Vitamin D is crucial to maintaining strong bones and healthy muscles, but it’s also one of the vitamins we most commonly find our bodies lacking.

Up to 35 percent of American adults have low vitamin D. The condition turns up more frequently in women after 50 and in all adults after 65. People with dark skin and those who spend most of their time inside are also at risk of vitamin D deficiency, which can lead to deep fatigue, weak bones and muscles.

“Vitamin D plays a key role in helping the kidneys, skin and digestive tract keep calcium in balance,” says Dr. Matthew McCoskey, a provider at Tidelands Health Family Medicine at Myrtle Beach. “Most of the symptoms really depend on how low your levels are and what stage of life you’re in.”

Vitamin D helps the body absorb calcium, a mineral necessary for bone strength, muscle contraction, nerve function and more. Without adequate vitamin D, even a calcium-rich diet can fall short.

Because vitamin D is fat‑soluble, the body stores it in tissue. But this also means deficiencies can build slowly and quietly.

“Most people expect a dramatic symptom if a vitamin is low, but vitamin D tends to chip away at your well‑being over time,” Dr. McCoskey says. “By the time someone comes to see me, they’re often dealing with several symptoms.”

In children, vitamin D deficiency can show up as rickets, a rare but serious condition that leads to soft, weak bones and, in some cases, bowed legs.

“Thankfully, rickets is uncommon today because many foods are fortified with vitamin D,” Dr. McCoskey says. “But it’s still something we watch closely in kids who are picky eaters or who have limited sun exposure.”

Adults usually experience more subtle symptoms of low vitamin D, including:

• Persistent fatigue
• Bone or joint pain
• Muscle weakness
• Mood changes, including depression
• Thinning hair
• A weakened immune system

Low vitamin D can affect anyone, but some health conditions make deficiency more likely:

• Kidney or liver disease, which alters vitamin D processing
• Obesity, which can trap vitamin D in fat cells
• Gastric bypass surgery, which reduces absorption
• Intestinal disorders such as Crohn’s disease, Celiac disease and cystic fibrosis

“Any condition that affects digestion or fat absorption puts someone at higher risk,” Dr. McCoskey says. “Vitamin D is absorbed in the gut, so anything that interferes with that can have a downstream effect. And these symptoms overlap with so many other conditions, which is why it often goes undiagnosed unless we’re looking for it.”

The simplest treatment for low vitamin D is also the most enjoyable: spending time in the sun.

“Sunlight is intimately involved in processing vitamin D,” Dr. McCoskey says. “I’m a huge proponent of getting people outdoors, not just for vitamin D, but for mental and physical health overall.”

Just 10–15 minutes of sun exposure on most days is enough for many people to activate the body’s vitamin D production. Factors such as sunscreen, clothing coverage, skin pigmentation and season can affect this, so individual needs vary.

If sunshine isn’t a reliable option, foods rich in vitamin D can help fill the gap. Natural or fortified sources include:

• Oily fish such as salmon, tuna and mackerel
• Eggs
• Milk and dairy products
• Fortified cereal
• Fortified orange juice

For many adults, though, diet alone still may not provide enough vitamin D.

Over‑the‑counter vitamin D3 supplements can safely boost levels for most individuals. However, Dr. McCoskey recommends talking with a provider before starting a new supplement.

“More isn’t always better,” he says. “Too much vitamin D can lead to kidney stones or calcium deposits in the blood vessels. A simple blood test can help determine exactly what dose you need.”