Vitamin D Explained: Key Questions Answered by a Preventive Cardiologist

By Dr. Joel Kahn, MD, FACC 

You hear a lot about vitamin D for immune health, bone strength, brain function, and heart health. But how important is it to maintain healthy blood and tissue levels?
You probably know that vitamin D is the “sunshine vitamin,” and we’ll get more as the calendar brings sunnier, longer days, and we are outside tanning.
But even with outside activities, it may be tough to pull in enough vitamin D. Over half of my clinic patients who are not taking vitamin supplements are low in D, and some are at rock bottom even during summer. That matters because vitamin D is made in the skin in response to the sun’s ultraviolet rays and plays a role in strong bones, healthy blood vessels, robust immune systems, and optimal brain and nervous system function.

Common Questions About Vitamin D
Here are eight questions I often hear:

How much vitamin D do I need?
The conventional answer is about 800 IU (international units) a day, the dose in most multivitamins. Blood testing by your doctor’s office can show whether that’s enough to achieve blood levels over 30 ng/ml, with 50-80 ng/ml being optimal. My routine in my preventive clinic is to start patients on 4,000 IU of vitamin D3 per day, usually combined with vitamin K2, to achieve optimal blood levels and health benefits. I advise this dose year-round, even for patients who spend plenty of time outdoors in summer, and I have not seen excessive blood levels on testing.

How much vitamin D do I get from the sun?
The amount of vitamin D your body produces depends on how directly sunshine hits your area. This is why you make more in summer and the most at midday. A good rule of thumb is to check your shadow. If it’s longer than you are tall, you’re not making much vitamin D.

What are the two kinds of vitamin D?
Vitamin D2 is found in mushrooms, while vitamin D3 is often found in animal sources, although there are vegan sources too. When you supplement, you want to choose a vitamin D3 supplement.

What foods contain vitamin D?
A cup of diced mushrooms daily provides about half the suggested daily dose of vitamin D and may cut certain cancer risks. Try to incorporate mushrooms into as many dishes as possible.

Very few other foods have much vitamin D. While vitamin D is in fatty fish, eggs, cheese, and beef liver, the cholesterol and saturated fat content is high and best avoided. I advise one cup of diced mushrooms daily, providing about 400 IU of vitamin D. This amount has been shown to cut the risk of certain cancers in half. However, I still recommend supplementing with vitamin D3 in those who incorporate mushrooms into their diet.

What happens if vitamin D levels are low?
Severe vitamin D deficiency (commonly called rickets) weakens bones. Lesser deficiencies are linked to diabetes, hypertension, dementia, autoimmune diseases, and multiple sclerosis. Vitamin D deficiency has also been linked to depression. In my clinic, I encounter many new patients with very low, and potentially dangerous, vitamin D levels. It is so easy to correct that deficiency with daily supplementation.

Are certain people prone to low levels of vitamin D?
Darker-skinned people and older individuals produce less vitamin D from sun exposure. People with bowel disease, like Crohn’s or celiac disease, often absorb fat poorly, and vitamin D is a fat-soluble vitamin. In obese individuals, vitamin D may be bound to fat stores, making it unavailable in the blood for testing or to support vital tissues. There are also measurable genetic influences. The best approach is to ensure blood work includes a vitamin D level.

Can I get too much vitamin D?
I have seen several cases of people taking capsules of over 10,000 IU daily for long periods, usually by mistake. This can cause kidney stones, changes in mood, abdominal pains, and blood pressure swings. However, at the 4,000 IU a day of vitamin D3 that I start patients on, I do not see excess levels or symptoms.

Should vitamin D be taken at a set time or with food?
Vitamin D is fat soluble, so it’s often absorbed more effectively when taken with a meal.