This study aimed to investigate the association between the DII and risk of PCOS. Our findings indicated a significant relationship between DII score and higher risk of PCOS. So that, increasing the intake of diets with a higher DII score increased the risk of PCOS by 1.75 times after adjusting for potential confounders. These findings support the current recommendations to increase the dietary intake of anti-inflammatory nutrients and restrict the consumption of foods with proinflammatory properties.
To the best of our knowledge, this is the first study investigating the relationship between DII and risk of PCOS in an adult population. However, previous studies have shown a positive and significant association between DII and other chronic diseases that appear to have common mechanisms such as increased inflammation and oxidative stress, insulin resistance, and metabolic syndrome disorders with PCOS. A prospective Korean population-based study was conducted by Khan et al. to investigate the relationship between inflammatory diet intake and increased risk of metabolic syndrome and its components showed that after a complete adjustment of potential confounders, higher intake inflammatory diet increases the risk of metabolic syndrome by 50% and this relationship is stronger in women than men. Also in a case-control study by Vahid et al. on patients with diabetes, the results showed that individual with a higher tertile of the DII index than the lower tertile, significantly higher serum levels of fasting plasma glucose, hemoglobin A1c, low-density lipoprotein, triglyceride, and body fat as well as after adjustment of potential confounders, the risk of diabetes increased 19 times. In addition, in another prospective study in 2020, after 5 years of follow-up, a significant association was found between the DII index and the increased risk of cardiovascular disease among 162,733 participants. In another case-control study published by Huang et al. in China found that a higher DII score was associated with a higher risk of breast cancer. Also in line with our results, this increased risk of cancer was observed in women with higher levels of the sex hormones oestrogen and progesterone.
In addition, studies show an association between different dietary pattern (which appear to be associated with increased levels of inflammation in the body) with an increased risk of PCOS.[28–30] For example, western diets and high glycemic index diets increase the risk of PCOS by increasing the levels of high inflammatory factors such as TNF-α and IL-6.[29,31] In a study of 100 women with PCOS, an anti-inflammatory diet consisting of 25% protein, 25% fat and 50% carbohydrates, reduced saturated fat, and moderate fibre with a significant improvement in body composition, homoeostasis glucose, hypertension and dyslipidemia, hormones, and menstrual cycle were associated and potentially reduced inflammation in these patients. Furthermore, according to published literature, receiving healthy eating patterns such as dietary approaches to stop hypertension and mediterranean-style diet with high content of antioxidant and anti-inflammatory vitamins as well as some substances with potential anti-inflammatory properties, such as garlic and quercetin, can reduce the burden of PCOS disease.[33,34]
Our study also showed that the intake of vitamin D, fibre were significantly lower in PCOS patients than the control group, but the intake of refined grains and cholesterol were higher. These results are consistent with other studies. In one study, increasing vitamin D intake by increasing the soluble receptor for advanced glycation end products (AGEs) levels (acts as a decoy by binding circulating AGEs) prevented the pathophysiological factors involved in PCOS, such as increased levels of inflammatory factors and insulin resistance, and reduced the risk of developing this disease. Studies also show the beneficial effects of vitamin B12 intake on reducing the incidence of PCOS and the risk factors observed in this disease such as increased homocysteine levels, insulin resistance and obesity. In addition, the beneficial effects of high fiber intake and reduced intake of carbohydrates, especially refined grains, on the prevention and reduction of recurrence of this disease through various mechanisms have been shown in various studies.[37–39] However, contrary to our study, in a study of 400 patients with PCOS, milk intake was associated with an increased risk of PCOS through increased insulin-like growth factor I levels and insulin resistance. However, no significant relationship was observed between total dairy intake and increased risk of this disease. Therefore, the investigation of the total intake of dairy products between the two groups in our study cannot sufficiently justify the opposite effect in this regard.
Inflammatory factors, such as TNF and CRP, have been shown to reduce insulin sensitivity and, consequently, insulin resistance by disrupting the phosphorylation of insulin receptor substrate 1 serine by protein kinase C. Insulin resistance also causes retention and increased levels of sex hormones through various factors such as increased visceral adipose tissue and disruption of the synthesis or release of hormones involved in the reproductive process and can increase the risk of PCOS. In addition, inflammatory cytokines stimulate the synthesis and proliferation of steroids as well as the release of follicles associated with hyperandrogenism, which in itself predisposes to more complex PCOS disorders. On the other hand, hyperglycaemia caused by inflammation increases the glucose available to mononuclear cells to increase the oxidation of this molecule and the production of reactive oxygen species, which effects on impaired sex hormone synthesis and metabolic syndrome factors involved in PCOS are clearly confirmed by studies.
Our study has several strengths. The current article is the first case-control study assessing the association between DII and risk of PCOS. Other strengths of this study is using a valid and reliable FFQ and physical activity questionnaire. In addition, in this study, we selected patients with a new diagnosis to reduce bias and the possibility of changing the dominant diet of patients. Also due to similarity of ethnicity and culture of people in Middle East and North Africa region, it is possible that the present study might be able to generalisable for women outside Iran. Despite these strengths, some limitations should be noted. Although we used a valid and reliable FFQ to estimate nutritional intakes, the probability of a measurement error is unavoidable. Similar to other case–control studies, causal relationships between DII and PCOS were not investigated. The inability to measure the plasma levels of anti and proinflammatory factors such as CRP, IL-1, IL-6, TNF, IL-10, as well as some of nutrient antioxidant and show its correlation with the DII index could be another limitation of our study. In addition, due to unknown confounding factors or without measurement, despite adjusting for the confounding variables in our study, residual confounding cannot be ruled out.
Clin Endocrinol. 2022;96(5):698-706. © 2022 Blackwell Publishing