, P=0.07, R2 for model=0.61, P for model<0.0001) trended to statistical significance. Dietary cholesterol, total trans fatty acids, elaidic acid, and trans-linolelaidic acid (all P>0.1) intake were no longer significantly associated with peak stimulated GH after controlling for these covariates.
Table 3.
| A. Multivariate modeling for peak stimulated GH controlling for age, gender, race, ethnicity, VAT, HOMA-IR, total caloric intake and vitamin C intake. R2 for overall model =0.50 and P value for overall model <0.0001. | |||
|---|---|---|---|
| B estimate | Standard error | P | |
| Age | −0.31 | 0.21 | 0.14 |
| Gender (male or not) | 0.79 | 2.24 | 0.73 |
| Race (Caucasian or not) | −0.45 | 1.98 | 0.82 |
| Ethnicity (Hispanic or not) | −2.55 | 3.13 | 0.42 |
| VAT | −0.13 | 0.02 | <0.0001 |
| HOMA-IR | −1.40 | 1.05 | 0.19 |
| Total caloric intake | −0.0009 | 0.003 | 0.72 |
| Vitamin C intake | 0.03 | 0.01 | 0.007 |
| B. Multivariate modeling for peak stimulated GH controlling for age, gender, race, ethnicity, VAT, HOMA-IR, total caloric intake and fiber intake. R2 for overall model =0.50 and P value for overall model <0.0001. | |||
|---|---|---|---|
| B estimate | Standard error | P | |
| Age | −0.33 | 0.21 | 0.12 |
| Gender (male or not) | 1.69 | 2.30 | 0.47 |
| Race (Caucasian or not) | −1.62 | 1.96 | 0.41 |
| Ethnicity (Hispanic or not) | −4.00 | 3.16 | 0.21 |
| VAT | −0.12 | 0.03 | <0.0001 |
| HOMA-IR | −1.56 | 1.06 | 0.15 |
| Total caloric intake | −0.005 | 0.003 | 0.15 |
| Fiber intake | 0.62 | 0.23 | 0.009 |
| C. Multivariate modeling for peak stimulated GH controlling for age, gender, race, ethnicity, VAT, HOMA-IR, total caloric intake and arachidic acid intake. R2 for overall model =0.48 and P value for overall model <0.0001. | |||
|---|---|---|---|
| B estimate | Standard error | P | |
| Age | −0.24 | 0.21 | 0.17 |
| Gender (male or not) | 0.25 | 2.29 | 0.91 |
| Race (Caucasian or not) | −1.85 | 2.01 | 0.36 |
| Ethnicity (Hispanic or not) | −3.25 | 3.20 | 0.31 |
| VAT | −0.13 | 0.03 | <0.0001 |
| HOMA-IR | −1.35 | 1.09 | 0.22 |
| Total caloric intake | −0.002 | 0.003 | 0.46 |
| SFA 20:0 (arachidic acid) | 43.8 | 21.90 | 0.049 |
| D. Multivariate modeling for peak stimulated GH controlling for age, gender, race, ethnicity, VAT, HOMA-IR, total caloric intake and behenic acid intake. R2 for overall model =0.48 and P value for overall model <0.0001. | |||
|---|---|---|---|
| B estimate | Standard error | P | |
| Age | −0.21 | 0.21 | 0.32 |
| Gender (male or not) | −0.03 | 2.29 | 0.99 |
| Race (Caucasian or not) | −2.16 | 2.03 | 0.29 |
| Ethnicity (Hispanic or not) | −2.80 | 3.21 | 0.39 |
| VAT | −0.13 | 0.03 | <0.0001 |
| HOMA−IR | −1.40 | 1.08 | 0.20 |
| Total caloric intake | −0.0001 | 0.003 | 0.96 |
| SFA 22:0 (behenic acid) | 33.92 | 16.65 | 0.045 |
A combined forward step-wise multivariate model for peak stimulated GH was assessed including the four individual nutrients shown to be independently related to peak stimulated GH in the above multivariate models (vitamin C, fiber, behenic acid and arachidic acid) to determine which nutrient is the most strongly related to peak GH stimulation. Independent variables of age, gender, race/ethnicity, VAT, HOMA-IR, total caloric intake, and vitamin C, fiber, behenic and arachidic acid intake were simultaneously tested for inclusion in the model. Of the independent variables, only VAT and vitamin C were selected for inclusion by the modeling as significant parameters independently related to peak stimulated GH (P<0.05). In this combined model, only vitamin C, but not fiber, behenic or arachidic acid was significantly related to peak stimulated GH.
