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The Journal of Clinical Endocrinology & Metabolism Vol. 84, No. 4 1296-1303
Copyright © 1999 by The Endocrine Society

Original Studies

Growth Hormone (GH) Therapy in GH-Deficient Adults Influences the Response to a Dietary Load of Cholesterol and Saturated Fat in Terms of Cholesterol Synthesis, But Not Serum Low Density Lipoprotein Cholesterol Levels1

M. Leonsson, J. Oscarsson, I. Bosaeus, B. K. Lundgren, G. Johannsson, O. Wiklund and B. Å. Bengtsson

Research Center for Endocrinology and Metabolism, Departments of Physiology (J.O.) and Medical Nutrition (I.B., B.K.L.), and Wallenberg Laboratory (O.W.), Sahlgrenska University Hospital, Goteborg, Sweden

Address all correspondence and requests for reprints to: Dr. Maria Leonsson, Research Center for Endocrinology and Metabolism, Sahlgrenska University Hospital, SE-413 45 Goteborg, Sweden. E-mail: maria.leonsson{at}ss.gu.se


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
An increased dietary load of cholesterol (ch) and saturated fat increases serum low density lipoprotein ch (LDL-ch) levels. GH therapy in GH-deficient adults decreases serum LDL-ch levels. In the rat, GH is important for resistance to dietary cholesterol in terms of serum cholesterol levels. The aim of this study was to investigate the influence of GH on the effects of an increase in the intake of cholesterol and saturated fat on serum lipoproteins and markers for cholesterol synthesis in man. Six GH-deficient adults were given an isocaloric diet enriched in cholesterol and saturated fat for 17 days with and without GH therapy (1–1.5 U/day). Serum cholesterol, LDL-ch, apolipoprotein B (apoB), and apoA1 levels increased during the diet period with GH therapy and tended to increase during the diet period without GH. However, GH therapy did not influence the dietary effect on serum cholesterol, LDL-ch, apoA1, or apoB levels. Serum levels of triglycerides, very low density lipoprotein ch, high density lipoprotein ch, and apoE were not affected by diet or GH therapy. GH therapy increased serum lipoprotein(a) levels, but did not affect the response to diet. The serum total {Delta}7-lathosterol/cholesterol ratio increased less during the diet period with GH therapy than during the diet period without GH. Serum 7{alpha}-hydroxy-4-cholesten-3-one levels tended to increase during both diet periods, but were not influenced by GH treatment. Serum plant sterol levels did not change. These results indicate that GH counteracts an increase in cholesterol synthesis induced by a high fat diet without affecting bile acid synthesis or sterol absorption. GH therapy did not have any major influence on the dietary effects on serum lipoprotein levels.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
AN IMPORTANT role for GH in terms of lipoprotein metabolism was suggested several years ago (1). Subsequent studies have demonstrated an unfavorable serum lipoprotein pattern in GH-deficient (GHD) adults, i.e. increased serum levels of cholesterol (ch), low density lipoprotein ch (LDL-ch), and triglycerides and decreased serum high density lipoprotein ch (HDL-ch) levels (2, 3, 4, 5). Changes in serum lipoprotein levels were also observed postprandially in women with untreated GHD (6). GH treatment of patients with GHD decreased serum ch and LDL-ch levels and increased HDL-ch levels, whereas serum triglyceride levels were not affected in most studies (4, 7, 8, 9, 10). The effect of GH on serum lipoprotein(a) [Lp(a)] is of special interest. The Lp(a) level is considered to be an independent marker of atherosclerosis and coronary heart disease (11, 12, 13). Lp(a) levels in patients with GHD did not differ from those in normal healthy subjects (10), but the Lp(a) levels were markedly increased after 6–12 months of GH therapy (9, 10).

GH has been shown to be of importance for LDL receptor expression in the liver of rats (14, 15) and humans (14). A loss of resistance to dietary ch was demonstrated in the hypophysectomized rat, resulting in a 6-fold increase in plasma ch levels and decreased hepatic LDL receptor expression. GH therapy in ch-fed, hypophysectomized rats attenuated hypercholesterolemia and normalized the expression of hepatic LDL receptors (15).

