点击显示 收起
1 From the Centre for Diabetes and Metabolic Medicine, Queen Mary School of Medicine and Dentistry, University of London, London, United Kingdom (NM and BJB), and the Department of Clinical Chemistry, Bart’s and The London NHS Trust, London, United Kingdom (WGJ and KN)
2 Supported by a grant from the North East- (later North-) Thames NHS Research & Development Directorate, London. 3 Reprints not available. Address correspondence to BJ Boucher, Department of Diabetes and Metabolic Medicine, Royal London Hospital, London, E11BB, United Kingdom. E-mail: bboucher{at}doctors.org.uk.
ABSTRACT
Background: Although hypovitaminosis D has been suggested to increase the risk of heart disease, its relation to components of the fasting lipid profile has not been clarified for specific ethnic groups.
Objective: The objective was to determine the relation of circulating 25-hydroxyvitamin D [25(OH)D] concentrations to fasting lipid concentrations in South Asian subjects at risk of hypovitaminosis D.
Design: The present study was conducted in 170 British Bangladeshi adults, 69 men and 101 women, from east London who were free of known diabetes or chronic disorders. Vitamin D repletion was assessed by measuring fasting serum 25(OH)D concentrations. Fasting lipid profiles were measured as part of a study of the risk factors for type 2 diabetes and ischemic heart disease, which included hypovitaminosis D.
Results: A univariate analysis showed that total cholesterol, LDL cholesterol, and both apolipoprotein (apo) A-I and apo B concentrations correlated directly with serum 25(OH)D concentrations. However, a multiple regression analysis, which included all the documented risk factors for diabetes and ischemic heart disease, showed that the 25(OH)D concentration (vitamin D status) was an independent predictor of increasing apo A-I concentrations (standardized coefficient ß = 0.3; P < 0.001) but not of fasting lipid concentrations.
Conclusions: In this study of British South Asians, the data showed a positive relation of fasting apo A-I concentrations to serum 25(OH)D concentrations, independent of glycemia and other dietary, anthropometric, and lifestyle risk factors for type 2 diabetes and ischemic heart disease after multiple regression analyses. Subjects with hypovitaminosis D are likely to have an increased risk of ischemic heart disease independent of their increased risk of type 2 diabetes.
Key Words: Vitamin D • fasting lipids • hypovitaminosis D • triacylglycerol • cholesterol • apolipoprotein A-I • apolipoprotein B • HDL cholesterol • LDL cholesterol • South Asians
INTRODUCTION
Vitamin D repletion is achieved by environmental factors, such as exposure to ultraviolet light and consumption of foods rich in fat-soluble vitamin D (eg, oily sea fish, meat, and eggs), and can be assessed by measurement of the concentration of serum 25-hydroxyvitamin D [25(OH)D] (1-3). Hypovitaminosis D is common in the Northern Hemisphere of the Western world but is also becoming increasingly common in the Southern Hemisphere because of changes in lifestyle, such as working indoors, wearing occlusive clothing, and increasing the use of sunscreen creams. Vitamin D deficiency is a risk factor for osteoporosis, certain cancers, type 1 diabetes, and hypertension. The association of vitamin D deficiency with ischemic heart disease (IHD) may be partly attributable to its association with the metabolic syndrome (4-6). The reductions in insulin secretion and insulin sensitivity that are found in persons with hypovitaminosis D, and the improvements in these abnormalities seen after vitamin D repletion, may explain the association with IHD, because IHD is increased in relation to glycemia in nondiabetic patients as well as in patients with diabetes (7-11). The reduction in the risk of type 2 diabetes and of IHD with increased fish consumption, especially consumption of oily fish, and the associated improvements in fasting lipid profiles are usually attributed to increases in the consumption of n–3 fatty acids (12, 13) but may also reflect any benefits from increased vitamin D intake (4). Hypovitaminosis D has also been reported to be associated with increased total cholesterol concentrations and with reduced apolipoprotein (apo) A-I concentrations in Belgian men (14, 15). Abnormal glucose tolerance adversely affects fasting lipid profiles, and some studies suggest that diabetes can also lower circulating 25(OH)D concentrations (16). More recently, hypovitaminosis D was shown to be associated not only with lowered insulin secretion and sensitivity but also with adverse effects on both total cholesterol and LDL-cholesterol concentrations in a study of healthy men and women from several racial and ethnic groups (8). Thus, hypovitaminosis D appears to be directly associated with adverse effects on lipid profiles independent of any adverse effects that might result from the increased risk of type 2 diabetes.
