Obesity is defined by the World Health Organization (WHO) as “an abnormal or excessive fat accumulation that may impair health.” It is the most significant nutrition-related health problem in both developed and developing countries. According to WHO data from 2016, 39% of adults are overweight and 13% are classified as obese.
Vitamin D is a fat-soluble vitamin that plays an essential role in bone metabolism and calcium balance by binding to vitamin D receptors. It is found in certain foods and supplements, but the primary source of vitamin D is synthesis in the skin through exposure to sunlight. In terms of diet, vitamin D is mainly found in fatty fish, liver, and egg yolks.
The best indicator of vitamin D nutritional status is serum 25(OH)D levels. A serum 25(OH)D level of ≥30 ng/ml is considered “sufficient,” 21–29 ng/ml is considered “insufficient,” and <20 ng/ml indicates “deficiency.” Vitamin D deficiency is extremely common worldwide.
Causes of Vitamin D Deficiency in Obese Individuals
- Less participation in outdoor activities due to physical inactivity and social withdrawal
- Inadequate exposure to sunlight, frequent use of sunscreen and hats
- Wearing more covered clothing compared to individuals with normal body weight
- Poor dietary habits, including low intake of fish, milk, yogurt, and cheese
- Consumption of unhealthy foods lacking vitamin D and rich in junk food
Relationship Between Obesity and Vitamin D
Body Mass Index (BMI), and particularly fat mass, are indicators of vitamin D status. As a result, vitamin D deficiency is very common among individuals with obesity. Studies show a two-way relationship between vitamin D deficiency and the development of obesity.
Since vitamin D is a fat-soluble vitamin, its storage increases proportionally with fat tissue in overweight individuals. This results in lower circulating levels, leading to vitamin D deficiency.
Moreover, vitamin D deficiency is associated with impaired insulin secretion, which is a major risk factor for diabetes. Vitamin D helps reduce insulin resistance in peripheral tissues, thereby lowering excessive insulin secretion in response to high blood sugar levels and improving insulin sensitivity.
Therefore, vitamin D deficiency and the resulting insulin resistance may contribute to difficulty in losing weight and the development of metabolic syndrome and type 2 diabetes.
