According to a new study from the Kingdom of Saudi Arabia, blood vitamin D concentrations in adult Saudis are higher during the winter than the summer.
The observational study examined the influence of seasonal variations in serum vitamin D levels, as well as its concomitant interactions with several metabolic parameters (1). Vitamin D concentrations were measured in blood samples of 121 adult, overweight and obese Saudis aged 18–70 years every three months for one year. The study results showed that mean 25(OH)-vitamin D levels in winter were significantly higher than those in summer. Men and women had an equally high prevalence of vitamin D deficiency. While negligible differences were observed as to parameters, such as BMI, arterial blood pressure, and fasting serum glucose levels, HDL cholesterol levels improved significantly as vitamin D levels increased in both summer and winter.
The researchers noted that seasonal differences in serum 25(OH)-vitamin D levels in Saudi Arabia and the surrounding region were counterintuitive, with blood levels being higher during the winter than the summer months. They concluded that while vitamin D supplementation is currently recommended throughout the year, these results support increased supplementation rather than increased sun exposure during the summer months for Saudis living in the central region. This would counteract the higher risk of decreased levels of vitamin D during this period.
Several studies conducted in other geographical locations suggest higher prevalence of vitamin D deficiency during fall/winter than spring/summer. The reverse conditions found in the current study may be due to the fact that between the months of April to October temperatures in the Kingdom are the highest and people are advised to stay indoors, with outdoor activities kept to a minimum. This is in stark contrast to the winter season (October–March), characterized by increased exposure to sunlight.