Because few studies of the relation between fasting lipid profiles and vitamin D status have been done, we report our findings in a group of British South Asians who were of Bangladeshi origin and were selected as being free of known diabetes and in whom we have already shown an association between increased inflammatory activity, in terms of circulating concentrations of both sensitive C-reactive protein and matrix metalloproteinase 9, and hypovitaminosis D (17).
SUBJECTS AND METHODS
Subjects
The present study was conducted in British men and women of Bangladeshi origin aged 35–65 y who were free of known diabetes, IHD, hypertension, or other ongoing illness. The subjects were recruited randomly during visits to their family physician’s offices with relatives or for minor intercurrent illness. The subjects gave informed consent both verbally and in writing for a study that was designed to examine the effect of vitamin D supplementation on the development of type 2 diabetes in subjects who were at the upper range of normoglycemia and were at high risk of vitamin D deficiency. The subjects who gave informed consent and who were defined as being at risk of diabetes were assessed as previously described (17, 18). Diabetes risk was defined as a random blood glucose concentration (which was measured with the use of a point-of-care hexokinase method and was validated with the use of standard autoanalyzer methods) of >6.4 mmol/L <2 h after food ingestion or >4.4 mmol/L >2 h after food ingestion. In brief, the subjects were recruited at their family physician’s office by one bilingual researcher (NM) over a period of time that included all 4 seasons of 1 year. Six hundred thirty-one persons who consecutively attended the doctor’s office and who met the initial selection criteria agreed to participate in the study and were screened by measurement of a random blood glucose concentration. Twenty-five subjects who had random glucose values of >12 mmol/L were determined to have previously undiagnosed diabetes and were thus excluded from additional study because overt diabetes may have confounded the analyses by lowering circulating 25(OH)D concentrations (16). Two hundred thirty subjects met the random blood glucose criteria. Of these, 38 subjects declined further participation in the study, 2 subjects reported a recent oral-glucose-tolerance test (OGTT), and 19 subjects were unable to attend a morning OGTT, which left 171 subjects who were enrolled into the study. The initial survey of these subjects provided the cross-sectional study, which was the basis for the present study. The study was approved by the local District Ethical Committee and was performed in accordance with its requirements.
Assessment of build, diet, and glucose tolerance
Anthropometric measurements were made with the use of standard techniques (19). In brief, a 15-mm fiberglass tape was used to measure waist circumference (to the nearest cm; between the costal margin and the iliac crests) and hip circumference (to the nearest cm; at the level of the greater trochanters). Height was measured (without shoes; to within 0.1 cm) with the use of standard National Health Service wall-fixed scales. Weight was measured (to the nearest 0.5 kg) with a single scale. The subjects completed a validated questionnaire, which was provided in both English and Bengali and that covered paan usage (number of paan quids, which contain betel nuts, chewed daily), aspects of diet that were relevant to vitamin D intake (egg, fish, meat, and yogurt consumption per week, and whether 15 g margarine/d was used), and cigarette smoking (no. cigarettes/d) (17). The 171 subjects, who were defined as at risk of developing diabetes, underwent a 75-g OGTT, which was carried out after an overnight fast in a hospital-based clinical research facility, as previously described (17, 18). One hundred seventy of the 171 subjects completed the assessments, including the 75-g OGTT. Vitamin D status was defined by the subjects’ serum 25(OH)D concentration (20).
