What Doctors Say About Vitamin D as the Days Get Shorter

As daylight shrinks, vitamin D deserves attention, not just because of winter sunshine, but because of the important jobs this nutrient performs inside the body.

It Helps Build and Maintain Strong Bones

Vitamin D helps the intestines absorb calcium and phosphorus, minerals the body uses to build and maintain bones and teeth. Without enough vitamin D, bones can become weak or soft. It also supports normal muscle function, which helps explain why adequate levels matter beyond the skeleton.

Your Body Makes It From Sunlight

When ultraviolet B rays reach the skin, they trigger vitamin D production. Shorter days and weaker sunlight can reduce that production in northern regions. Older adults generally make less vitamin D in their skin than younger people, while darker skin pigmentation can also reduce production from a given amount of sunlight.

Food Sources Are Surprisingly Limited

Fatty fish such as salmon, fortified milk, some plant-based drinks and egg yolks provide vitamin D. A typical 3-ounce serving of cooked salmon can supply around 400–600 IU, depending on the variety. Recommended daily intake is 600 IU for most adults through age 70 and 800 IU after 70.

It Does More Than Support Bones

Vitamin D also helps regulate calcium levels and supports normal immune-system function. However, researchers have not established that taking extra vitamin D reliably prevents a wide range of illnesses in otherwise healthy adults.

Testing and Supplements Aren’t One-Size-Fits-All

A blood test measuring 25-hydroxyvitamin D can assess vitamin D status, but experts generally advise against routine testing for every healthy adult. Supplements may be appropriate for some people, yet excessive amounts can cause dangerously high calcium levels and kidney problems. A healthcare professional can help determine whether testing or supplementation makes sense.

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