Vitamin D: Deficiency Signs and the Risks of Overdose

Recent research has identified a previously unknown interaction between different forms of vitamin D supplements, suggesting that some common additions to a health regimen may inadvertently undermine the body’s immune defenses.

A study conducted by the University of Surrey, the John Innes Centre and the Quadram Institute Bioscience found that taking vitamin D2 may actually lower the body’s levels of vitamin D3. This finding, published in Nutrition Reviews, indicates that vitamin D3 is the more effective form for supporting overall health and the immune system.

The Difference Between Vitamin D2 and D3

The human body naturally produces vitamin D3 when ultraviolet rays from sunlight convert a protein called 7-dehydrocholesterol in the skin. While both D2 and D3 are used in supplements to help individuals meet daily health recommendations—such as the UK government’s guidance of 10 micrograms (µg) per day—they are not processed by the body in the same way.

According to the research from the University of Surrey, vitamin D3 is more efficient and specifically enhances the immune system’s first line of defense against infections. The study’s discovery that vitamin D2 can deplete these D3 levels raises new questions about which type of supplement should be prioritized to avoid weakening immunity.

Identifying Vitamin D Deficiency

Despite the risks of certain supplements, deficiency remains a widespread public health issue. In the United States, reports indicate that 94.3% of the population fails to meet the daily intake requirement for vitamin D.

Medical experts, including endocrinologist and vitamin D expert Dr. Michael Holick, note that the most telltale sign of this deficiency is constant exhaustion. This fatigue often persists even after a person has received adequate sleep.

Other subtle signs and risk factors for deficiency include:

• Increased risk for those with darker skin tones or those who work primarily indoors.
• Higher susceptibility among vegans, vegetarians, and individuals with obesity.
• Potential links to chronic headaches, depression, and muscle aches.
• Increased risk for older adults and those with inflammatory bowel disease.

To combat these deficiencies, Dr. Holick has suggested a daily intake of 1,500 to 2,000 IU (or 37.5 to 50 micrograms) of vitamin D.

The Risks of Over-Supplementation

While deficiency is common, health professionals warn that taking excessive amounts of vitamin D can lead to toxicity, known as hypervitaminosis D. This condition can cause a dangerous buildup of calcium in the blood, a state called hypercalcemia.

Excessive intake can lead to a variety of severe health complications, including:

• The development of kidney stones (nephrolithiasis).
• Nausea, constipation, and fatigue.
• Interference with bone metabolism.
• In severe cases, potential cardiovascular issues such as blocked arteries.

Toxicity is particularly dangerous for older adults and young children. Most individuals recover after the supplements are discontinued and they receive medical intervention, such as IV fluids.

Some individuals may also be genetically predisposed to toxicity. Research indicates that defects in the CYP24A1 gene can make certain people more susceptible to hypercalcemia even at lower doses of vitamin D3.