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Department of Medicine (K.W.), Karolinska Institutet, SE-17177 Stockholm; Division of Nutritional Epidemiology (S.C.L., A.W.), The National Institute of Environmental Medicine, Karolinska Institutet, SE-17177 Stockholm; Department of Public Health and Caring Sciences (B.V.), Unit of Clinical Nutrition, Uppsala University, SE-75125 Uppsala; and Department of Molecular Medicine (K.B.), Karolinska Institutet, SE-17176 Stockholm, Sweden
Address all correspondence and requests for reprints to: Katarina Wolk, Department of Medicine, Karolinska Institutet, P.O. Box 210, SE-171 77 Stockholm, Sweden. E-mail: katarina.wolk{at}ks.se.
| Abstract |
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| Introduction |
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Serum IGFBP-1 levels vary considerably among healthy individuals (12, 13). A monozygotic twin study showed that nongenetic factors explained 64% of total variation in serum IGFBP-1 levels (14). Insulin and IGF-I explained 28% and 8%, respectively, of the nongenetic variation in IGFBP-1 (15). Hence, approximately 30% of the variation in IGFBP-1 levels remains unexplained, and this variation might be due to dietary and other lifestyle factors.
IGFBP-1 has been found to be inversely correlated with fasting levels of insulin and IGF-I as well as with body mass index (BMI) (4, 5, 14). Serum IGFBP-1 levels are sensitive to energy and nutrient intake. Whereas food intake markedly reduces serum IGFBP-1 levels, dietary protein restriction and fasting have an opposite effect (6, 16). A study among 292 women (meat-eaters, vegetarians, and vegans) showed that both serum IGFBP-1 and IGFBP-2 levels were 2040% higher in vegan women compared with meat-eaters and vegetarians (17).
To our knowledge, no previous study has examined the relation between IGFBP-1 levels and long-term nutrient intake in healthy men. Given the potential important role of IGFBP-1 on overall health, better knowledge about the associations of modifiable nutritional and anthropometric factors with circulating IGFBP-1 levels is needed. We therefore conducted a study to evaluate these associations in a general population of free-living middle-aged and elderly men.
| Subjects and Methods |
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This study included healthy men, ages 4276 yr, randomly selected from the population register of Uppsala (city) and Knutby (countryside) in central Sweden. The men were invited to participate in a validation study of a food-frequency questionnaire (18). A total of 469 men filled in the form; and among them, 308 participated in 14 24-h diet recall interviews performed during 1 yr (once per month). In this group, within 2 wk after completion of the last interview, we collected blood samples and data on weight, height, waist circumference, and sagittal measures for 237 men. We excluded from the analyses 11 subjects who had measurements 2 SDs below or above the mean of the reference population: nine with extremely high insulin values (>28.5 mU/liter), one with high IGFBP-1 value (149 µg/liter), and one with very high IGF-I value (358 µg/liter). These exclusions left 226 men for analyses. The study was approved by the ethical committees at Uppsala University Hospital and at Karolinska Institutet, Stockholm.
Blood collection and laboratory assays
Venous blood samples were drawn in the morning, after a 12-h overnight fast. Serum was separated by centrifugation at 1200 x g after coagulation of the blood during 2 h at room temperature. Samples were stored at 70 C until analyzed for insulin, glucose, IGFBP-1, and IGF-I. IGFBP-1 and IGF-I were analyzed after storage of up to 2 yr. IGFBP-1 was determined by RIA according to the method of Póvo et al. (19) The antibodies used were raised in rabbits against purified human amniotic protein, and the cross-reaction with IGFBP-2 and IGFBP-3 was less than 0.1%. The assay measures both phosphorylated and nonphosphorylated forms. The sensitivity of the assay was 3 µg/liter with within-assay and between-assay coefficients of variation of 3% and 10%, respectively.
IGF-I was assayed, within 1 yr after blood collection, by RIA after separation of IGFs from IGFBPs by acid-ethanol extraction (20). The sensitivity of the assay was 2 µg/liter, and the within-assay and between-assay coefficients of variation were 4% and 11%, respectively. There was no significant cross-reactivity between IGF-I, IGF-II, or other peptide hormones with IGF-I antiserum. No significant interference of IGFBPs was detected. Insulin was assayed by ELISA with reagents from Hoffman-La Roche LTD Diagnostic Division (Basel, Switzerland) immunodiagnostics for the ES 300. Plasma glucose was assayed by the glucose dehydrogenase method (Advia 1650, Bayers Business Group Diagnostics, Tarrytown, NY).
