This common vitamin deficiency becomes far more dangerous when paired with belly fat

The findings, published in the journal Diabetes, Obesity and Metabolism, suggest that the two conditions may be especially harmful when they occur together. Although each was associated with a higher risk of death on its own, the combined effect was substantially greater.

"Abdominal obesity is a well-known risk factor because it's associated with inflammation and metabolic problems. Vitamin D, on the other hand, is a hormone that acts on various organs, and its deficiency impairs several bodily functions. When these two conditions occur together, one amplifies the effects of the other, further increasing the risk of death," explains Tiago Silva Alexandre, a professor in the Department of Gerontology at the Federal University of São Carlos (UFSCar) in Brazil, who coordinated the study. "For that reason, monitoring vitamin D levels and treating excess abdominal fat are essential measures to prevent premature death, especially after age 50," he adds.

Tracking Health Risks Over Six Years

The research was carried out in collaboration with University College London (UCL) in the United Kingdom and funded by FAPESP. Researchers followed 5,520 people age 50 and older for six years. All of the participants were enrolled in the English Longitudinal Study of Ageing (ELSA), one of the world's largest studies focused on aging.

The data showed that vitamin D deficiency (levels below 30 nmol/L) was associated with a greater risk of death on its own than abdominal obesity. Abdominal obesity was defined as a waist circumference greater than 102 centimeters for men and 88 centimeters for women.

People with abdominal obesity alone had a 47% higher risk of death, while vitamin D deficiency by itself was associated with an increase of up to 91%. When both conditions were present, however, the risk of death rose by more than twofold.

How Belly Fat May Reduce Available Vitamin D

Alexandre says abdominal obesity and vitamin D deficiency may reinforce one another in a harmful cycle. Abdominal fat can "sequester" vitamin D circulating in the body, storing it in adipocytes (fat cells) instead of leaving it available in the bloodstream.

"This means that although the body may have the vitamin stored in fat, it isn't freely available in the blood to perform vital functions in other organs and systems," he says.

Obesity may also interfere with the way the body processes vitamin D. People with obesity have lower expression of enzymes involved in vitamin D metabolism, further limiting how much of the vitamin is available for the body to use.

"Abdominal obesity reduces circulating vitamin D, and that deficiency impairs the immune system, exacerbating the chronic inflammation caused by excess fat and drastically increasing the risk of mortality," Alexandre explains to Agência FAPESP.

Aging, Inflammation, and Vitamin D

The combination may be particularly important later in life because aging itself is associated with inflammaging, a state of low-grade chronic inflammation.

"Under normal conditions, vitamin D acts as a regulator of the immune system, preventing inflammation from getting out of control. When vitamin D levels are low and there's excess abdominal fat, an unfavorable systemic environment develops that accelerates cardiovascular and metabolic diseases, as well as muscle loss," he stresses.

Cascade Effect

The new findings are part of a broader series of studies examining how vitamin D affects health during aging.

"In previous studies, we identified a cascade effect. Vitamin D deficiency leads to a loss of strength, which results in reduced walking speed, causing a loss of independence and greater dependence in daily activities," Alexandre explains.

The research group has also found in a separate study that vitamin D deficiency increases the risk of cognitive decline.

"Vitamin D is a hormone with various functions. It plays a role in regulating blood pressure, heart rate, the central nervous system, the immune system, and the endocrine system. Therefore, when its levels are low, several essential bodily functions are compromised," he says.