In the general population, the effect of ch in the diet on serum ch levels, especially LDL-ch, differs (16). The reason for the individual responsiveness is probably multifactorial and includes genetic factors (16). However, hormonal factors may also contribute to the individual sensitivity to dietary manipulations. The aim of this study was to investigate the role of GH in the individual response to an increased dietary load of ch and saturated fatty acids. The subjects received a diet enriched in both ch and saturated fats, as LDL-ch levels in humans in general are more sensitive to an increase in the intake of saturated fatty acids than are ch levels (16, 17). We hypothesized that there is a loss of resistance to dietary fat in GHD subjects that might be restored by GH therapy.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Six patients, four men and two women, with known pituitary deficiency were asked to participate in the study (Table 1Go). GH deficiency was diagnosed by an insulin tolerance test (blood glucose, <2.2 mmol/L). The mean peak GH response was 0.74 mU/L (range, 0.073–2.95; 3 mU = 1 µg). All of the subjects were receiving stable replacement therapy with cortisone acetate, L-T4, and/or sex steroids. Subjects received GH (Humatrope) as a daily sc injection in the evening. The mean daily dose of GH was 1.08 U (range, 1–1.5). The individual dose of GH was dependent on serum insulin-like growth factor I (IGF-1) and clinical response (18). Two men (no. 3 and 4) had been treated for hypertension for more than 10 yr. One woman (no. 5) had previously been adrenalectomized due to Cushing’s disease before she underwent pituitary surgery. None of the patients had serum ch concentrations above 7.8 mmol/L at baseline. Written informed consent was obtained from all of the patients, and the study was approved by the ethics committee at the University of Goteborg.


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  		Table 1. Subjects

 
Study design

The study was a 17-day open (no placebo injections) study with a cross-over design (Fig. 1Go). The GH wash-out period was at least 11 weeks (11–13 weeks). Four patients (no. 1, 2, 4, and 5) entered the study after 1–2 yr of GH replacement therapy, and two patients (no. 3 and 6) entered without any previous GH treatment. These subjects (no. 3 and 6) received GH for 13 and 11 weeks, respectively, before the second diet period (Fig. 1Go). All blood samples were drawn in the morning after an overnight fast. Lipoproteins were analyzed at the start and on days 9 and 17 of each diet period. To determine whether the lipoprotein concentrations were stable until the start of the second diet period, lipoprotein concentrations were also analyzed 8 weeks after the first diet period, i.e. during the GH wash-out period (n = 4) or after 8 weeks of GH therapy (n = 2), respectively. Total {Delta}7-lathosterol, plant sterols, and 7{alpha}-hydroxy-4-cholesten-3-one in serum were determined on days 0 and 17 of each diet period. Oral glucose tolerance tests (OGTTs) were performed in the morning after an overnight fast on days 0 and 17 of each diet period. Blood samples were taken before the oral dose of glucose (100 g) and after 30, 60, 90, and 120 min.



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  		Figure 1. Schematic presentation of the study design. The duration of the high fat diet period was 17 days. Subjects 3 and 6 had no GH therapy before they entered the study. The durations of GH therapy before the second diet period were 13 and 11 weeks for subjects 3 and 6, respectively. The GH wash-out period was between 11–13 weeks for the other subjects. The shaded area depicts GH therapy.

 
Diet

The experimental diets were made up of conventional food items and were designed to resemble the habitual diet of the subject. The composition of the habitual diet was determined from a 3-day food record combined with an interview about eating habits. A 2-day test menu based on this information preceded the study to make sure that the experimental diet was palatable and acceptable to the participants. Individual energy intake was estimated both from the 3-day food record and from age, height, weight, and activity history, as a food record alone often underestimates energy intake (19).

