Why Vitamin D in Winter Is More Than the "Sunshine Vitamin" – It May Be One of Your Best Allies for Staying Well

Here is a number worth remembering before the next grey week sets in: when researchers from the Nutrition Institute in Ljubljana measured vitamin D levels in adults who were not taking supplements, 63 percent had insufficient levels in winter. The findings come from the longitudinal Nutri-D study, published in Frontiers in Nutrition (Hribar et al., 2023). In summer, as TIME reported, the same problem affected only about 6 percent of participants. Same people, same country, completely different picture – and the only thing that changed was the sun.

Even one of the world's best-known infectious disease experts has spoken about this. In a widely reported 2020 Instagram Live conversation with actress Jennifer Garner, Dr. Anthony Fauci, then director of the U.S. National Institute of Allergy and Infectious Diseases, said he takes a vitamin D supplement himself. He later explained that, because he did not get out in the sun much, his vitamin D level had generally been low; after he started supplementing, it returned to normal. He also noted that people with low vitamin D appear to be more prone to infections when they are going around (Insider, Business Insider). In the same conversation, he was notably sceptical about most other "immune-boosting" products.

Autumn Is Already Here – and the Sun Is Fading Fast

The autumn weather is getting gloomier, and the sun's rays are becoming increasingly rare. We are also approaching the period when we will not see the sun for days at a time, and low vitamin D is already becoming an issue for many people. In Slovenia, which lies between 45{-15696} and 46{-15696} north, the Nutri-D researchers point out that UVB radiation from November to April is not strong enough to trigger vitamin D production in the skin. Once you read what this vitamin does, it will be clear why, in autumn and winter, we need to make sure we get enough vitamin D on our own.

What Vitamin D Does in the Body

Over the years, vitamin D has been studied in connection with an impressive list of conditions: colorectal and breast cancer, melanoma, bone strength, LDL ("bad") cholesterol, type 2 diabetes, heart health, psoriasis, dental caries, rickets, osteoporosis and depression. For some of these, the results are encouraging; for others, large clinical trials have been more sobering. That is why the NIH Office of Dietary Supplements describes much of this evidence as mixed and still developing.

What is firmly established, however, is that the main task of vitamin D is to regulate and support the absorption of calcium (and phosphorus). According to the NIH, adequate vitamin D prevents rickets in children and osteomalacia in adults, and together with calcium it helps protect older adults from osteoporosis.

At this time of year, it is also one of the most talked-about nutrients for the immune system. Research suggests that the immune system may struggle to switch on properly without vitamin D. In a widely cited laboratory study published in Nature Immunology (von Essen et al., 2010), scientists at the University of Copenhagen reported that T cells – the immune system's "soldiers" – rely on vitamin D to become fully activated. As ScienceDaily summarised, without enough vitamin D in the blood, these cells may remain dormant. It is worth adding that other scientists questioned some of the study's interpretations, so it is best seen as an important piece of the puzzle rather than the final word.

Colds, Flu and Sore Throats: What the Studies Suggest

Keep this in mind the next time you are recovering from the flu, a cold or a sore throat. Many people notice that one infection seems to follow another during winter, often because the body has not yet fully recovered. Good overall nutrition, rest and adequate vitamin D are all part of helping your defences get back on their feet.

The strongest evidence so far comes from a large analysis published in The BMJ (Martineau et al., 2017), led by Queen Mary University of London. It pooled individual data from 25 randomised controlled trials with more than 11,000 participants and found that vitamin D supplementation was linked to a modest overall reduction in acute respiratory infections. The benefit was greatest in people who were very deficient and in those taking vitamin D daily or weekly rather than in occasional large doses. According to ScienceDaily, daily or weekly supplementation roughly halved the risk among people with the lowest starting levels.

When Vitamin D Stays Low for Too Long

In studies, people whose vitamin D levels remain low over longer periods more often have autoimmune and infectious conditions, and researchers have also explored possible links with multiple sclerosis, low mood, osteoporosis, fertility issues and migraines. Keep in mind that an association does not prove cause and effect – sometimes illness itself lowers vitamin D levels.

One of the most interesting results comes from the large American VITAL trial, run by researchers at Brigham and Women's Hospital and Harvard. In nearly 26,000 older adults, taking 2,000 IU of vitamin D daily for about five years was associated with a 22 percent lower rate of newly diagnosed autoimmune diseases compared with placebo, as reported in The BMJ (Hahn et al., 2022).

How Much Vitamin D Do You Actually Need?

Experts generally recommend a daily intake of 400 to 800 international units (IU) of vitamin D. The NIH sets 600 IU (15 µg) for most adults and 800 IU (20 µg) for people over 70, while the European Food Safety Authority (EFSA) sets an adequate intake of 15 µg (600 IU) per day for adults.

You may also come across claims that much higher amounts, such as 4,000 to 8,000 IU per day, can help fight serious illnesses like cancer. Several studies have explored higher intakes in relation to breast and colorectal cancer, multiple sclerosis and diabetes, but the results have been mixed. In the main VITAL trial, published in the New England Journal of Medicine (Manson et al., 2019), 2,000 IU daily did not lower the overall rate of cancer or cardiovascular events. Vitamin D is not a treatment for cancer, and higher doses belong under a doctor's supervision.

Where to Find Vitamin D on Your Plate

Of course, capsules are not the only source of vitamin D. There is a lot of it in fish oil, salmon, tuna, mackerel, sardines, trout and egg yolks. For comparison, using figures from the NIH vitamin D fact sheet:

• Cod liver oil, 1 tablespoon: about 1,360 IU


• Rainbow trout, 85 g (3 oz), cooked: about 645 IU


• Sockeye salmon, 85 g, cooked: about 570 IU (older tables list around 447 IU – wild salmon generally contains far more than farmed)


• UV-exposed mushrooms, half a cup: about 366 IU


• Canned light tuna, 85 g: about 40 IU


• Beef liver, 85 g, cooked: about 42 IU


• One large egg: about 44 IU, almost all of it in the yolk

To reach 4,000 IU from salmon alone, you would have to eat roughly 600 grams of it every single day. For more ideas, see our guide The Sunshine Vitamin Without the Sun: 7 Foods That Help Top Up Your Vitamin D.

It is also good to know that there are two types of vitamin D: animal-derived calciferol, also called cholecalciferol or vitamin D3, and plant-derived ergocalciferol, or vitamin D2, which is found mainly in mushrooms. Both are well absorbed, but experts quoted by TIME note that D3 raises blood levels more effectively and keeps them up for longer. Because vitamin D is fat-soluble, eating it with a meal that contains some fat helps absorption.

The Other Side of the Coin: Too Much Is Not Better

Unfortunately, vitamin D also has a downside. Too much of it raises the level of calcium in the blood (hypercalcaemia), which can cause nausea, loss of appetite, weakness and, in serious cases, kidney damage. Excess calcium has its own unwanted effects as well. Importantly, this happens from high-dose supplements, not from sunshine.

The commonly cited safe upper limit for adults is 4,000 IU (100 µg) per day, set by both the NIH and EFSA. Doctors sometimes prescribe higher doses for a limited time to correct a confirmed deficiency, but this should always be done under medical supervision. If you take other medicines, have kidney problems or a history of kidney stones, talk to your doctor or pharmacist first. Remember that a healthy recommended daily amount for most adults is 600–800 IU.

A Simple Winter Plan

• Catch the midday light on bright days – a walk around noon is good for your mood even when the sun is too weak for vitamin D.


• Eat fatty fish about twice a week and include eggs and mushrooms regularly.


• Support your immune system as a whole – try 7 Natural Juices to Help You Stay Strong Through the Autumn Cold Season or Warm Water with Turmeric Against Winter Troubles, and read When Olive Oil Becomes a Shield: What Scientists Discovered About Fats and the Immune System.


• Ask about a blood test (25-hydroxyvitamin D) if you are unsure about your levels, especially if you are over 65, spend most of your time indoors, or have darker skin.


• If you supplement, choose a moderate daily dose and stay within the upper limit unless your doctor advises otherwise.

If you were thinking of getting all your vitamin D from food alone, you will really have to eat a lot... For most of us in the darker half of the year, a combination of smart food choices, daylight whenever possible and, where appropriate, a sensible supplement is the most realistic path.