Laboratory assays
Blood samples from fasting subects were taken without venous constriction during the OGTT, and serum and plasma aliquots were frozen at –20 °C before the samples were blinded for assay within single analytic runs. Insulin profiles were assessed during the OGTT (fasting and 30-min concentrations of insulin, proinsulin, and 32:33 split proinsulin) as well as the calculated insulin secretion index [(30 – 0 min plasma glucose concentrations)/(30 – 0 min serum insulin concentrations)] and the calculated fasting 32:33 split proinsulin-to-insulin ratios (21, 22), as previously reported (23). Fresh blood samples were analyzed for fasting serum triacylglycerol, cholesterol, and HDL-cholesterol concentrations (after precipitation with magnesium and phosphotungstate) with the use of enzymatic methods according to the manufacturer’s procedures (Instrumentation Laboratory UK Ltd, Warrington, United Kingdom). LDL-cholesterol concentrations were calculated with the use of the Friedewald formula. Apo A-I and apo B concentrations were measured by immunoturbidimetry (Instrumentation Laboratory UK Ltd) on previously unthawed serum samples (24). Fasting serum 25(OH)D concentrations were measured by immunoassay (IncStar, MN; within- and between-assay CVs <3.8%); vitamin D concentrations <11 ng/mL were defined as vitamin D deficiency (23). Fasting serum concentrations of intact parathyroid hormone were measured with a radioimmunometric assay (Nichols Institutes Diagnostics, San Juan Capistrano, CA; within- and between-assay CVs were <7% and <10%, respectively; normal range: 48–119 nmol/L). Vitamin D receptor (VDR) genotyping, which tested for ApaI, BsmI, TaqI, and FokI polymorphisms, was available for the subjects as previously reported (25).
Statistical analyses
Analyses were performed with SPSS version 11.0 (SPSS Inc, Chicago, IL). A multivariate analysis with stepwise logistic regression (to P < 0.05) was used to examine the influence of vitamin D status and of relevant risk factors, which were treated as potentially confounding variables. Continuous variables that were not normally distributed were normalized by logarithmic transformation before analysis. Pearson correlations were examined, and nonparametric Spearman’s rank tests were used if the data were not normally distributed. Differences between men and women in means for each variable were examined with one-factor analysis of variance or with nonparametric Kruskal-Wallis tests if the data were not normally distributed.
RESULTS
Although subjects with known diabetes were excluded from the study, diabetes was found in 14 subjects (8.19% of the total number of subjects) and impaired oral glucose tolerance was found in 28 subjects (16.37% of the total number of subjects) during the OGTT with the use of the 1985 World Health Organization criteria, which were the current criteria when the study was designed (25, 26). Because diabetes has been reported to be associated with reduced 25(OH)D concentrations (16), the 2-h plasma glucose concentration measured as part of the OGTT was included as a potential confounder in the multiple regression analyses, which were used to identify the independent predictors of each component of the fasting lipid profile. The variables examined are shown in Table 1. If the confounders varied by sex, then the means for men and women are shown separately. Regression analyses were carried out both including and excluding the subjects who were found to have type 2 diabetes from the OGTT. The fasting lipid profile data are shown in Table 2; if the outcome varied by sex, then the means for men and women are shown separately. Forty subjects were classically vitamin D deficient [25(OH)D concentration <11 ng/mL], and an additional 71 subjects were vitamin D insufficient but not deficient [25(OH)D concentrations were >11 but <20 ng/mL] (23).
View this table:
TABLE 1. Clinical characteristics and dietary habits of the subjects1
View this table:
TABLE 2. Fasting lipid profiles and vitamin D status (serum 25-hydroxyvitamin D concentration) of the South Asian (Bangladeshi) subjects1
Relation of 25(OH)D to lipids from fasting subjects
A univariate analysis of the relation of serum 25(OH)D, which represents vitamin D status, with each component of the fasting lipid profile is shown in Table 3. Also shown in Table 3 are the predictors of the variables of interest, which were identified as independent determinants (P < 0.05) with the use of a stepwise multiple logistic regression analysis that included the same variables used in the analyses reported by Chiu et al (8). When the additional risk factors investigated in the present study were included in these analyses, the 25(OH)D concentration was identified as an independent predictor for apo A-I but for no other lipid variables. Other predictors that were identified as having independent adverse effects on components of the fasting lipid profile in the analyses [which included the full range of variables that were examined as relevant risk factors for type 2 diabetes (see footnote to Table 3)] were waist size for total cholesterol and triacylglycerol concentrations (P = 0.008 and 0.001, respectively), smoking for LDL-cholesterol concentrations (P = 0.048), male sex for apo B concentrations (P = 0.046), season of the year for apo B and HDL-cholesterol concentrations (P = 0.01 and 0.045, respectively), serum creatinine concentrations for HDL-cholesterol concentrations (P = 0.029), and the fasting proinsulin-to-insulin ratio for total cholesterol and triacylglycerol concentrations (P = 0.001 and 0.012, respectively). Triacylglycerol concentrations were directly correlated with fasting glucose concentrations (P = 0.015), but glycemia was not a predictor of increased triacylglycerol concentrations in a multiple stepwise regression analysis. Two-hour blood glucose concentrations during the OGTT (P = 0.005) and female sex (P = 0.05) were the only independent predictors, other than vitamin D status (P < 0.0001), that were identified as increasing apo A-I concentrations. No significant change in the findings was observed when these analyses included paan (betel nut) consumption, VDR genotype (Apa1, Bsm1, Taq1, and Fok1 polymorphisms), or both, even if the subjects with type 2 diabetes diagnosed by the OGTT were included in the analyses.