The experimental diet contained more saturated fatty acids and ch than the habitual diet (Table 2Go). Compared with the habitual diet, our aims were to increase saturated fat intake by 10 energy% to obtain a total fat intake of 40–50 energy% and a ch intake of 900 mg/day. All food was prepared in advance in duplicate portions and was kept in individual batches at -18 C until the day of consumption or analysis, respectively. One of the duplicate portions made for each patient was subjected to homogenization and was subsequently freeze-dried. The energy content was analyzed using bomb calorimetry. The nitrogen content was analyzed using a modified Kjeldahl method, and ch and plant sterols were analyzed using gas liquid chromatography as previously described (20).


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  		Table 2. Total energy intake, fat intake, cholesterol intake, and intake of unsaturated (U) and saturated (S) fatty acids in the habitual diet at baseline and during the experimental diet

 
Two daily menus were made and consumed in alternating order during the study periods. The participants collected all food to be consumed at the metabolic ward at 1- to 2-day intervals. They were carefully instructed not to leave any food and not to eat anything but the experimental food. Any leftovers were to be returned for analysis. This strict level of control was maintained from Monday to Friday of each study week, but, to enhance compliance, foods of the patient’s own choice were allowed from an individually made diet sheet on the weekends. The subjects were thereby encouraged to maintain the increased intake of ch and saturated fat during the weekends. To maintain energy balance, the subjects were weighed on each visit, and energy intake was adjusted if necessary.

Body composition

At the start and end of each diet period, physical examinations and measurements of body composition were performed. Body composition was measured by dual energy x-ray absorptiometry. A Lunar DPX-L scanner (Lunar Corp., Madison, WI) was used. A total body scan was performed at the scan speed suggested by the system for each subject and was analyzed using software version 1.31. Precision errors on the scan, as determined by double examinations in 10 healthy subjects, were 1.7% for fat mass, 0.7% for lean tissue mass, 1.9% for total bone mineral content, and 1.5% for total body bone mineral density. Body weight was measured using a balance scale to the nearest 0.1 kg three to five times a week during diet periods and once in the middle of the wash-out period.

Lipids and lipoproteins

Serum ch and triglycerides were determined using fully enzymatic methods (Boehringer Mannheim, Mannheim, Germany), using a Cobas Fara autoanalyzer (Hoffmann-La Roche, Inc., Basel, Switzerland). The within-assay coefficients of variation (CVs) were 0.9% and 1.1%, respectively. HDL-ch was determined after the precipitation of apoB-containing lipoproteins with manganese chloride and heparin (21). Very low density lipoprotein ch (VLDL-ch) and VLDL triglyceride concentrations were determined in the fraction with a density of less than 1.006 g/mL obtained by ultracentrifugation. LDL-ch concentrations were calculated from total serum ch, HDL-ch, and VLDL-ch concentrations. The analyses mentioned above were performed consecutively during the study.

Serum was stored at -20 to -30 C for the analysis of apolipoproteins and Lp(a) in one assay run after the study. ApoA1 and apoB were determined by immunoturbidometric assays (UniKit Roche, Hoffmann-La Roche, Inc., Basel, Switzerland). The within-assay CVs were 2.3% and 1.9%, respectively. ApoE concentrations were determined using an electroimmunoassay. The within-assay CV of this analysis was 4.8% (9). Lp(a) was determined by RIA (Pharmacia Biotech, Uppsala, Sweden) with a within-assay CV of 4.4% (22, 23).

Total {Delta}7-lathosterol and plant sterols (sitosterol and camposterol) in serum were determined using gas liquid chromatography (24, 25). In short, after the addition of an internal standard (5{alpha}-cholestane, 1 mg/mL), nonsaponifiable serum lipids were derived with trimethyl-silyl-ether. The sterols were separated by gas liquid chromatography on a 50-m long capillary column with hydrogen as the carrier gas, an initial temperature of 170 C, and a final temperature of 270 C (ultra performance capillary column Ultra1, Hewlett-Packard Co., Little Falls, Philadelphia, PA). The within- and between-assay CVs were 5% and 10%, respectively. Levels of 7{alpha}-hydroxy-4-cholesten-3-one in serum were determined according to the method of Axelsson et al. (26). In short, after the addition of [3H]25-hydroxyvitamin D3 as an internal standard and dilution with saline to avoid the protein binding of steroids, samples were purified by sorbent extraction and analyzed by high performance liquid chromatography using a Kromasil 100–5 column and UV detection at 254 nm. The between-assay CV, as determined from pooled sera run with each batch, was 11%.

