Polycystic ovary syndrome (PCOS) is associated with reproductive disorders and cardiometabolic consequences including obesity, type 2 diabetes and cardiovascular disease risk factors [1
]. Low vitamin D levels are similarly associated with increased obesity and cardiometabolic disorders [3
]. We have previously reported that vitamin D levels are lower in overweight women with PCOS compared to overweight women without PCOS and that low vitamin D levels are associated with insulin resistance in women with PCOS [5
], hypothesising that poor vitamin D levels may contribute to the association between PCOS and poor cardiometabolic health.
We have previously shown that a 12-week moderate to vigorous intensity exercise program failed to normalise insulin sensitivity for overweight women with PCOS compared to overweight women without PCOS [6
]. It is possible that low vitamin D levels in women with PCOS contribute to this reduced exercise responsiveness. Low vitamin D levels are associated with reduced fat oxidation during exercise in women [7
], and also smaller exercise-associated improvements in body composition and muscle function in older adults [8
Low vitamin D may impact on poor cardiometabolic health and exercise responsiveness via infiltration of skeletal muscle tissue with adipose tissue. Intra- and intermuscular adipose tissue (IMAT) likely contributes to insulin resistance through local secretion of pro-inflammatory adipokines and impaired insulin signalling in muscle [10
]. We have previously proposed that low vitamin D status contributes to increased accumulation of IMAT [11
], and higher IMAT levels are also associated with poorer exercise responsiveness in older adults [12
]. It is therefore possible that the low vitamin D status of women with PCOS contributes to higher IMAT levels, and that IMAT explains blunted improvements in insulin resistance following exercise.
The aims of this study were to determine cross-sectional associations of vitamin D, IMAT and insulin resistance in women with and without PCOS, and to explore whether baseline vitamin D status and IMAT influence changes in insulin resistance in overweight women with PCOS compared to overweight women without PCOS following a 12-week exercise intervention.
In our cross-sectional study, women with PCOS and low 25OHD levels had significantly increased insulin resistance compared with counterparts with normal 25OHD, and also those without PCOS. Low 25OHD levels were also associated with higher thigh IMAT, and thigh IMAT was associated with higher triglycerides and insulin, and lower HDL cholesterol and higher insulin resistance. Overweight women with and without PCOS and with higher, but not low, pre-training 25OHD levels demonstrate improvements in body composition and insulin resistance following an exercise intervention.
We have demonstrated that PCOS has an intrinsic insulin resistance irrespective of obesity; and age, BMI, testosterone and sex hormone-binding globulin could not explain all of the insulin resistance [20
]. We have also previously demonstrated that 25OHD is associated with insulin resistance in women with PCOS [5
], and have expanded our findings here by demonstrating in our cross-sectional study that women with PCOS and low 25OHD status (<50 nmol/L) have significantly increased insulin resistance compared to counterparts without PCOS and/or low 25OHD levels. These findings suggest that vitamin D may play an important role in insulin resistance, particularly in PCOS. In a study of 38 women with PCOS, Muscogiuri and colleagues demonstrated that higher BMI and total body fat mass were the strongest predictors of low 25OHD amongst metabolic and hormonal parameters [21
], suggesting that the association of vitamin D with insulin resistance in PCOS may simply reflect increased adiposity. Nevertheless, our previous study indicated that the relationship of low vitamin D with insulin resistance in PCOS women is independent of total body fat mass [5
], which might indicate that insulin resistance in women with PCOS and low vitamin D is not attributable to higher fat mass alone.
Total body fat mass may not be a reliable indicator of the extent of ectopic fat depots which are potentially more strongly related to cardiometabolic outcomes than subcutaneous fat depots [22
]. Although there were no differences in visceral fat according to PCOS and 25OHD levels in this study, women with PCOS and low 25OHD had significantly lower thigh muscle density (indicating higher fat infiltration) than women with PCOS and normal 25OHD levels. We have previously proposed that vitamin D reduces deposition of IMAT in skeletal muscle because vitamin D deficient states have been shown to induce myoblasts to transdifferentiate into an adipogenic lineage [11
]. Our findings are also consistent with previous research reporting that 25OHD levels were significantly associated with lower muscle density in 90 women aged 16 to 22 years [23
], with similar findings reported for inter-muscular fat in older adults [24
]. Thus, low vitamin D may contribute to increased IMAT, although it is also possible that higher IMAT levels result in decreased circulating vitamin D given that fat tissue acts as a storage site for vitamin D [4
]. Furthermore, low vitamin D levels result in increased levels of parathyroid hormone which promotes the influx of calcium into adipocytes [25
]. Intracellular calcium in adipocytes may enhance lipogenesis and so this may be another pathway by which low vitamin D status contributes to increases in IMAT and also insulin resistance.