View this table:
TABLE 3. Regression analysis of the effect of vitamin D status on components of the fasting lipid profile in British Bangladeshi South Asians1
DISCUSSION
Our data showed that the concentration of 25(OH)D, which reflects vitamin D repletion, as examined with the use of a multiple logistic regression analysis that included all the relevant risk factors investigated (16), had a positive relation to and is an independent predictor of apo A-I concentrations but not of other components of the fasting lipid profile of British South Asians who were in good health. When we included only the classic risk factors used by Chiu et al (8), the serum 25(OH)D concentration, which reflected vitamin D status, was a strong independent predictor of increases in apo A-I and apo B concentrations, of increases in total cholesterol and LDL-cholesterol concentrations, and of decreases in serum triacylglycerol concentrations. In contrast, Chiu et al (8) found negative relations of total cholesterol and LDL-cholesterol concentrations with serum concentrations of 25(OH)D. The reasons for this discrepancy are not obvious. The relations between vitamin D status and fasting lipid concentrations were not reported for separate ethnic groups by Chiu et al (8), but ethnicity approached significance as a predictor of insulin sensitivity in that study. It is possible, therefore, that these discrepancies may reflect the particular ethnicity of our subjects, who were from a close-knit community that originated in Sylhet, Bangladesh. The increased triacylglycerol concentrations in our subjects is a feature that is commonly reported in randomly selected South Asian subjects from the same population group in the United Kingdom, and this feature correlates directly with central obesity in these subjects and is increased in relation to type 2 diabetes, which is found in >20% of Bangladeshi South Asian subjects compared with 4% of white control subjects (19). The elevated triacylglycerol concentrations found in the present study do not, therefore, relate simply to the selection of subjects in the higher rather than the lower range for normoglycemia.
Limitations of this study included the relatively small sample size, but this was mitigated by the lack of confounding by inclusion of only 1 ethnic group. The restriction of the glycemic range through the exclusion of subjects with overt diabetes or with low normoglycemia may have narrowed the range of lipid measurement findings; a larger study group with a wider glycemic range might uncover additional relations between components of the lipid profile and vitamin D status. The risk factors assessed in our study were those previously identified for type 2 diabetes, IHD, or both and included smoking, serum creatinine concentrations, homocysteine concentrations, fish consumption, the use of margarine (which is vitamin D–fortified in the United Kingdom), tea and coffee drinking, and glycemic status, as defined by the 2-h plasma glucose concentrations measured after the 75-g OGTT [increases in glycemia are a risk factor for IHD in healthy as well as hyperglycemic subjects (10-13, 27-31)]. Remarkably few of these factors were predictors of the lipid profile in our study group. Increases in waist circumference were, as expected, related to increases in total cholesterol and triacylglycerol concentrations. Increased hip circumference predicted decreases in these lipids and in LDL-cholesterol concentrations, but these findings were probably artifactual because waist and hip size are related and the waist-to-hip ratio, when used as a variable rather than these 2 variables separately, did not appear as a predictor. Smoking related only to increases in LDL-cholesterol concentrations, whereas an increased fasting 32:33 split proinsulin-to-insulin ratio (22), which reflects reduced insulin release, related to increases in total cholesterol, triacylglycerol, and apo B concentrations. Our findings were not confounded by the habit of chewing betel nuts, which are found in paan quids and are a recognized risk factor for type 2 diabetes both in CD1 mice and in humans (32-34). Nor was the presence of previously undiagnosed type 2 diabetes in our subjects a confounder, because reanalyses of the data that excluded subjects found to have type 2 diabetes at the initial OGTT did not affect the findings. Furthermore, a reanalysis that included the VDR genotype (Apa1, Bsm1, Taq1, and Fok1 polymorphisms), which were previously shown to relate to the insulin secretion index (25) of the subjects, did not affect the findings. This suggests that vitamin D activity, insofar as it affects lipid metabolism directly, does not vary with VDR genotype and that the variation in insulin secretion because of a VDR polymorphism, which we reported previously in this cohort of subjects, has no significant effect on the fasting lipid profiles.