Hormones, insulin, and blood glucose

IGF-1 and IGF-binding protein-3 (IGFBP-3) were determined using RIAs (Nichols Institute Diagnostics, San Juan Capistrano, CA) (27). The serum IGF-1 levels of each subject were compared with their predicted serum IGF-1 values (based on sex and age), derived from a large population sample (27). The formulae for the predicted IGF-1 value are 292.7 - (2.1 x yr of age) for men and 375.7 - (3.7 x yr of age) for women. Serum insulin concentrations were determined by a RIA (Phadebas, Pharmacia Biotech), and blood glucose concentrations were determined using a glucose-6-phosphate-dehydrogenase method (Kebo Laboratory, Sweden).

Statistics

Values in text and tables are given as the median and 25–75 percentiles, unless otherwise stated. The effect of the diet was analyzed by comparing the values on days 0 and 17 of each diet period using Wilcoxon’s rank sum test. The impact of GH therapy on baseline values was tested by comparing the values at the start of each diet period using Wilcoxon’s rank sum test. The influence of GH therapy on effects induced by the diet was analyzed using Wilcoxon’s rank sum test by comparing the changes from days 0–17. When analyzing the OGTT, we compared the areas under the response curves. Comparisons using the values on day 9 of the dietary periods were avoided, because the subjects had not always stabilized their responses to the dietary challenge at this point. P < 0.05 was considered significant.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Diet

The total energy intake in the experimental diet exceeded the intake calculated from the food records in all subjects (Table 2Go). One subject (no. 3) did not complete the initial food record. The calculations of the change in diet composition did not include this subject. The experimental diet given to this subject was similar to those given to the other subjects. The diet composition shown in Table 2Go is the mean of two or four diet menus. Subjects 1, 5, and 6 had two diet menus, whereas subjects 2, 3, and 4 had two additional diet menus due to small fluctuations in body weight.

The amount of saturated fat nearly doubled, from a mean of 12.5% to 23.1% of total energy intake. The ch intake increased to a mean of 785 mg/day (median increase, 2.5-fold; 1.9–2.7). The median daily protein intake was 84 (80–94 g) at baseline and 88 g (74–119 g) in the experimental diet. The daily intake of carbohydrates was 242 g (230–256) at baseline and 230 g (204–284 g) during the diet periods. The daily intake of saturated fat was 28.4 g (26.5–34.3 g) at baseline and 59.2 g (54.3–71.2 g) during the diet. The daily intake of unsaturated fats was 38.5 g (31.7–47.9 g) at baseline and 41.3 g (37.5–48.8 g) during the experimental diet.

Lipoproteins and ch metabolism

Serum lipoproteins were measured once during the wash-out period. The median serum apoA1 level 8 weeks after the first diet period was higher than that at the start of the second diet period (1.43 and 1.38 g/L, respectively). Serum levels of measured components of lipoproteins were otherwise stable at the start of the second diet period (data not shown).

Table 3Go summarizes the effects of the experimental diet and GH on serum lipoprotein concentrations. The experimental diet had no significant effect on total ch, VLDL-ch, LDL-ch, or HDL-ch without GH treatment, but when GH therapy was given, there was a significant increase in total ch and LDL-ch levels during the diet period. However, the changes in total ch and LDL-ch during the diet periods did not differ between the two periods. The individual LDL-ch responses are given as a line plot in Fig. 2Go, A and B. Serum triglyceride, VLDL-triglyceride, or VLDL-ch levels were not affected by either diet or GH treatment (Table 3Go).


