Why Vitamin D isn't just about bones, it decides whether calories should be fat or muscle

Indian Express

10-07-2025

Written by Dr Shovana Vaishnavi
Many of us usually associate Vitamin D with bone health. But that's just half the truth. In fact, its deficiency can affect how you store fat because this key vitamin decides whether the calories you eat turn into fat or muscle.
You may have been harbouring the greatest myth if you thought that calories are just numbers and that your body treats all of them the same. Truth is your body's response to calories depends heavily on your levels of Vitamin D. It works behind the scenes, regulating insulin sensitivity, inflammation and muscle metabolism.
So how does this happen? Sufficient Vitamin D levels stimulate protein synthesis and activate muscle repair processes, especially after physical activity. Let's look at how Vitamin D acts on proteins. For example, myostatin limits muscle growth. Vitamin D can help decrease myostatin production, allowing muscles to grow larger and potentially burn more calories.
The satiety hormone leptin signals to the brain about the body's fat reserves. When leptin levels are too high, your brain becomes resistant to its signals, leading to constant hunger and more fat storage. Leptin resistance keeps us in a vicious cycle of overeating and lethargy. Vitamin D improves leptin sensitivity, helping your brain regulate your hunger better and potentially reducing overeating. By influencing these hormones, Vitamin D may help your body prioritise muscle growth when excess calories are available, instead of storing them as fat.
This micronutrient also supports hormones like testosterone and growth hormones which are essential for building lean mass. Without it, even with proper training and nutrition, muscle gain is limited.
Low Vitamin D levels increase the body's tendency to store fat. It not only controls how many fat cells should be created but how many should be broken down. Deficiency can tilt the balance towards fat retention —even if you're not overeating.
For older adults, maintaining muscle mass is crucial for preventing falls and injuries. Vitamin D can support muscle strength and function in older adults, helping them stay active and independent.
To optimise body composition, get your levels tested—and corrected if needed. If you're taking a vitamin D supplement, you won't need more than 600 to 800 IU per day. Some people may need a higher dose, however, including those with a bone health disorder and those with a condition that interferes with the absorption of vitamin D or calcium. Consult a doctor then. Avoid taking more than 4,000 IU per day, which is considered the safe upper limit.
Sometimes, sunshine is more powerful than a protein shake.
(The author is principal consultant, internal medicine, Max Super Speciality Hospital, Noida)