Study Finds Association Between Vitamin D Levels, Obesity in Infertile Women

Lower vitamin D levels may link to a greater prevalence of obesity in infertile women, which emphasizes the importance of vitamin D supplementation in obese infertile women, according to a study published by Gynecological Endocrinology. The investigators noted that obesity and infertility are among the most significant global public health epidemics, with incidence rates growing for both conditions.

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They added that there is limited research that sought to determine the association between serum vitamin D levels and obesity/abdominal obesity in female infertility populations and data are limited regarding a potential linear relationship between serum vitamin D levels and obesity/abdominal obesity.

“Obesity and abdominal obesity contribute to a variety of chronic diseases. Meanwhile, obesity and abdominal obesity have multifaceted negative effects on reproductive potential, ranging from menstrual irregularity, endometrial pathology, oocyte, embryo, to infertility,” the study authors wrote. “Infertility is defined as the inability to conceive after 12 months of unprotected sexual involvement. About 15% of the world’s population suffers from infertility. In the United States, approximately 12.7% of women of childbearing age seek fertility treatment each year. To reduce the impact of obesity on infertility, weight loss is the first-line treatment for infertile patients.”

The investigators screened data from the National Health and Nutrition Examination Survey (NHANES) 2013-2016. They identified 201 infertile women between 20 and 40 years of age for inclusion. The independent association of vitamin D with obesity and abdominal obesity was evaluated using weighted multivariate logistic regression models and cubic spline analyses.

The researchers defined infertility as individuals who responded affirmatively to the reproductive health question ‘Have you ever tried to conceive for more than a year without becoming pregnant?’ and any woman who answered ‘No’ was deemed fertile.

“Infertility is a widespread problem that has major implications for individuals, families, and society,” the study authors wrote. “Modifiable lifestyle factors, such as overconsumption of food and vitamin deficiency can affect pregnancy. It has been well-established that obesity and vitamin D deficiency are two major pandemics undermining global public health.”

Among infertile women in the NHANES 2013-2016 database, the investigators found that serum vitamin D levels had a significant and negative association with body mass index (ß= -0.96, 95% CI: -1.40, -0.51, p < 0.001) and waist circumference (ß= -0.40, 95% CI: -0.59, -0.22, p < 0.001), respectively. A multivariable adjustment showed that lower vitamin D levels were linked to a greater prevalence of obesity (OR: 8.290, 95% CI: 2.451-28.039, p for trend = 0.001) and abdominal obesity (OR: 4.820, 95%CI: 1.351-17.194, p for trend =0.037). Spline regression showed the linearity of the associations between vitamin D and obesity/abdominal obesity (p for nonlinearity > 0.05).

“Although the cause-and-effect relationship between vitamin D deficiency and obesity is controversial, evidence from meta-analyses has consistently shown a negative association between vitamin D and obesity,” the authors wrote. “Our study in an infertile population also demonstrated this point and further showed that the negative correlation between vitamin D levels and obesity/abdominal obesity was linear, namely the lower vitamin D levels, the greater the chance of obesity, which also suggests that when to treat such patients, doctors should pay special attention to emphasize the importance of weight loss as well as the vitamin D supplements.”

Study limitations included that NHANES is a cross-sectional study, and the link between vitamin D levels and obesity or abdominal obesity was evaluated in a population of infertile women without the involvement of a fertile control group. As such, the investigators noted that the data should be interpreted carefully. Also, the study design created a challenge in establishing a definitive causal relationship between vitamin D and the risk of obesity/abdominal obesity.

However, the investigators added that their evidence-based study highlights the importance of recognizing the inverse association between vitamin D and obesity/abdominal obesity in female infertility to use as a reference for developing strategies for infertility treatment guidelines.

“In conclusion, obesity and abdominal obesity are associated with low vitamin D status and are inversely linear,” the authors wrote. “The causal relationship between vitamin D and obesity/abdominal obesity is controversial. Future large-scale prospective observational or interventional studies are needed.”

Reference

Zhao J, Fu S, Chen Q. Association between the serum vitamin D level and prevalence of obesity/abdominal obesity in women with infertility: a cross-sectional study of the National Health and Nutrition Examination Survey data. Gynecol Endocrinol. 2023 Dec;39(1):2217251. doi: 10.1080/09513590.2023.2217251. PMID: 37267998.