Sensitivity analyses excluding subjects with vitamin C intake >1,000 mg/day in Study 1
Four of the 108 subjects had vitamin C intake >1,000 mg/day. Sensitivity analyses excluding these four subjects did not significantly alter the results. Vitamin C intake remained significantly associated with peak stimulated GH (r=+0.29; P=0.01) amongst the remaining 104 subjects.
Clinical characteristics and nutrient intake values of Study 2 subjects
The subjects ranged in age from 21 to 58 years with median age of 49 years (IQ range: 35.75–55 years). All subjects were male and 83.3 % were Caucasian. BMI ranged from 20.9 to 33.6 kg/m2 with median BMI of 27.6 kg/m2 (IQ range: 24.3–32.1 kg/m2). Details of their nutritional intake are presented in Table 4. Three subjects (25%) did not meet the recommended US RDA for vitamin C intake in Study 2.
Table 4.
Nutrient intake values of subjects in Study 2 (n=12). Values are presented as median (IQ range).
| All Subjects (n=12) | |
|---|---|
| Total energy (kcal) | 2647 (2569–3005) |
| Carbohydrate (g) | 342 (292–392) |
| Protein (g) | 116 (83–135) |
| Fat (g) | 99 (67–112) |
| Cholesterol (mg) | 309 (146–468) |
| Saturated Fatty Acids (g) | 29 (17–41) |
| Trans Fatty Acids (g) | 3.6 (2.5–6.1) |
| Fiber (g) | 22 (18–42) |
| Vitamin C (mg) | 133 (78–254) |
| SFA 20:0 (arachidic acid) (g) | 0.14 (0.08–0.19) |
| SFA 22:0 (behenic acid) (g) | 0.06 (0.04–0.22) |
| TRANS 18:1 (elaidic acid) (g) | 2.9 (2.1–5.0) |
| TRANS 18:2 (trans-linolelaidic acid) (g) | 0.5 (0.4–0.8) |
The association of nutrient intake with parameters of endogenous spontaneous GH secretion in Study 2
The four nutrient indices, vitamin C, fiber, arachidic acid and behenic acid intake, that were identified as independently associated with peak stimulated GH on standard stimulation testing in Study 1, were evaluated for their association with parameters of endogenous GH secretion. Vitamin C intake was associated with basal GH secretion (r=+0.95; P<0.0001), GH half-life (r=+0.75; P=0.005), total GH production (r=+0.76; P=0.004), GH area-under-the-curve (r=+0.89; P=0.0001), mean log10 GH pulse area (r=+0.67; P=0.02), and overnight GH maximum (r=+0.62; P=0.03), nadir (r=+0.97; P<0.0001), and mean (r=+0.89; P=0.0001) in univariate analysis (Table 5). Fiber intake was associated with basal GH secretion (r=+0.76; P=0.004), mean log10 GH pulse area (r=+0.67; P=0.02), and overnight GH nadir (r=+0.60; P=0.04) in univariate analysis. Arachidic acid intake was negatively associated with the physiologic half-life of GH (r=−0.59; P=0.04) but no other parameters of endogenous GH secretion. Behenic acid intake was negatively associated with overnight maximal GH secretion (r=−0.61; P<0.05) and trended to significance with physiologic half-life (r=−0.56; P=0.06) and mean log10 GH pulse area (r=−0.54; P=0.07).
Table 5.
