Vitamin D is essential for keeping our bones and muscles strong. It helps the body absorb key minerals like calcium and magnesium. While we get some vitamin D from sunlight, many people also take supplements to meet their daily needs. Some studies have even suggested that vitamin D might protect against serious illnesses like cancer and heart disease.
Now, a new large study from scientists at Brigham and Women’s Hospital brings more clarity—and raises some important questions about how body weight may affect vitamin D’s benefits.
The researchers wanted to know whether taking vitamin D or marine omega-3 supplements could lower the risk of cancer, heart disease, or stroke. They used data from a major nationwide clinical trial that included over 16,000 adults. They also took a closer look at nearly 2,750 people who had their blood tested again two years after starting the supplements.
The team measured levels of vitamin D in the body and looked at signs that show how well the body is using the vitamin. They tracked both total vitamin D and free vitamin D, which is the form that’s available for the body to use.
The results were surprising. Vitamin D supplements did increase vitamin D levels in all participants. But here’s the catch: the increase was smaller in people with a higher Body Mass Index (BMI).
BMI is a number that estimates body fat based on height and weight. A BMI under 25 is considered normal. The study found that people with a BMI under 25 saw clear benefits from vitamin D supplements. These included a 30–40% reduction in cancer deaths. But people with higher BMIs didn’t see the same benefits.
This suggests that body weight might affect how well vitamin D supplements work. The researchers believe that fat tissue may trap more of the vitamin, making less of it available to the body’s organs and systems.
This is important because many people take vitamin D supplements expecting a broad range of health benefits. But if the supplement isn’t working as well in people with higher body weight, they might not be getting the full effect.
This also helps explain why earlier studies on vitamin D had mixed results—especially since they didn’t always consider participants’ body sizes.
The researchers say we may need to rethink the “one-size-fits-all” approach to vitamin D supplementation. Instead of giving everyone the same dose, it might be more effective to adjust vitamin D intake based on body weight. This way, more people—regardless of their BMI—could get the full benefits.
Lead author Deirdre K. Tobias emphasized that while more research is still needed, this study offers a clear step toward personalized nutrition.
This study strengthens the idea that the way our bodies process vitamins isn’t always the same for everyone. It highlights how important it is to tailor supplements to individual needs—just like we already do with medication dosages.
For now, people with higher BMIs who are taking vitamin D might consider speaking to their doctor about whether their dosage is high enough to be effective. However, it’s not recommended to increase doses on your own without medical advice, as too much vitamin D can be harmful.
This research also opens the door for more studies on how body size affects other vitamins and nutrients—and how we can better personalize nutrition for different people.
If you’re curious about how supplements and nutrition affect your health, other studies have explored:
- The best time of day to take vitamins for heart health
- How zinc may reduce the risk of getting COVID-19
- How certain plant nutrients can help lower blood pressure
- And how antioxidants may protect against dementia
The full findings from this vitamin D study were published in JAMA Network Open. It’s an exciting step toward smarter, more tailored nutrition for all.
If you care about weight loss, please read studies that hop extract could reduce belly fat in overweight people, and early time-restricted eating could help lose weight.
For more information about weight loss, please see recent studies that Mediterranean diet can reduce belly fat much better, and Keto diet could help control body weight and blood sugar in diabetes.
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