Assessment of nutrient intake
Long-term nutrient intake was estimated on the basis of the mean nutrient intake from 14 24-h dietary telephone interviews performed over 1 yr. The 24-h diet recall technique was completed with probing questions. An administrative program had been choosing random days for consecutive dietary interviews for each individual, covering all weekdays, including weekends. Portion sizes were described in household measures. The interviews were entered using personnel computer nutrient software package MATS (Rudans Lättdata, Västeras, Sweden) (21). For nutrient calculations, we used the Swedish Food Administration Food Database PC version 1992, which includes 1,593 foods and whole dishes (22). For dishes reported but not included in this database, the dietician obtained recipes from the participants and entered appropriate amounts of the component foods.
Anthropometric measurements
The waist was measured with the subjects in the supine position. The abdominal sagittal circumference was recorded at the umbilical level as the height of the abdomen measured from the examination couch when lying down with the legs straight. BMI was calculated by dividing body weight (in kilograms) by the square of height (in meters).
Statistical analyses
Because of the slightly positively skewed nature of the IGFBP-1 distribution in our dataset, we conducted a logarithmic transformation of this variable to improve normality. Homeostasis model assessment (HOMA)-insulin resistance was calculated according to the formula of Matthews et al. (23), i.e. HOMA: [fasting insulin (mU/liter) x fasting plasma glucose (mmol/liter)]/22.5. Spearman rank correlation coefficients and multivariate regression analyses (adjusted for age, insulin, IGF-I, and BMI) were used to estimate association of metabolic, anthropometric, and nutritional factors with IGFBP-1 levels. To evaluate how much of the variability in serum IGFBP-1 levels might be explained by anthropometric and nutritional factors, we calculated partial Spearman correlation coefficients controlled for age, insulin, IGF-I, and BMI. All statistical analyses were performed using SAS software (SAS Institute Inc., Cary, NS). Two-sided P values < 0.05 were considered statistically significant.
| Results |
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We calculated partial Spearman correlation coefficients to evaluate how much of the variability in serum IGFBP-1 levels might be explained by different metabolic, anthropometric, and nutritional factors (Table 3
). In these analyses, we used BMI as a measure of body composition, because this was the anthropometric variable most strongly correlated with IGFBP-1 levels. BMI, insulin, and carbohydrate intake could together explain 39% of the variability in serum levels of IGFBP-1 in men of age 4254 yr, with BMI being the strongest single determinant, explaining 21% of the variability. In men of age 5564 yr, insulin was the strongest single predictor of IGFBP-1 levels. None of the single parameters explained more than 8% (IGF-I) of IGFBP-1 variability in men 6576 yr of age. Regarding the macronutrients, carbohydrate was the most important determinant of IGFBP-1 levels in men of age 4254 yr and 5564 yr (explained 7% and 3%, respectively, of the variability), whereas total fat and protein appeared more important in men of age 6576 yr, explaining 4% of the variability.
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| Discussion |
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In men less than 65 yr of age, BMI, insulin, and carbohydrate intake explained most of the variability in IGFBP-1 levels; whereas in men 65 yr and older, IGF-I, independent of insulin, appeared to be the major determinant. The lower explanatory power of metabolic, anthropometric, and nutritional factors in the elderly men suggests that with advancing age, genetics or other factors not measured by us might have an increasing role in explaining IGFBP-1 levels.
Our finding of an inverse correlation between IGFBP-1 and IGF-I is consistent with previous investigations (4, 12, 13, 24, 25). Benbassat et al. (13) reported that IGFBP-1 and IGF-I levels were related in older men but not in younger, which is in accordance with our study. This observation might suggest that the relation between IGFBP-1 and IGF-I is strongest when the secretion of GH is limited, as in older age. Because no IGF-I receptors have been detected in the liver (26, 27), the inverse correlation between IGFBP-1 and IGF-I in older men is unlikely due to a suppressing effect of IGF-I on liver IGFBP-1 production. Instead, one can speculate that IGF-I might increase the clearance of IGFBP-1.
Numerous investigations have confirmed that insulin, through inhibition of IGFBP-1 transcription, is the primary determinant of IGFBP-1 expression both in vitro and in vivo (28). Similarly to other studies (3, 4, 13, 24, 29, 30), we found that IGFBP-1 was strongly inversely related to insulin levels. However, in our data, this correlation was confined to middle-aged men (4265 yr).
This study further confirms previous findings of a positive relation between IGFBP-1 and age in men (12, 13, 29, 31). The positive correlation between IGFBP-1 and age might be caused by higher production rates or slower clearance of IGFBP-1 from plasma in older men.
We found that BMI was strongly inversely correlated with IGFBP-1 levels, which is in agreement with earlier studies (3, 4, 24, 25, 29, 32). In our study, anthropometric factors reflecting abdominal obesity (waist and sagittal measure) were more strongly inversely related to IGFBP-1 in men less than 65 yr of age than in the elderly men, with the exception for weight, which showed the same correlation in both age groups.
To our knowledge we, for the first time, addressed the association between long-term nutrient intake and serum IGFBP-1 levels in middle-aged and older men. Moreover, this is the first study to investigate how much of the variability in IGFBP-1 levels might be explained by nutritional factors. We found that intakes of energy and carbohydrates were positively correlated with IGFBP-1. Given that IGFBP-1 was inversely correlated with HOMA (a measure of whole-body insulin resistance), one can speculate that nutritional factors that increase insulin sensitivity and thus reduce insulin levels are associated with higher IGFBP-I levels. Indeed, Lemne and Brismar (5) reported that men with insulin resistance had lower IGFBP-I levels than men without insulin resistance. Furthermore, studies in healthy children (33) and adults with normal glucose homeostasis (1) and in individuals with type 2 diabetes (30) have shown that IGFBP-1 is positively correlated with insulin sensitivity. Recently, Frystyk et al. (34) reported that glucose infusion during a 24-h period in healthy nonobese individuals resulted in increased insulin sensitivity. We found that intake of carbohydrates or more specifically starch, which is broken down into glucose, was positively correlated with IGFBP-1 levels. Thus, a potential explanation for this finding might be that a high carbohydrate diet leads to increased insulin sensitivity. Alternatively, one might speculate that a long-term high carbohydrate diet may induce hepatic insulin resistance and increased IGFBP-1 levels.
IGFBP-1 is highly sensitive to short-term nutritional status; its serum levels decrease rapidly after a meal and increase markedly after a few hours of fasting (35). Plasma and hepatic mRNA levels of IGFBP-1 increased in rats restricted of protein (36). Smith et al. (37) examined the influence of energy restriction (50% reduction of intake) for 6 d in eight pubertal children and eight adults. Energy restriction resulted in a significant increase in IGFBP-1 levels in adults but not in children. In another study, 16 healthy obese adult women were followed for 14 d of energy restriction (fasting or a hypocaloric diet enriched in protein, fat, or carbohydrate) (38). Although, IGFBP-1 levels increased during energy restriction in all groups, the increment was significantly lower on the high carbohydrate diet. However, these results are difficult to translate to our findings because the men in our study were generally well-nourished and because our results were based on long-term nutrient intake.
Major strengths of our study include its population-based design, relatively large sample size, and the extensive information on nutrient intake collected over 1 yr. Serum levels of IGFBP-1, IGF-I, and insulin were analyzed by experienced laboratory personnel, and all anthropometric measures were taken by a well-trained nurse from the metabolic clinic. However, this study was limited by its cross-sectional nature; the analyses were based on only one blood sample collected in the fasting stage. Nonetheless, it has been found that fasting IGFBP-1 levels correlate well with the 24-h mean serum IGFBP-1 levels. Kaaks et al. (39) demonstrated that the Spearman correlation coefficient for measurements of IGFBP-1 levels between two serum samples 1 yr apart was 0.63.
In conclusion, this study corroborates and extends previous findings of an inverse correlation between serum IGFBP-1 levels and anthropometric factors reflecting obesity. IGFBP-1 levels were significantly inversely correlated with whole-body insulin resistance, as measured using HOMA. We have showed that intakes of energy and carbohydrates were positively correlated with IGFBP-1 in well-nourished middle-aged men. A large proportion of the variability in IGFBP-1 levels might be explained by metabolic, anthropometric, and nutritional factors. These results suggest that factors such as metabolic, anthropometric, and nutritional are important determinants of IGFBP-1 levels in healthy men.
| Footnotes |
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Abbreviations: BMI, Body mass index; HOMA, homeostasis model assessment; IGFBP, IGF binding protein.
Received August 27, 2003.
Accepted January 8, 2004.
| References |
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