Vitamin D

Vitamin D plays a crucial role in calcium absorption, which is essential for building and maintaining strong bones and teeth. Deficiency can result in calcium being reabsorbed from the bones, reducing their density and leading to osteomalacia (bone softening) and osteoporosis (reduced bone mass).

There are several high-quality studies looking at whether vitamin D supplementation improves bone health.

• Many large scale population studies (like NHANES-III that included 13,432 participants) have shown a correlation between low vitamin D levels and reduced bone density
• This Cochrane Review examined 53 trials involving 91,791 participants and found that vitamin D supplementation reduced the risk of fractures
• These findings are replicated in other systematic reviews, although the benefit is likely greater for older or institutionalised people and the risk of fracture in a younger, healthy population is generally low.
• People taking steroid medications are particularly at risk of osteoporosis, and this review shows clear evidence of benefit from taking vitamin D in improving bone density

Vitamin D acts upon most immune cells in the body, and there is some debate about whether it reduces the risk of developing autoimmune disorders.

• The VITAL trial studied 25,871 participants and found that vitamin D reduced autoimmune conditions by 22% (particularly rheumatoid arthritis and polymyalgia rheumatic) over a period of 5 when compared with placebo
• Similarly, this study found that US military recruits with low levels of vitamin D had a two-fold increased risk of developing Multiple Sclerosis later in life
• On the other hand, whilst there were some initially promising results suggesting benefit from vitamin D in preventing asthma exacerbations, this recent Cochrane review assessing 20 trials has concluded there was no benefit

There have also been many studies looking at whether vitamin D helps to prevent infections, but a causal relationship has not been established.

Vitamin D may play a role in muscle strength and function and several observational studies suggest a correlation between vitamin D and muscle strength. However, of the the RCTs that have been done to date, the results have been varied.

• This meta-analysis of 30 randomized controlled trials involving 5615 individuals showed a small but significant improvement in muscle strength, but the studies were very heterogenous in their doses, durations and methods.

There's emerging research on the potential link between Vitamin D levels and mood.

Population studies have shown a correlation between depression and low levels of vitamin D, however there is not enough high quality evidence to suggest a causal link between vitamin D supplementation and improving mood.

Vitamin D is generally very safe, however the NHS advises not exceed doses of 4000iu per day due to risks of toxicity and excessively high calcium levels.

If in doubt, consult with your doctor or pharmacist.