Vitamin D deficiency is linked to chronic fatigue, says Nichola Ludlam-Raine, a specialist dietitian and the author of How Not To Eat Ultra-Processed. She points to a study of female nurses in 2015, where a connection was found between low levels (89% of the participants were deficient in this vitamin) and self-reported fatigue. And a 2019 study of 480 older adults also linked low levels with fatigue symptoms.

“Supplementation can improve energy levels by supporting immune function and reducing inflammation,” says Ludllam-Raine.

It’s a controversial subject, however. Ashley Grossman is an emeritus professor of endocrinology at Oxford University, and says it’s important to remember that a common symptom such as tiredness can have lots of causes. So he would only link the two if the level of vitamin D measured in a blood test is below the level of 50nmol/L.

2. Catching colds and infections

Research indicates there is a link between vitamin D deficiency and impaired functioning of the immune system, which may increase the risk of infections.

Some research, including a 2019 article, found that vitamin D supplementation helped reduce the risk of acute respiratory tract infections. However, other research states that this protective effect could be small.

Our expert weighs in, describing the recent recommendations on the amount of the vitamin we should take as complete and wide-ranging.

“Empirical evidence implies that vitamin D supplements might generally help reduce the risk of respiratory infection in normal children,” says Grossman. “But again it’s controversial, the recommendations are between 400 IU and 2000 IU [international units], but 800-1000 IU is the most popular amount sold.”

For anyone suffering frequent infections or illness without any known cause, it’s best to speak to your GP about the appropriate supplements for your overall health. This is crucial if you’re pregnant or over 75.

“There is no harm in taking reasonable supplements, and I generally recommend 1000-2000 IU units a day if the level is less than 50nmol/L in most patients,” says Prof Grossman. “Multivitamin supplements are fine, but most of the other vitamins and minerals are unnecessary, and you should check if the amount of vitamin D included is sufficient.”

3. Bone pain

Vitamin D plays a crucial role in calcium absorption and bone metabolism, and a deficiency in it has been linked to an increased risk of developing bone diseases such as osteoporosis.

However, often osteoporosis isn’t diagnosed until a bone is broken, perhaps after a sudden fall. “Suffering pain in your bones is not a symptom of osteoporosis,” says Grossman. “But in someone with bone pain (and no other cause), I would advise a supplement if their level is less than 50nmol/L. Severe vitamin D deficiency causes ‘osteomalacia’, not osteoporosis, which is chronic bone pain and often muscle weakness.

“In children with known bone disease there could be strong indications for treatment” he adds.

4. Muscle fatigue and soreness

It’s not always easy to pinpoint the causes of muscle pain, but a vitamin D deficiency may be a potential cause. As vitamin D is a nutrient that supports muscle function, a deficiency of it can contribute to muscle weakness and soreness, says Ludlam-Raine.

The vitamin D receptor is present in nerve cells called nociceptors which sense pain. This vitamin may also be involved in your body’s pain signalling pathways which may play a role in chronic pain. A 2014 study found that 71% of people with chronic pain had a deficiency in the vitamin.

5. Slow wound healing

A 2014 study involving 221 people found that those with severe vitamin D deficiency were more likely to have higher levels of inflammatory markers that can negatively affect healing.

Ludlam-Raine explains that vitamin D plays an important role in:

So if you’re lacking in it, then it makes sense that the healing process can be impaired.

Adequate levels may speed up wound recovery according to Ludlam-Raine, and in a group of 60 people with diabetes-related foot ulcers, those who took vitamin D supplements over the course of 12 weeks experienced significant wound healing improvements compared with the placebo group.

“Some people are more at risk of having very low levels, especially the elderly, the institutionalised, those physically confined to their home, and people with darker skin who are living in darker climates, as we are in UK,” Grossman notes.

6. Hair loss

There seems to be a connection between vitamin D deficiency and hair loss, particularly in conditions like alopecia areata, says Ludlam-Raine. “Alopecia areata is an immune disease characterised by severe hair loss. In one study, 48 people who suffered with the condition found that applying a synthetic form of vitamin D topically (over 12 weeks) reported a significant increase in hair regrowth”.

In another separate review, higher vitamin D levels were connected to less hair loss.

7. Weight gain

The most likely reason for the scales creeping up is that you’re consuming more calories than your body actually needs rather than lacking any vitamin. Still, vitamin D deficiency can play a role. In fact, Grossman says that “there is some evidence that in patients with ‘pre-diabetes’, empiric treatment [supplementation] may slightly slow down the progression to diabetes”.

Ludlam-Raine notes: “Many people living with obesity do have low levels of vitamin D. The relationship between vitamin D and weight gain isn’t fully understood, and the supplementation’s effects on weight loss require more research.”

How can you get more vitamin D?

As well as through sunlight (ideally in the earlier part of the day) or supplementation, we can boost our levels through our diet. Ludlam-Raine recommends the following tips to up our intake: