Vitamin D: Too Much of a Good Thing?

There is no question that vitamin D is essential—it helps to maintain healthy bones and can protect against a variety of diseases. This vitamin, that we naturally produce from spending time in the sun, helps contribute to a healthy life.

However, researchers are now asking, is there such a thing as too much vitamin D? Daily recommendations for vitamin D supplementation range from 400 to 2,000 international units (IU). However, some adults report daily vitamin D supplementation above 4,000 IU. If vitamin D is healthy and necessary, shouldn’t more be better? Not necessarily, say authors of a recent Harvard study.

The Harvard study, published in the Journal of the American Medical Association, found that increasing the amount of vitamin D past a certain point may not be beneficial. The study even posed a new hypothesis for future study, that high-dose vitamin D may have a negative effect on bone health.

What Is Vitamin D?

Vitamin D is different from other vitamins. It is not readily available in most foods and it is actually technically a hormone. When your skin is exposed to sunlight, your body produces vitamin D through a process involving both your liver and kidneys. If you wear sunscreen, keep in mind that vitamin D is only produced when it is exposed to ultraviolet B (UVB) sun rays.

In addition to absorption from the sun, vitamin D may be available through supplements and in a limited amount of foods (including some fortified foods). Vitamin D is a fat-soluble vitamin and can be stored in your body for a long time.

There are two main types of vitamin D, vitamin D2 and vitamin D3. Fortified foods can contain either type of vitamin D. Vitamin D3 is found in a limited number of animal-sourced foods and is produced by your body when your skin is exposed to sunlight. Vitamin D2 comes from plant sources.

Why Do We Need Vitamin D?

Vitamin D has multiple roles in the body. Your body needs vitamin D to absorb calcium and maintain healthy bones and teeth. Vitamin D helps with regulating insulin levels and diabetes management. Vitamin D supports cardiovascular, immune, brain, and nervous system health. There is also research suggesting that vitamin D and calcium may help with the aging process.

How Do I Get Vitamin D?

If you are not getting enough vitamin D there are several options available, including: sunshine, food, and supplements.

• Sunshine – The number one source of vitamin D is the sun. Your body can produce large quantities of vitamin D when your skin is exposed to the sun’s UVB rays. However, many factors affect this process including age, skin color, and geographic location.
• Food – Vitamin D is available in a few natural food sources. Natural sources of vitamin D include beef liver, cod liver oil, and egg yolks. Fatty fishes with vitamin D include mackerel, salmon, sardines, swordfish, and tuna. Cheeses can also provide you with your daily dose of vitamin D. Mushrooms treated with UV light may also provide high doses of vitamin D. Fortification is the addition of nutrients to food. Food fortification can be used to correct identified nutrient deficiencies in the population. In the United States there are several foods fortified with vitamin D that you can eat to supplement your diet. These fortified foods include orange juice, soy milk, cereals, margarine, yogurt, and milk.
• Supplements – If your lifestyle does not allow for you to consume vitamin D through sunlight or foods, then you may also get vitamin D through supplements. Vitamin D supplements may also be necessary if you have certain conditions such as Crohn’s disease, celiac disease, liver and kidney diseases, or had gastric bypass surgery.

What if I Don’t Get Enough Vitamin D?

Too little vitamin D, or vitamin D deficiency, can have severe health consequences in both children and adults.

In children, severe vitamin D deficiency can result in rickets, a rare condition in which the bone tissue does not properly mineralize, leading to soft bones and skeletal deformities.

In adults, severe vitamin D deficiency can result in osteomalacia. Osteomalacia is a softening of the bones from a loss of bone mineral content. It includes both muscle weakness and bone pain.

Signs of vitamin D deficiency can include mood changes, bone loss, muscle cramps or weakness, bone and joint pain, depression, and fatigue. The older you get, the more likely you are to have vitamin D deficiency because as you age, it becomes harder for your body to naturally produce vitamin D.

The Study

The Harvard study, “Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength A Randomized Clinical Trial,” was conducted over a three-year period by Lauren A. Burt, Ph.D.; Emma O. Billington, M.D., F.R.C.P.C.; Marianne S. Rose, Ph.D.; Duncan A. Raymond, M.S.; David A. Hanley, M.D., F.R.C.P.C.; and Steven K. Boyd, Ph.D.

The study included 311 community-dwelling healthy adults without osteoporosis, aged 55 to 70 years. Its objective was to assess whether higher-dose vitamin D supplementation improves bone mineral density and bone strength. The authors hypothesized that a higher dose of vitamin D has a positive effect on measures of bone mineral density and strength.

During the double-blind, randomized clinical trial, the adults were treated with vitamin D during that period and calcium intake was monitored. Those adults not consuming the recommended dietary allowance of calcium received calcium supplements. The adults were treated with vitamin D in dosages of 400 IU per day, 4,000 IU per day, or 10,000 IU per day.

The study found that “at a dose of 4,000 IU per day or 10,000 IU per day, compared with 400 IU per day, resulted in statistically significant lower radial bone mineral density; tibial bone mineral density was significantly lower only with the daily dose of 10,000 IU.” Further, “there were no significant differences in bone strength at either the radius or tibia.”

The results of the study were the opposite of what the researchers hypothesized. The researchers had hypothesized that an increase in vitamin D supplementation would result in an increase in bone strength and density. However, the results of the study were a negative dose-response. Higher doses of vitamin D supplementation were associated with significantly greater loss of bone.

The researchers cautioned that this result could only be confirmed with additional research. Thus, “the appropriate interpretation of this study is that for maintenance of bone quality in healthy vitamin D-sufficient adults, these results do not support a skeletal benefit of vitamin D doses well above the recommended dietary allowance.”

Criticism

Responses to the Harvard study have not all been positive. At the beginning of 2020 doctors in Chongqing, China commented on the study. Their criticism was regarding the authors’ conclusion that “among healthy adults, supplementation with higher doses of vitamin D did not result in improved bone health.” The doctors said that this statement “could be misleading.”

Their reasoning looked to the patients’ medical histories and their relationship to vitamin D deficiency. The doctors said that the study population did not include healthy adults because the patients had medical histories that included “cancer, cardiovascular-related disease, type 2 diabetes, and rheumatoid arthritis.” These diseases may relate to vitamin D deficiency and therefore “such patients should have been excluded.”

How Much Vitamin D Is Safe?

It is clear that some vitamin D is necessary for your health; however more research is needed to determine whether higher doses may be harmful. In the meantime, how much vitamin D should be taken by an individual is a determination that should be made by each individual and his or her doctor.