Within the United States, obesity has reached epidemic proportions. According to the National Health and Nutrition Examination Survey (NHANES), one-third of the US population is affected by obesity. Furthermore, of those in the United States over the age of 60 years old, 70% are overweight or obese. Despite the known genetic contribution to obesity, this epidemic is primarily attributed to modifiable lifestyle factors, of which nutrition plays a prominent role.
A potential link between vitamin D and obesity was first observed in 1971 when Rosenstreich et al. demonstrated an association between increased body fat and low serum vitamin D. This study laid the groundwork for many more studies that investigated the link between vitamin D and weight gain. Does obesity affect vitamin D status? Does vitamin D influence weight gain or weight loss? Keep reading to find out what the research says.
Vitamin D Basics
There are two primary forms of vitamin D that you have likely heard about, vitamin D2 and D3. Vitamin D2 (ergocalciferol) is attained from some plant sources, such as mushrooms, and is the most common form found in fortified food products. Vitamin D3 (cholecalciferol) is produced in the skin after sun exposure and can be acquired by consuming animal products such as oily fish, egg yolk, and butter.
Vitamin D’s formation, activation, and catabolism is a complex process that involves two hydroxylation steps. First, vitamin D2 and D3 are converted to 25-hydroxyvitamin D (25(OH)D). Second, 25(OH)D is converted into the active form of vitamin D, 1,25-dihydroxy-vitamin D, which executes an action on vitamin D receptors (VDRs) and human physiology.
Vitamin D is stored in adipose tissue, and its release from fatty storage may serve as an endogenous source of vitamin D when cutaneous production is low. In the fatty tissue, vitamin D is stored in its native form (vitamin D2 and vitamin D3) and 25(OH)D. 25(OH)D is the dominant circulating form of vitamin D and is most often measured as an indicator of vitamin D status. There is evidence that vitamin D metabolism, storage, and action both influence and are influenced by adiposity.
Does Obesity Affect Vitamin D Status?
Studies across the board seem to concur that low levels of circulating vitamin D are associated with increased fat mass and body mass index (BMI). A 2010 cross-sectional and longitudinal intervention study found a significant negative correlation between BMI and serum 25(OH)D at baseline as well as at the end of the yearlong study.
A systematic review and meta-analysis completed in 2015 confirmed these findings, concluding that vitamin D deficiency was associated with obesity irrespective of age and latitude. Furthermore, studies have found that weight loss is associated with increased serum 25(OH)D concentrations in overweight or obese women.
Taken together, these studies suggest that weight is an important factor for vitamin D levels. Yet, the underlying reasons for this are not fully agreed upon, and several hypotheses have been made.
There are Several Reasons Obese People Have Low Vitamin D Levels
- Lower dietary intake: Low vitamin D intake is typical for people all over the globe, not only individuals who are overweight or obese. However, one study did conclude that BMI and vitamin D intake were negatively associated in both sexes. Moreover, they found that the lowest quartile of vitamin D intake was an independent predictor of obesity in both genders.
- Reduced cutaneous synthesis: Although results on this matter are conflicting, obese individuals may expose less skin to the sun less often than non-obese individuals, resulting in reduced vitamin D synthesis.
- Reduced intestinal absorption: Low vitamin D is well-documented in those who have had bariatric or gastric bypass procedures. However, there is no evidence to date suggesting that obesity itself results in reduced absorption of dietary vitamin D.
- Altered metabolism: Research has found that two enzymes responsible for vitamin D metabolism in subcutaneous adipose tissue are impaired in obesity. Based on their findings, the authors proposed that fatty tissue passively stores vitamin D and dynamically changes its capacity for activation and deactivation.
- Negative feedback loop: Intact parathyroid hormone, which stimulates the enzyme that converts 25(OH)D to 1,25-dihydroxy-vitamin D, is elevated in obesity. This could, to some extent, contribute to lower 25(OH)D concentrations in obese individuals. Furthermore, negative feedback from elevated 1,25-dihydroxy-vitamin D and parathyroid hormone levels could affect the hepatic synthesis of 25(OH)D.
- Sequestration of vitamin D in the adipose tissue: Studies have found an attenuated increase in plasma vitamin D levels in obese subjects in response to a single dose of vitamin D. One hypothesis is that vitamin D is accumulated and sequestered within the increased amounts of fatty tissue in obese individuals and not readily available for the circulation. However, this has been challenged by another hypothesis. Drincic et al. showed that differences in serum vitamin D levels between lean and obese subjects could be explained by simple volumetric dilution.