Previous work conducted in white subjects in Belgium showed that fasting apo A-I concentrations correlate directly with serum 25(OH)D concentrations in men but not in women (15), but we have found no other studies of apolipoprotein in relation to serum 25(OH)D concentrations to date. The present study in British Bangladeshis (South Asians) showed no significant variation in the fasting lipid concentrations with regard to sex: vitamin D status was an independent predictor for serum apo A-I concentrations in both men (P = 0.003) and women (P = 0.011). The relation found between vitamin D status and HDL-cholesterol concentrations in white subjects by Auwerx et al (15) was not observed in our subjects.
It has been postulated in a review of the literature (4) and, in particular, in epidemiologic and cross-sectional studies (8, 35-38) that hypovitaminosis D contributes to the risk of the metabolic syndrome (syndrome X). This postulate is supported by previous findings of correlations between serum 25(OH)D concentrations and both HDL-cholesterol concentrations and apo A-I concentrations (15) and by data from the Coronary Artery Risk Development in Young Adults study (39), which showed that the consumption of dairy-based foods, including the vitamin D–fortified milk sold in the United States, is inversely related to the risk of type 2 diabetes and IHD. This finding was, paradoxically, noted in overweight subjects, whereas obesity is normally regarded as a powerful risk factor for the metabolic syndrome (40, 41). Obese subjects are now thought to be at an increased risk of hypovitaminosis D because of sequestration of vitamin D in adipose tissue, which may contribute to the risk of the metabolic syndrome that is associated with obesity (42). Inverse associations between vitamin D status and both diabetes and insulin resistance were recently reported in Mexican Americans and non-Hispanic whites but not in non-Hispanic blacks (43). Serum total cholesterol concentrations have not been found to relate to serum 25(OH)D concentrations in the few studies reported over the past 25 y (44-46). The additional finding that insulin sensitivity and ß cell function are both adversely affected by worsening hypovitaminosis D in normoglycemic subjects as well as in those with dysglycemia (8, 7, 9, 14, 37) is important because increases in insulin resistance and in glycemia are associated with an increased risk of cardiovascular disease even in normoglycemic subjects (10, 11, 40). Although the contribution of hypovitaminosis D to dyslipidemia in British South Asians in the present study did not appear to extend beyond an effect on apo A-I concentrations, our findings support the postulate that avoidance of hypovitaminosis D could contribute to reductions in the risk of the sequelae of the metabolic syndrome. Because apo A-I is involved in the reverse transport system that clears tissue cholesterol (47), lowering the availability of apo A-I will increase the risk of vascular damage associated with overt type 2 diabetes and, specifically, of IHD and stroke, which are also associated with the metabolic syndrome. These findings further justify the need for inclusion of variables of the metabolic syndrome in future considerations of the overall benefits of the avoidance of vitamin D deficiency (8, 48, 49). However, larger studies that provide estimates of the preventable proportion of the metabolic syndrome–related disease at both the personal and population levels are still needed for public health planning, and such studies appear to be long overdue.
ACKNOWLEDGMENTS
The authors are grateful to BV North, Academic Department of Psychiatry, Queen Mary School of Medicine & Dentistry, for statistical advice.
WGJ advised on the biochemical methodology and carried out the lipid analyses; KN advised on the immunoassay methodology and carried out the vitamin D metabolite and parathyroid hormone immunoassays; NM assisted in the study design and organization, assisted in the preparation of bilingual material (including the questionnaires), carried out all subject assessments, assisted with the oral-glucose-tolerance tests, made the data records, and assisted with preliminary data analysis; and BJB was responsible for the inception of the study, raising grant support, statistical analyses, and for drafting and finalizing the report. All authors contributed to the preparation of the manuscript and all authors saw and agreed on the final submitted version. None of the authors had a conflict of interest.
REFERENCES