How Body Weight Affects Vitamin D Levels Over Time

Introduction

Vitamin D is essential for bone health, immunity, and many bodily functions. Doctors often check vitamin D levels a few months after people begin supplements, assuming the levels have stabilized by then. But a recent study published in JBMR Plus by researchers from Tufts University found that this timeline may not apply to everyone—especially people living with obesity.

What the study looked at

The research was part of the Boston STOP IT study. It involved 167 adults over the age of 65 who took 700 IU of vitamin D3 and 500 mg of calcium every day for one year. The participants were divided into two groups:

Normal weight (BMI between 18.5–24.9)

Overweight and obese (BMI 25 or higher)

Vitamin D levels were measured at the start, after 6 months, and after 12 months.

 

Key findings

People with normal body weight reached a steady level of vitamin D by 6 months. However, those with overweight or obesity continued to see their vitamin D levels rise even after 6 months. This suggests they needed more time to reach a steady level, even with consistent supplementation.

Interestingly, the higher the body weight, the longer it took to stabilize vitamin D levels. This means that people with obesity may not benefit as quickly from supplements compared to those with lower body weight.

 

Why does this happen?

There are a few reasons why body fat may slow the response to vitamin D:

1. Storage in fat tissue: Vitamin D is fat-soluble, so it may get trapped in body fat instead of staying in the bloodstream.

2. Larger distribution volume: More body mass means the vitamin D spreads out more, diluting its effect.

3. Liver changes: Obesity may reduce how well the liver processes vitamin D, slowing the body’s ability to convert it to its active form.

 

Why this matters

Most doctors check vitamin D levels after 3 months of starting supplements. But for people with obesity, this check might happen too early and not reflect the full benefit of supplementation. This could lead to unnecessary changes in dose or concerns that the supplement isn’t working.

 

Conclusion

This study suggests that body weight plays a key role in how long it takes to reach steady vitamin D levels. For people with overweight or obesity, it might take longer than the usual 3 to 6 months. Understanding this can help healthcare providers make better decisions about when and how to monitor vitamin D levels.

This research highlights the importance of tailoring healthcare based on individual factors like body size. More studies are needed to refine how we monitor vitamin D in people with different body types.

Study link: https://academic.oup.com/jbmrplus/article/9/4/ziaf030/8005811?login=false