- Volumetric dilution: A study of 686 community-dwelling individuals showed that a volumetric dilutional model accounted for essentially all the variability in serum vitamin D levels attributable to obesity. Once serum vitamin D levels are adjusted for body size, there is no longer a difference between obese and non-obese individuals. In other words, even if obese and lean subjects have similar amounts of vitamin D, in overweight people, vitamin D is distributed into a larger volume, making serum concentrations lower. A rather sizable systemic review of the literature that included human trials reporting changes in 25(OH)D, weight, or body composition concluded that the outcomes support a volumetric dilution of vitamin D.
The association between reduced vitamin D concentrations and obesity is well established and may be accounted for by a volumetric dilutional model. Correction of low vitamin D concentrations in obese individuals will likely require higher doses than those often advocated to the general population. Here, it seems, one size does not fit all.
Can Vitamin D Influence Weight Change?
Nuclear and membrane VDRs are expressed in adipocytes, suggesting that adipose tissue responds to vitamin D. However, research in this area is still conflicting and unclear. Several studies, including Mason et al. and Wamberg et al., have shown no effect of vitamin D supplementation on body weight and composition. Several reviews (1, 2, 3, 4) have supported these findings and concluded that there was no effect of vitamin D supplementation on weight loss. These findings suggest that even though low vitamin D concentration is associated with obesity, it is not a cause of obesity.
However, not every study has come to these same conclusions. One observational study in older women found that higher vitamin D levels were associated with lower weight gains over time, suggesting that low vitamin D status may predispose one to fat accumulation. Additionally, Salehpour et al. found that increasing vitamin D concentrations in healthy overweight and obese women did lead to body fat mass reductions. Although, body weight and waist circumference did not change significantly.
While results are conflicting, most research suggests that excess adiposity leads to lower 25(OH)D levels. Still, lower 25(OH)D levels are not likely to influence body weight or increase adiposity. More research is required in this area to determine with certainty the influence vitamin D levels have on weight gain and loss.
How Might Vitamin D Influence Weight Gain?
Vitamin D is a vital nutrient on its own, but it also intimately involved with several other nutrients and health outcomes that may influence adiposity. Vitamin D is critical for calcium metabolism, and it isn’t easy to separate the effects of vitamin D from those of calcium.
It appears that calcium may play a role in adiposity as well. A randomized, double-blind study of 171 overweight and obese adults found that adding calcium and vitamin D daily was associated with a significant reduction in visceral adiposity compared with the placebo group. Furthermore, experimental data suggest that an increased parathyroid hormone level due to vitamin D deficiency may promote lipogenesis by greater calcium inflow in adipocytes.
Moreover, it’s important to pay attention to vitamin D’s role in other aspects of health that may indirectly influence body weight. Excessive fatigue, tiredness, depression, and anxiety have all been associated with vitamin D deficiency.
Vitamin D deficiency has also been linked to type 2 diabetes and cardiovascular disease, two common comorbidities associated with chronic obesity. It’s possible that if adequate vitamin D levels could positively influence fatigue, depression, and other chronic illnesses it could indirectly affect obesity by supporting a more healthy, active lifestyle.
To Wrap it All up…
Obesity and vitamin D deficiency are both modifiable risk factors for several chronic diseases. Although there is a clear association between higher BMI and lower vitamin D status, there is no clear evidence for a causal role of vitamin D in the development of obesity. The evidence suggests that although increases in vitamin D status are not likely to help with weight regulation directly, vitamin D deficiency could contribute to other adverse health effects associated with obesity, and therefore, should be corrected.
People with larger amounts of body fat may need greater amounts of vitamin D to maintain optimal vitamin D status. Experts suggest that those who are overweight may need 1.5 times more, and those who are obese may need 2-3 x more vitamin D to reach optimal serum levels.
Besides those with excess adiposity, others at risk for vitamin D deficiency include older adults, breastfed infants, those of non-white race due to darker skin pigment, and those with limited sun exposure. The best way to know how much vitamin D is required to meet your needs is to test your current levels. Once you know where you stand, you can change your diet and lifestyle to support optimal amounts of this critical nutrient. Always speak to a trusted health care professional to determine your best course of action.