In the cross-sectional cohort, thigh IMAT was positively associated with triglycerides and negatively associated with HDL cholesterol in women with PCOS. Erector spinae IMAT also correlates positively with triglycerides in middle-aged sedentary adults, potentially due to increased inflammation and lipolysis rates in muscle [26
]. The lower HDL cholesterol in those with higher thigh IMAT may also reflect that IMAT is lower in more physically active individuals, even amongst those with type 2 diabetes [27
]. Erector spinae IMAT has been shown to be associated with higher homeostasis model assessment of insulin resistance (HOMA-IR) and insulin levels in older adults [28
], and thigh IMAT is also associated with poorer insulin sensitivity in obese individuals with and without type 2 diabetes [29
]. The present study demonstrated a significant association between IMAT and insulin resistance in women with PCOS. Of note, this association was independent of visceral fat area and was not present in women without PCOS. IMAT may potentially have a greater impact on insulin resistance than visceral fat due to its proximity to skeletal muscle, which is the largest insulin sensitive tissue in the body accounting for 85% of glucose utilisation [30
]. Indeed, intrinsic abnormalities in glucose transport and insulin-signalling have been observed in cultured skeletal muscle from women with PCOS [31
]. It is also possible that the association between IMAT and insulin resistance in women with PCOS, but not those without PCOS, is explained by the higher levels of systemic inflammation associated with PCOS [32
], which may contribute to adipose tissue inflammation and dysregulation [33
In our exercise trial which included overweight women with and without PCOS, we observed that 94% of women had low vitamin D levels (<50 nmol/L). Women with 25OHD levels equal to or above the median (30 nmol/L) for this cohort had significant decreases in fat mass and improvements in insulin sensitivity over 12 weeks that were not observed in women with 25OHD below the median. Our previous research suggests that older adults with higher 25OHD levels combined with higher physical activity have the smallest age-related gains in body fat over five years [8
], and this may be related to evidence suggesting that fat oxidation is enhanced during exercise in women with higher vitamin D levels [7
]. Further research is needed in this area given the potential clinical implications of enhanced responsiveness to exercise regimens.
Insulin sensitivity can be improved through vitamin D supplementation in individuals with pre-diabetes [34
]. Interventions investigating whether vitamin D supplementation can also enhance exercise-induced improvements in metabolic health are necessary in overweight women with PCOS, who demonstrate increased risk for vitamin D deficiency, and who are inherently insulin resistant [2
]. A randomised controlled trial of 104 overweight and obese women with PCOS has demonstrated that calcium and vitamin D co-supplementation over eight weeks reduced insulin levels, HOMA-IR and triglycerides and LDL cholesterol levels [35
]. However, given that the present study demonstrated no association between pre-training IMAT levels and changes in insulin resistance, or between pre-training vitamin D levels and changes in IMAT, it does not appear that potential benefits of higher vitamin D in PCOS are attributable to lower muscle fat infiltration. Nevertheless, addressing low vitamin D and physical inactivity in women with PCOS may provide significant improvements in quality of life through reductions in adiposity [36
]. This may be relevant to other features of PCOS, given that higher vitamin D levels are associated with greater reproductive success in women with PCOS [37
], and that there is evidence that vitamin D reduces cytokine secretion through its effects on the NF-κB pathway in adipocytes [38
], which may reduce systemic inflammation observed in PCOS.
The findings of this study are subject to limitations. Muscle density is an indirect assessment of muscle lipid content and future studies assessing these relationships would be strengthened by the use of muscle biopsy with assessment of local inflammatory and insulin signaling markers. The strengths of the study include the use of gold-standard assessments for body composition and insulin resistance, and the 90% adherence amongst those who completed the exercise intervention with no significant difference between women with and without PCOS [6
In conclusion, women with PCOS and low 25OHD levels have increased insulin resistance. This may be partly explained by low 25OHD and the relationship with higher thigh inter- and intra-muscular adipose tissue. We report that overweight women with higher pre-training 25OHD levels demonstrate improvements in body composition and insulin resistance following an exercise intervention. Further research is required to determine potential benefits of vitamin D supplementation for reducing insulin resistance and for potentially enhancing metabolic responses to exercise in women with PCOS.