What Vitamin D Deficiency Does to Your Body—and How to Fix It Fast

Vitamin D is a simple nutrient with a massive job. When you're low, it can disrupt nearly every system in your body. Osteopathic physician Ryan Greene, D.O told GQ it supports more than 200 functions in the body. Longevity expert Neil Paulvin, D.O., added that it’s essential for everything from hormone balance to immune health and inflammation control.

Here’s what low vitamin D actually does, and how to fix it.

Why Vitamin D Matters So Much for Men’s Health

Internal medicine physician Neha Sangwan, M.D., told GQ that vitamin D behaves more like a hormone than a typical vitamin — meaning it acts as a regulator, not just a booster. It influences neurotransmitters tied to motivation and mood, helps modulate inflammation, supports bone integrity, and even impacts cholesterol formation.

Signs You May Be Low in Vitamin D

The most reliable way to know is with the 25-hydroxy vitamin D test, says Sangwan. But symptoms are a red flag.

Low vitamin D has been tied to joint pain driven by inflammation, brain fog, reduced immune function, and depression. A 2020 study cited in GQ found that low D levels were associated with cognitive impairment and dementia risk. Greene also notes vitamin D stimulates melatonin, so deficiency may throw off sleep cycles.

Smart Ways to Fix Low Vitamin D Starting Today

Try these expert-backed strategies to boost your levels fast, and keep your body performing at its best year-round.

1. Get sunlight daily. Short, consistent exposure supports natural vitamin D production. Still wear sunscreen, evidence does not show it stops vitamin D synthesis.
2. Eat real sources. Dairy, bananas, nuts, beans, broccoli, Brussels sprouts, sardines, herring, salmon.
3. Supplement strategically. Look for Vitamin D3 paired with Vitamin K2 and choose products that are NSF Certified for Sport.
4. Get tested yearly. Optimal ranges vary, but measuring it removes guesswork.

Why Taking Too Much Vitamin D Can Backfire

But more doesn’t mean better. Harvard Medical School’s JoAnn E. Manson, M.D., told Harvard Health that supplement use has surged — even though only a small subset of people need high-dose therapy. A research letter in JAMA found a growing number of Americans now take over 4,000 IU/day, which exceeds the safe upper limit. And a 2010 JAMA study linked extremely high dosing in older women with more falls and fractures.

Megadosing can also lead to hypercalcemia, too much calcium circulating in the bloodstream, raising risks for kidney stones or calcium buildup in arteries.

Bottom line: Aim for 600–800 IU/day unless your doctor directs otherwise. Optimize your levels, don’t overshoot them.