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  		Table 3. Effects on serum lipoprotein fractions

 


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  		Figure 2. Line plot of the percent change in LDL-ch levels from day 0 in the six subjects during the diet period without GH therapy (A) and during the diet period with GH therapy (B). Open circle, Subject 1; filled triangle, subject 2; open square, subject 3; filled diamond, subject 4; half-filled square, subject 5; half-filled diamond, subject 6. The dotted line depicts the individual starting value, which was designated 0.

 
Serum apoB, apoA1, and apoE levels were not affected by the diet when no GH was given. When GH therapy was given, apoB and apoA1 levels increased as a result of the diet (Table 4Go). However, the changes in apoB and apoA1 levels during the two diet periods did not differ from one another. Lp(a) levels increased as a result of GH therapy (Table 4Go). However, no significant change in Lp(a) levels occurred during the diet periods. The changes in Lp(a) levels from days 0–17 did not differ between the periods with and without GH therapy.


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  		Table 4. Effects on serum apolipoproteins and lipoprotein(a)

 
The total {Delta}7-lathosterol/ch ratio in serum was determined as an indication of total ch biosynthesis (24, 25). The lathosterol/ch ratio increased during the diet period without GH. On day 0, the value was 161 µmol/100 mmol (144–177), and on day 17, it was 226 µmol/100 mmol (180–279). The diet had no effect on the lathosterol/ch ratio when the subjects were receiving GH therapy. On day 0 in that diet period, the value was 141 µmol/100 mmol (121–210), and on day 17, it was 197 µmol/100 mmol (129–255). Figure 3Go shows the individual values of the lathosterol/ch ratio on day 17, both diet periods. The changes in the lathosterol/ch ratio differed between the diet periods. The change in the lathosterol/ch ratio was 52 µmol/100 mmol (37–102) without GH therapy and 20 µmol/100 mmol (8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42) when the subjects were receiving GH therapy. This finding indicates that GH therapy suppressed the enhanced ch biosynthesis induced by the diet.



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  		Figure 3. Individual lathosterol/ch ratios on day 17 of the diet without (-GH) and with GH (+GH) therapy. Open circle, Subject 1; filled triangle, subject 2; open square, subject 3; filled diamond, subject 4; half-filled square, subject 5; half-filled diamond, subject 6.

 
We also determined serum plant sterol levels (sitosterol and camposterol) as an indication of intestinal sterol absorption (24). Neither the diet nor GH therapy influenced serum plant sterol concentrations. During the diet period without GH, the serum concentration of plant sterols was 11.5 µmol/L (9.4–19.8) at the start and 10.5 µmol/L (7.0–16.9) on day 17. During the diet period with GH therapy, the serum concentration was 12.4 µmol/L (8.7–19.3) at the start and 11.4 µmol/L (7.8–22.9) on day 17. There was no difference between the periods when comparing the changes in serum plant sterols from days 0–17 (data not shown). The values on day 17 of the diet periods were also compared, as the diets were the same on the days before these measurements. However, the serum plant sterol concentrations on day 17 were not different from one another.

No significant effect of diet or GH therapy was observed in terms of the serum levels of 7{alpha}-hydroxy-4-cholesten-3-one, an intermediate product in bile acid formation that correlates to the rate of bile acid synthesis (26, 28). Serum levels of 7{alpha}-hydroxy-4-cholesten-3-one were 30 ng/mL (19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50) at the start of the diet period without GH and 44 ng/mL (24–54) on day 17. During GH therapy, the value was 32 ng/mL (23–41) at the start of the diet period and 55 ng/mL (29–60) on day 17. The change from days 0–17 without GH therapy was 7.5 ng/mL (0.0–17), whereas it was 14 ng/mL (1.0–37) when the subjects were receiving GH therapy. These changes did not differ from one another.

