Understanding Vitamin D and Why We Test for It

Vitamin D is essential for our well-being. For years, vitamin D has been credited with playing a significant role in bone health, immune system support, muscle function, cognitive function and nervous system health. New research suggests that vitamin D might help slow aging and the progression of multiple sclerosis precursor, while another study found that tailored doses of vitamin D can lower the risk of heart attack by 50% in people with heart disease.  

Unfortunately, getting enough vitamin D is difficult. This is why about 40% of Americans have a low level. It’s also why your doctor probably tests your levels as part of the MDVIP Wellness Program.

Why Vitamin D Has a Strong Influence On Your Health  

Vitamin D is involved in many aspects of our health because it’s technically a hormone, not a vitamin. What are the differences? Hormones are chemical messengers produced by glands on demand that target specific cells or organs to regulate processes such as growth, mood and metabolism. Vitamins are nutrients from supplements, food or fortified food and support a wide range of functions, including energy production and immune health.  

There are two forms of vitamin D: D2 and D3. They’re the same nutrient but with slight structural differences that affect their availability.  

D3 is referred to as the “sunshine vitamin” because its synthesis is triggered when ultraviolet (UV) rays hit the skin. Most of your vitamin D in your body is synthesized from exposure to sunlight. Experts recommend that you get between 10 and 30 minutes of midday sunlight several times per week to maintain an adequate vitamin D level. Unfortunately, this recommendation doesn’t work for everyone. Factors such as the season, latitude and your skin's pigmentation can affect vitamin D production. Of course, it can run counter to skin cancer prevention recommendations, and wearing sunscreen can lower vitamin D production.  

D3 also behaves like a fat-soluble vitamin in that it’s available from supplements, animal-based foods such as egg yolks, and fatty fish (salmon, mackerel, trout, herring and sardines) and fortified foods (e.g., milk, cereal and orange juice). Keep in mind, only a small percentage of vitamin D is absorbed through food. You can get D2 from supplements and vegetable-based sources, particularly mushrooms. And there may be some D2-fortified products on the market; however, most of these foods are fortified with D3.    

Your body has a built-in safety system to help regulate vitamin D. When your calcium levels drop, a hormone tells your kidneys to make more active vitamin D. This natural process prevents an overdose from sun exposure alone. However, taking too many vitamin D supplements can be dangerous because vitamin D is stored in body fat, and can rise to toxic levels.  

How Vitamin D Works in the Body  

After vitamin D enters your body, it converts into two forms: 25-hydroxyvitamin D in the liver and calcitriol in the kidneys. Calcitriol is the active form that performs “vitamin D responsibilities.” It travels through the blood, binds to vitamin D receptors (VDR) in cell nuclei of tissues throughout the body and influences gene expression — turning some on and others off.  

As a result, vitamin D is involved with many functions, including:  

Bone health. Vitamin D helps your body absorb calcium and phosphorus from your intestines and helps maintain these minerals at bone growth and mineralization levels. Vitamin D signals the kidneys to retain calcium, as opposed to excreting it into urine, and it works with zinc and magnesium to promote bone formation. Without enough vitamin D, your bones become weak and soft, raising the risk of bone diseases like osteomalacia or osteoporosis.  

Immune regulation. Vitamin D helps control the immune system’s response to germs and supports defense. It enhances the body's immediate defenses by producing natural germ-killing substances, and it helps fine-tune the immune response to fight infections without causing harmful inflammation. Without enough, you're more vulnerable to infections.  

Muscle function. Vitamin D is needed for muscle development, strength and performance. Supplementation can help lower fall risks among older adults.  

Cell growth and differentiation. Vitamin D influences how cells grow and decide what each cell becomes. This is why vitamin D is an important topic in cancer prevention research.  

Brain health. Vitamin D has anti-inflammatory, antioxidant and neuroprotective properties, which may help explain why low vitamin D levels are linked with a risk for dementia and stroke, according to a study published in the American Journal of Clinical Nutrition.  

Maintaining Vitamin D Levels  

You should adopt a multi-pronged approach to manage your vitamin D levels. For instance, you should eat foods rich in vitamin D and spend time in the sun if possible. It’s also important that you work with your doctor, especially to determine if you need to supplement.    

MDVIP-affiliated physicians measure vitamin D through a blood test that checks vitamin D storage levels. There isn’t a universally accepted optimal level, but doctors generally like to see your vitamin D level above 20 to 30 ng/mL.  

If your level is low, your doctor may recommend taking a high dose for a brief period. They also may order tests or refer you to a specialist to ensure you don’t have a malabsorption condition such as inflammatory bowel disease, short bowel syndrome or chronic pancreatic insufficiency.  

If you don’t have a physician, consider joining an MDVIP-affiliated practice. MDVIP-affiliated physicians have the time and resources to develop a personalized wellness program that can focus on maintaining healthy vitamin D levels. Find a physician near you and begin your partnership in health »