Univariate analyses of vitamin C intake to parameters of endogenous spontaneous GH secretion among all Study 2 subjects (n=12).
| r | P | |
|---|---|---|
| Basal GH secretion | +0.95 | <0.0001 |
| GH half-life | +0.75 | 0.005 |
| Number of overnight GH secretion pulses | −0.30 | 0.35 |
| Total GH production | +0.76 | 0.004 |
| Percent pulsatile GH production | +0.13 | 0.68 |
| GH area-under-the-curve | +0.89 | 0.0001 |
| Mean log10 GH pulse area | +0.67 | 0.02 |
| Overnight GH maximum | +0.62 | 0.03 |
| Overnight GH nadir | +0.97 | <0.0001 |
| Overnight GH mean | +0.89 | 0.0001 |
Discussion
We report the first association between various macro- and micronutrient intake variables and discrete measures of GH secretion in otherwise healthy men and women. Our data demonstrate a significant association between vitamin C and peak stimulated GH on standard clinical stimulation testing and various representative parameters of endogenous spontaneous GH secretion. The results suggest that decreased vitamin C intake may be independently associated with reduced GH secretion.
Previous studies evaluating different regulators of GH secretion have focused on various physiologic hormones including insulin, glucose, free fatty acids and neuropeptides such as GHRH, somatostatin, and various GH-releasing factors. While van Vught et al. have performed detailed studies evaluating the contribution of protein intake on acute GH stimulation5, 6, limited studies have evaluated the role of macronutrient intake in long-term GH regulation. An earlier study in 35 elderly subjects was not able to detect any association between macronutrient intake and morning GH. However, in this study, GH was assessed after 2–3 hours of fasting and measured between 08:00 – 12:00 (noon) and neither standard clinical stimulation tests nor overnight frequent sampling was performed20. While one interventional study demonstrated suppression of summated GH secretion after an isocaloric high-carbohydrate diet7, one week of a low carbohydrate, high protein diet had no effect on 24 hour GH concentrations measured every 20 minutes21. Similarly, two weeks of either high fat or high carbohydrate diet had no effect on fasting GH22. However, these studies investigating the relationship of macronutrient intake to GH are limited by their small size, the short duration of intervention and method of assessing GH.
Micronutrients, specifically retinoic acid or vitamin A, has been shown to play a role in GH secretion in previous studies. In vitro studies have demonstrated stimulation of GH secretion in human and rat pituitary cells by vitamin A23, 24, and one clinical study has demonstrated a positive correlation between plasma vitamin A levels and nocturnal GH secretion in children with impaired nocturnal GH secretion25. We now demonstrate a potential novel role for dietary vitamin C intake in regulating GH secretion in two independent clinical studies. In our first study, we demonstrate a significant positive association between vitamin C intake and peak stimulated GH secretion on standard stimulation testing in 108 otherwise healthy men and women that remains significant upon controlling for total caloric intake and variables whose role in GH secretion have previously been published (age, gender, race/ethnicity, visceral adiposity, and insulin resistance). The previously demonstrated negative association between GH and age and visceral adiposity was confirmed in our study. We therefore controlled for these possible covariates in the multivariate modeling and found the positive association between vitamin C and GH to remain significant. Furthermore, the positive association between vitamin C intake and GH secretion is confirmed by our second study, in which endogenous spontaneous GH secretion was examined by q10 minutes overnight frequent sampling. Our demonstration of a positive association between vitamin C intake and GH secretion is in concordance with and would help explain a previous study’s finding of a positive dose-response association between dietary vitamin C intake and IGF-1 concentrations15. While we did not observe a direct relationship between dietary vitamin C intake and IGF-1 concentration in our study, this may be due to the smaller sample size in our study (108 subjects compared to the 1,542 subjects in the study by Tran et al.15).
This study was not designed to assess the mechanism of how dietary vitamin C intake affects GH secretion but rather as a hypothesis-generating study to identify macro- and micronutrients that may have a physiologic role in the regulation of GH. Nonetheless, previous studies suggest a physiological connection may exist between vitamin C intake and GH secretion. Vitamin C is well known as an antioxidant but it also functions as a co-factor for peptidylglycine α-amidating mono-oxygenase (PAM), which is responsible for amidation of various neuropeptides26. Tissue expression of PAM has been studied in animal models and very high tissue concentrations of PAM activity have been demonstrated in both the hypothalamus and pituitary27,28. Furthermore, PAM activity in rat serum and tissue homogenates as well as cultured rat anterior pituitary cells has been shown to be responsive to treatment with vitamin C27,29. In addition, vitamin C is actively transported across membranes using the sodium-ascorbate co-transporter (SVCT) 1 and 230. SVCT2 in particular is expressed in neurons and endocrine cells31, as well as hypothalamic tanycytes32, allowing for high concentrations of local vitamin C compared to circulating vitamin C. While PAM activity has not been directly assessed in regards to regulators of GH secretion, GH releasing hormone (GHRH) is known to be a C-terminal amidated peptide33–35, suggesting a possible role of the micronutrient vitamin C in regulating GH secretion via GHRH post-translational processing. SVCT2 may therefore allow higher local concentration of vitamin C in the hypothalamus leading to an increase in the activity of PAM resulting in greater efficiency of amidation and activation of GHRH, thereby increasing GH signaling. Although speculative, these prior studies in combination with the present study, suggest further investigations into the relationship between vitamin C and GH would be warranted.
The strengths of our study are its examination of detailed measures of stimulated and endogenous spontaneous GH secretion, the use of a relatively large population of 108 obese and non-obese men and women in study 1, and its validation in a separate more detailed physiology study that confirms the associations using a different method to assess GH secretion. Limitations include its observational design, our inability to account for food additives that may have an energetic or nutritional effect on GH secretion, and the use of self-administered food records. In addition, we were not able to measure specific micronutrient levels in the circulation to relate to GH parameters. Although self-administered dietary intake assessment tools are widely used, they can be associated with some imprecision36–40. Specifically, subjects tend to underreport total caloric and protein intake; however, Bingham et al. demonstrated a significantly high correlation (Spearman rank correlation of 0.86) between plasma vitamin C level measured by liquid chromatography with electrochemical detection and recorded dietary intake of vitamin C, suggesting this micronutrient may be less affected by general underreporting seen in dietary recall36. As such, we were able to uncover significant relationships between GH secretion and vitamin C intake in two independent studies. In this study, vitamin C intake was significantly associated with VAT, a measure of central adiposity. Given the known association between GH and measures of central adiposity3, this association may be mediated entirely by VAT. However, in multivariate regression modeling, we controlled for VAT and the relationship between dietary vitamin C intake and peak stimulated GH remained significant, making this unlikely. It is also possible that the relationships we demonstrate between GH secretion and vitamin C intake are a proxy for a healthy lifestyle, such as high fruit and vegetable intake, because we were unable to control for this variable in the present study. This study is an exploratory retrospective study. As such, future studies manipulating the dietary intake of vitamin C would be valuable in assessing their true contributions to GH secretion. Future studies employing Daily Recommended Intake percentages for nutrient intake values in order to take into account the change in nutrient requirements according to age and physical activity may also be of benefit.
In summary, we demonstrate for the first time that vitamin C intake is significantly associated with discrete parameters of stimulated and endogenous spontaneous GH secretion. Furthermore, this relationship appears independent of potential covariates such as age, gender, VAT, HOMA-IR, total caloric intake and other micronutrient intake. The associations between GH and vitamin C identified in this study suggest the potential utility of an interventional study to determine whether increasing vitamin C intake may affect endogenous GH secretion.
ACKNOWLEDGEMENTS
We gratefully acknowledge the MGH bionutrition and nursing staffs and the research volunteers for their participation in the study.
Funding: National Institutes of Health grant T32DK007028 to SDB, K23DK089910 to TLS, R01HL085268 to SKG, R01DK63639 to SKG, K24DK064545 to SKG, K23DK087857 to HM, and M01RR01066 and UL1RR025758, Harvard Clinical and Translational Science Center, from the National Center for Research Resources. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Center for Research Resources or the National Institutes of Health.
Clinical Trials Registration: This study was registered at www.clinicaltrials.gov as NCT00562796 and NCT00850564.
Footnotes
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Disclosure: The authors have nothing to disclose
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