Glucose metabolism

Neither the diet nor GH therapy had any effect on fasting blood glucose or insulin levels (data not shown). The area under the response curve was calculated for blood glucose and serum insulin response during the OGTT. No significant changes were observed in blood glucose response as a result of the experimental diet. The blood glucose response was 453 mmol/min·L (447–526) at the start and 500 mmol/min·L (470–680) on day 17 when no GH was given. During the diet period with GH therapy, the blood glucose response was 662 mmol/min·L (600–681) at the start and 667 mmol/min·L (606–686) on day 17. The change in response from days 0–17 was 15 mmol/min·L (-9 to 22) without GH therapy and 12 mmol/min·L (-10 to 22) with GH therapy. These changes did not differ from one another.

The insulin response displayed no significant changes due to diet. The insulin response without GH therapy was 4,824 mIU/min·L (4,802–5,486) at the start and 5,476 mIU/min·L (4,815–5,780) on day 17. During GH therapy, the insulin response was 8,571 mIU/min·L (7,593–9,736) at the start and 7,755 mIU/min·L (6,099–12,295) on day 17. The change in insulin response from days 0–17 was -9.0 mIU/min·L (-686 to -14.0) without GH therapy and 1,242 mIU/min·L (106–2,232) with GH therapy. These changes were not statistically different from one another. When comparing the baseline OGTTs performed at the start of each diet period, GH therapy tended to increase both the blood glucose (P = 0.08) and the insulin (P = 0.08) response.

Body weight, body composition, IGF-1, and IGFBP-3

Body weight at the start of the diet period without GH therapy was 82.9 kg (78.6–91.7) and 83.2 kg (79.7–91.6) on day 17. During the diet period with GH therapy, body weight was 82.6 kg (80.4–92.5) at the start and 82.8 kg (80.3–91.6) on day 17. Body weight was not significantly affected during the study. The experimental diet did not change body composition as measured by dual energy x-ray absorptiometry, and body composition was stable between the two diet periods (data not shown).

Serum IGF-1 and IGFBP-3 levels were not affected by the diet. Serum IGF-1 levels were 100 µg/L (69.6–107) at the start and 103 µg/L (81.7–120) on day 17 of the period without GH. The corresponding IGFBP-3 values were 1.86 mg/L (1.43–2.45) at the start and 1.98 mg/L (1.33–2.41) on day 17. During GH therapy, serum IGF-1 levels were 243 µg/L (229–341) at the start and 269 µg/L (190–291) on day 17 of the diet period. The corresponding IGFBP-3 levels were 2.80 mg/L (2.36–3.00) and 2.64 mg/L (2.51–2.88). GH therapy did not influence the small changes between days 0–17 induced by the diet (data not shown). However, GH therapy increased both parameters significantly when comparing values at the start of each period. The mean IGF-1 level of all subjects was 68% above the predicted level (27) when they received GH therapy and 43% below the predicted IGF-1 level without GH therapy.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The main finding in this study was that GH had no major effect on the responsiveness to an increased load of dietary ch and saturated fats in terms of serum LDL-ch levels. However, due to the small number of subjects, the possibility that GH has a minor effect on the response by LDL-ch or other serum lipoprotein components to a high-fat diet cannot be ruled out. Another limitation of this investigation is that it was not blind. However, due to the high level of food intake control, it is unlikely that the dietary challenge was different during the two periods. A further limitation of the study is that we were not able to determine whether the subjects were in steady state with respect to changes in serum lipoproteins after 17 days of a high fat diet, as some subjects experienced different effects on days 9 and 17. For this reason, we avoided comparisons using the values on day 9 of the dietary periods. In other studies, the effect of diet on serum ch levels occurred within 3 weeks, and no further changes occurred thereafter (29), thereby indicating that it is not likely that further changes in serum ch levels would have occurred if the diet period had been longer. The subjects in this study increased their median serum LDL-ch levels by approximately 0.5 mmol/L during the two diet periods. This increase is similar to that observed in other studies of increased saturated fat or ch intake (29, 30, 31).

