It’s Halloween ... the time of year when everyone should take vitamin D

Studies suggest depression, Alzheimer’s and bacterial infections could be linked to vitamin D deficiency

Dr Catherine Conlon

Tue Oct 27 2026 - 05:31 • 4 MIN READ

The days are shortening and the temperature is dropping. Dark, dismal, damp weather is predicted for the next few months. As Halloween approaches, the HSE advises that everyone – no matter what age or skin colour – should be taking a vitamin D supplement.

Vitamin D plays an important role in keeping bones and muscles in good shape and the immune system working well. Low levels can lead to bone conditions such as rickets in children, and pain and weakness in adults caused by a condition called osteomalacia.

Right until the middle of the last century, rickets was common in Ireland, often linked to poverty and urban malnutrition. Just two decades ago, severe nutritional rickets made media headlines when two major Dublin paediatric hospitals reported 23 diagnosed cases within a two-year period, mostly tied to a severe lack of vitamin D and children spending more and more time indoors.

Ireland’s northerly latitude means the population cannot make enough vitamin D from sunlight between October and March. And a paper in the Journal of Nutritional Science (2022) by Trinity researchers found that up to one in four children in Dublin are deficient in vitamin D.

Subsequently, the Food Safety Authority of Ireland and the HSE brought in new guidance advising all infants under 12 months be given daily vitamin D supplements to prevent re-emergence of disease.

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Other than sunlight, vitamin D is also found in a small number of foods, including oily fish (salmon, sardines, herring and mackerel); red meat, liver and egg yolks. Some foods, such as certain fat spreads and breakfast cereals, are fortified with vitamin D. However, the amounts in food are quite small, which is why people are advised to take a vitamin D supplement.

The intake you need depends on your age, skin tone, the time of year and your situation. It can be taken in different forms: as a tablet, a chewable tablet, or one that dissolves in your mouth; or as drops or sprays.

Dosage varies from 5 micrograms a day for babies under 12 months (if they are breastfed or receive less than 300ml of infant formula a day), to 15 micrograms a day for adults aged 65 years and over.

That sounds complicated, but the bottom line is that everyone in Ireland is at risk of vitamin D deficiency in winter, whereas some people are at risk of deficiency all year round. You need to take a vitamin D supplement all year round if you are not often outdoors (housebound or in a nursing home), usually wear clothes covering up most of your skin when you are outside, have black or brown skin, are over 65, pregnant, or you are a baby.

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In Ireland, 29 per cent of 18- to 30-year-olds, and one in five people over 50, are vitamin D deficient in winter and spring. One in eight people over the age of 50 are deficient all year round, as are half of people over 85.

Vitamin D has key functions in the body beyond maintaining strong bones. It also supports muscle strength, the ability of nerves to carry messages from the brain and a robust immune system to fight off infection. During the Covid-19 pandemic, vitamin D was found to play a role in reducing severity of infection, including reducing deaths.

The vitamin is involved in hundreds of processes throughout the body. Some really interesting research has emerged this year that has expanded our understanding of the vital role that vitamin D plays in mood, hearing loss and stomach infection.

Depression

A recent study published in Clinical Nutrition (2026) suggested that vitamin D status may play a key role in mood. The researchers followed more than 96,000 adults with hearing impairment over 12 years and found that those with vitamin D deficiency (less than 29ng/mL) had a 57 per cent higher risk of developing depression than those with no vitamin D deficiency.

Future clinical trials are needed to see if correcting a deficiency reduces these risks.

Brain health

Vitamin D may play a role in preventing cognitive decline. In a study of nearly 800 adults without dementia, published in Neurology (2026), participants’ vitamin D levels were measured during early midlife followed by brain PET scans 15 years later to assess markers associated with Alzheimer’s disease.

Participants with higher vitamin D levels in midlife were found to have lower levels of tau protein build-up in the brain that is associated with Alzheimer’s disease. The researchers concluded that maintaining adequate vitamin D levels midlife may be one factor associated with healthier brain ageing, although the exact mechanism is as yet unclear.

Stomach infection

Low vitamin D levels may increase risk of a bacterial infection (H pylori) that impacts the lining of the stomach in up to half the population globally. A paper in Gastroenterology (2024) found that vitamin D deficiency was much more common in those that tested positive for H pylori. Vitamin D is known to maintain the integrity of the mucosal lining of the stomach as well as supporting a robust immune system to fight off infection.

Taking more than the recommended amount of vitamin D in a supplement can weaken bones, kidney and heart due to a build-up of calcium in the body. That is why you should not take more than the recommended amount unless your GP advises you to do so.

You can buy vitamin D supplements without a prescription in your local pharmacy. There is a wide range of vitamin D and multivitamins available on the market, so check which one is right for you with your pharmacist or healthcare professional.

With the days getting shorter and darker, make sure vitamin D supplements are included on your Halloween list this year for all your family, young and old alike.

• Dr Catherine Conlon is a public health doctor in Cork and former director of human health and nutrition at Safefood

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