Vitamin D deficiencies are widespread among people who are overweight or have type-2 diabetes. However, researchers have not been able to explain the exact link. A study done by researchers from Baylor College of Medicine, Houston, Texas, suggests that it has something to do with vitamin D’s role in the brain, where the nutrient helps control both weight and blood sugar levels.
Summer sun is our primary source of vitamin D. However, our modern lifestyle with too many indoors activities, our fear of the sun and overuse of sunscreen, lack of sunlight during the winter period, and the ageing process leave many people deficient. Add to that the fact that dark-skinned people synthesize less vitamin D than do people with lighter skin. These factors alone can contribute to the growing obesity epidemic.
The brain cells have vitamin D receptors
Most cells in the body have vitamin D receptors (VDR). In fact, the vitamin may be viewed as a hormone, which is synthesized from cholesterol just like other steroid hormones. Science has identified vitamin D receptors in the brain’s hypothalamus, which functions like a command center for our hormone system and autonomic nervous system. It is already known that hypothalamus controls our appetite and metabolism, and according to Stephanie Sisley, the researcher who headed the above-mentioned study, hypothalamus controls our appetite and metabolism.
The study revealed improved glucose tolerance
In order to look closer at the relation, Stephanie Sisley and her team of researchers conducted a study of 26 obese male rats. They injected the active form of vitamin D (1,25-dihydroxyvitamin D3) directly into the hypothalamus in 12 of the rats. The 14 remaining rats, who were also obese, were injected with a matching placebo. None of the rats in the study were fed for four hours. That way, the scientists could measure the rats’ fasting blood sugar. One hour later, the rats had their glucose tolerance tested. They got a glucose injection followed by a new blood sugar measurement.
It turned out that the rats who had received vitamin injections in their brains had improved glucose tolerance and were better able to absorb glucose (blood sugar) in their cells. This was not the case with the rats in the placebo group.
Improved insulin sensitivity
In another study, obese rats that got vitamin D also showed signs of significantly improved glucose tolerance and insulin sensitivity. Insulin is a hormone that is produced by the pancreas. Its role it to channel glucose (blood sugar) into the cells. However, excessive intake of calories, carbohydrates in particular, may lead to insulin resistance, a condition characterized by impaired glucose uptake by cells. This prevents the cells from getting the energy they need. The blood sugar goes up, more insulin is produced (but the body does not respond properly to it), and the excess calories are eventually cleared from the blood and stored as fat. Insulin resistance increases the risk of untimely hunger and overweight, and the condition is part of metabolic syndrome, an early stage of type-2 diabetes. Getting enough vitamin D is therefore highly important for both blood sugar levels and weight management, because the nutrient increases insulin sensitivity that helps the cells take up glucose, making them more energy-efficient.
Less hunger and lower food intake
In a four-week study where researchers gave vitamin D to three obese rats and matching placebo to four similarly fat rats, they observed a substantial drop in food intake and body weight in the rats that got vitamin D compared with the rats on placebo. The rats in the vitamin D group ate nearly three times less food on average and lost almost 24% of their weight compared with the rats in the placebo group. Their weight remained unchanged.
As Stephanie Sisley sees it, vitamin D will never become a magic bullet for losing weight, but the vitamin may help improve the outcome of other strategies such as diet improvements and exercise. It is also worth mentioning that the trace element chromium improves insulin sensitivity.
Widespread vitamin D deficiency – not least among overweight people
At our latitude, we have difficulty with producing enough vitamin D during the winter, and experts believe that the majority of people fail to get enough vitamin D from the diet. Vitamin D deficiencies have become increasingly common in the past 20 years due to our modern lifestyle. As shown, overweight and diabetes are also associated with an increased risk of vitamin D deficiency, and it appears that the official guidelines for vitamin D intake are insufficient.
The body’s vitamin D status can be determined by a blood sample
Why overweight people and diabetics need more vitamin D
The official recommendations for vitamin D intake are indiscriminate and fail to take into account such things as differences in body weight. However, as all cells in the body need vitamin D, weighing more alone increases your need for the nutrient. Moreover, many overweight individuals and diabetics have difficulty with converting vitamin D into its active form, 1,25-dihydroxyvitamin D3.
Upper safe intake level for vitamin D (updated in 2012)
Experts disagree on the actual need for vitamin D, which hinges on age, skin type, BMI, and many other factors. The upper safe intake limit for vitamin D has been established by the European Food Safety Authority (EFSA) at 100 micrograms for children from 11 years and older and adults, including pregnant and lactating women.
Sisley SR et al. Hypothalamic Vitamin D Improves Glucose Homeostasis and Reduces Weight. Diabetes 2016
Iowa State University: New promise for diabetics with vitamin D-deficiency. ScienceDaily. 2016
Pernille Lund. Sådan får du styr på dit blodsukker og din vægt. Ny Videnskab 2013
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