The doses of GH used here (1–1.5 U/day) were more than enough to normalize serum IGF-1 levels (27). As a result, it is not likely that higher doses of GH would have produced effects other than those observed in this study. This study had a cross-over design. Two of the subjects received GH for a shorter time than the other subjects before the dietary challenge. However, we did not observe any reactions to indicate that these subjects responded differently from the others to the experimental diet during GH therapy. Other studies have revealed that most of the effects of GH on serum lipoproteins occur within a few weeks of therapy (9, 23), which indicates that the duration of GH therapy is not of major importance.

The lack of effect by GH therapy on the LDL-ch response to the dietary challenge in this study is in contrast to the marked suppressive effect of GH on a ch-induced increase in LDL-ch observed in the hypophysectomized rat (15). However, GH influenced the effect of a dietary load of ch and saturated fat on the increase in the lathosterol/ch ratio, indicating that GH decreases ch synthesis. The subjects had the same diet during the diet periods, and the serum concentrations of plant sterols at the end of the diet periods were similar, indicating that GH therapy did not influence the intestinal absorption of ch (32). The unchanged serum 7{alpha}-hydroxy-4-cholesten-3-one response indicated unaltered elimination of ch via bile acid synthesis (26). The indication of a suppression of total body ch biosynthesis during GH therapy therefore probably has an explanation other than a change in the intestinal ch absorption (32) or bile acid formation (28). The liver is important for the entire animal ch homeostasis. The hepatic synthesis of ch, but not the ch synthesis in other organs, is influenced by dietary ch and fatty acids (33). For this reason, the inhibitory effect of GH on the enhanced ch synthesis induced by diet probably involves the liver.

Previous studies demonstrate that precursors to ch in serum, such as lathosterols, reflect ch biosynthesis (34, 35). A good correlation between several ch precursors in serum, including total {Delta}7-lathosterol levels, and HMG-CoA-reductase activity in liver biopsies was demonstrated by Björkhem et al. (25). The total {Delta}7-lathosterol serum level was expressed relative to ch, as this ratio has been shown to correlate better to the HMG-CoA reductase activity (25). The increase in the lathosterol/ch ratio in response to the diet thus indicates an increase in hepatic HMG-CoA-reductase activity. The increase in ch biosynthesis is probably explained by the increased intake of saturated fatty acids, as an increase in the intake of ch alone would have resulted in decreased ch synthesis (32).

The subjects received a diet enriched in both saturated fat and ch for two reasons. Firstly, it is well known that serum ch and LDL-ch in humans are more sensitive to dietary saturated fat than dietary ch (16, 17, 36). Secondly, there is a parallel responsiveness to ch and saturated fat, at least in humans with normal ch intake (37). The mechanisms for the increase in LDL-ch and ch synthesis during a high intake of saturated fat in man are not completely understood. Studies in experimental animals have shown that most fatty acids increase ch synthesis (38, 39), but only saturated fatty acids increase LDL-ch levels via a decrease in LDL receptor expression (33, 40, 41). The indication of an increase in ch biosynthesis was therefore expected in view of the increase in ch intake from moderate to high and the increase in intake of saturated fat.

The mechanisms by which GH decreased the enhanced ch synthesis without affecting the change in serum LDL-ch can be speculated upon on the basis of the known actions of the hormone in hypophysectomized rats. As previously mentioned, LDL receptor expression was less down-regulated during a challenge with a high ch diet when the hypophysectomized rats were receiving GH therapy (15). If the LDL receptor activity was less down-regulated during the fat challenge when the subjects were receiving GH therapy, a larger receptor-mediated uptake of LDL-ch, followed by a decrease in ch biosynthesis, would have occurred. It is thus possible that GH therapy prevented a down-regulation of LDL receptor activity to some extent and thereby reduced the enhanced ch synthesis induced by the dietary load of saturated fatty acids.

However, the maintenance of LDL expression must have been balanced by an increase in the production of LDL-ch, as one would otherwise expect lower serum LDL-ch levels during GH therapy. An increase in VLDL secretion would result in an increase in the production of LDL. There are several experimental indications in the rat that GH in vivo increases VLDL secretion (42, 43, 44). Data from human studies are limited, but one report indicates an increase in VLDL secretion after GH therapy in GHD adults (45).

A change in bile acid formation might affect the rate of ch synthesis. Studies by Axelsson and co-workers have demonstrated that plasma 7{alpha}-hydroxy-4-cholesten-3-one levels reflect changes in bile acid synthesis during conditions of both increased and decreased ch synthesis (26). The effects of GH on bile acid synthesis in man are not clear. In line with our observations, Olivecrona and co-workers observed no changes in bile acid synthesis in healthy males after 3 weeks of GH treatment (46). In contrast, Heubi and co-workers found increased bile acid synthesis after 6 months of GH substitution in children with GHD (47). Although it has been shown that GH increases 7{alpha}-hydroxylase activity in hypophysectomized rats (48), the present results and previous results in adults (46) do not lend support to the hypothesis that GH increases LDL receptor expression in man by increasing bile acid synthesis. Dietary effects on 7{alpha}-hydroxylase activity have been demonstrated in the rat, whereas dietary alterations have small effects in humans (49). In our study, four of the subjects experienced an increase in their serum level of 7{alpha}-hydroxy-4-cholesten-3-one during the diet periods, indicating an increase in bile acid synthesis. Although the changes did not reach statistical significance, the possibility cannot be excluded, on the basis of the present data, that a diet rich in ch and saturated fat increases bile acid synthesis.

Serum Lp(a) levels are highly dependent on the apo(a) phenotype in the population (50, 51). However, GH therapy has been shown to markedly increase Lp(a) levels (9, 10) regardless of the apo(a) phenotype (10). In this study, we again observed the stimulatory effect of GH on serum Lp(a) levels. A diet enriched in ch and saturated fat has been shown to increase the serum levels of Lp(a) in baboons (52), but the effect of a change in fat intake in man is less consistent (53, 54, 55). In this study, no significant effect by GH therapy on the change in Lp(a) levels during the high fat diet was observed. If anything, Lp(a) levels tended to decrease during the diet period without GH therapy and to increase during the diet period with GH therapy. As these tendencies were more pronounced on day 9 of the diet periods, it is possible that any effect by GH on the response to a high fat diet might be transient and not possible to detect during a longer period of a high fat diet, even in a larger study population.

We did not observe any dietary effects on serum IGF-1 concentrations. IGF-1 levels have previously been shown to be affected by the energy and protein intake (56). However, to the best of our knowledge, no previous studies have tested whether a diet enriched in ch and saturated fatty acids affects serum IGF-1 levels. There was no effect of the diet on IGFBP-3 levels, thereby indicating that free IGF-1 levels were also unaffected. This observation is of interest, as it has been shown that low free IGF-1 levels may be associated with an increase in the prevalence of atherosclerotic plaques and coronary heart disease (57).

In conclusion, GH therapy in GHD adults attenuated the increase in ch synthesis induced by the dietary load of saturated fat, but the changes in serum LDL-ch levels were not affected. Therefore, although GH probably decreases hepatic ch synthesis, the lack of effect of GH on the serum LDL-ch response illustrates the complexity of the effects of GH on hepatic lipoprotein metabolism in man.


   Acknowledgments
 
We are indebted to Anne Rosén, Ingrid Hansson, Sigrid Lindstrand, and Lena Wirén at the Research Center for Endocrinology and Metabolism for their skillful technical support. We also thank Aira Lidell, Anita Lund, and Christina Ullström at the Wallenberg Laboratory for their skillful technical assistance.


   Footnotes
 
1 This work was supported by Eli Lilly & Co. (Sweden) and a grant from the Swedish Medical Research Council (Project 11621). Back

Received May 7, 1998.

Revised October 16, 1998.

Accepted November 17, 1998.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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