Obesity and Vitamin D Deficiency- Correlation vs. Causation

Before we dive into this issue, I want to define a few terms so we are all using the same language. 

Correlation - a mutual relationship or connection between two or more things.

Causation - the action of causing something.

Obesity – (can be defined many ways, this way is using body mass index or BMI) - 

For adult men and women, a BMI between 18.5 and 24.9 is considered healthy. Overweight is defined as a BMI between 25.0 and 29.9; and a BMI of 30 or higher is considered obese; and a BMI of 40 or higher is considered severe or morbid. Further sub categorized as Class 1 = BMI of 30 - <35, Class 2 = BMI of 35 -<40, and Class 3 = BMI of 40 or higher.

Body Mass Index or BMI – calculated by a person’s weight in kilograms divided by the square of height in meters. Is a screening tool but does not diagnose body fatness or health. You can calculate your BMI using the CDC’s online tool. 

For many years it has been well documented that obese individuals have a high frequency of vitamin D deficiency. One meta-analysis (a study that compares results from lots of other studies) showed vitamin D deficiency was 35% more likely in obese individuals compared to non-obese individuals (Obes Rev., 2015).  

This has been thought to most likely be due to dilution of vitamin D that occurs into the fat (adipose) tissue. Think about that as non-obese and obese individuals may have the same amounts of vitamin D but the vitamin D is spread throughout a larger volume in an obese individual (therefore the amount of vitamin D found in the blood sample will be less than a non-obese individual). However, if that were the case then weight loss should cause the vitamin D levels to increase, even without supplementation; and studies cannot consistently show this result. 

There are also studies indicating that obese individuals do not spend as much time in sunlight compared to non-obese individuals.

Sunlight exposure is necessary for vitamin D synthesis, as discussed in my previous article. There is data indicating that vitamin D deficiency is involved in the development of obesity and not a result of it. Some data shows vitamin D deficiency causing an increased level of parathyroid hormone, which promotes the creation of fat because of increased calcium flow into fat cells (adipocytes). Other research shows that normal levels of vitamin D can decrease or stop the creation of fat and fat cells (Curr Obes Rep., 2021). 

Regardless of the exact physiological cause, studies have shown that a lower baseline vitamin D level is associated with more weight gain compared to individuals with a higher baseline vitamin D level (Am J Epidemiol, 2012).

Studies also examined if vitamin D supplementation improved low-grade inflammation. While this was shown to be true in experimental studies (think in a lab), the results were not replicated in clinical studies (think with real human bodies). However, this does not disprove the theory, it only indicates that more studies are needed.

The reason the research into the decrease of inflammation is important is due to the increased risk for metabolic syndrome in obese individuals. 

Metabolic Syndrome is also known as Syndrome X and dysmetabolic syndrome. Metabolic syndrome is present when multiple conditions occur together in one individual and increase their risk for heart disease, stroke, and type 2 diabetes. These conditions are obesity (particularly central obesity), increased blood pressure, abnormal cholesterol or triglyceride levels, and elevated blood sugar levels. Chronic inflammation may be a trigger in the beginning of metabolic syndrome.

Another fact to think about is in obese individuals there is a high incidence of non-alcoholic fatty liver disease (NAFLD). NAFLD has become the most common form of chronic liver disease and therefore the leading cause of cirrhosis. Studies have shown an association between vitamin D levels and NAFLD. Vitamin D has antifibrotic effects which can slow the progression of changes in the liver tissue leading to cirrhosis (Drug Des Devel Ther., 2015).

Unfortunately, as is often the case in medicine, there is no black and white answer regarding vitamin D deficiency and obesity. While there is a known correlation between the two, it is difficult to prove causation. “Correlation does not imply causation” is a phrase used in response to the “questionable-cause logical fallacy” in which someone may deduce a cause-and-effect relationship between two occurrences based only on the relationship observed between them.

Studies do not indicate an increased risk of disease development or worsening of symptoms of diseases present (e.g. hypertension) if an obese individual with low blood serum vitamin D levels receives supplementation. This is a discussion to have with your medical provider, who can help you determine the best course of action. 

An important thing to remember if you‘re obese and you and your healthcare provider decide to supplement your vitamin D, obese individuals require higher loading doses of vitamin D to achieve the same serum levels as non-obese individuals (Curr Opin Endocrinol Diabetes Obes., 2017).