Why Vitamin D Deficiency Is So Common — and What You Really Need to Know

But low levels of vitamin D are not limited to the UK, and this is a widespread global health issue, affecting a staggering almost 1 in 2 of the entire world’s population, more than 3.7 billion people. 1

Vitamin D influences far more than bone health. It plays a role in immunity, inflammation, muscle strength, hormone regulation, and metabolic wellbeing and even mood. Low vitamin D means poor immunity, frequent coughs, colds and — even worse – increased susceptibility to viruses such as COVID.

Immunity is just one of the systems which requires vitamin D to function optimally. Other systems dependent on vitamin D include the skeletal system-bones and muscles, the nervous system, affecting mood, nerve function and cellular health. The disease rickets, common until the early 20th century, was virtually eradicated in the 1930s. But a resurgence due to vitamin D deficiency has occurred, due to changes in lifestyle with sun avoidance, migration patterns and diet changes. We are now seeing children with severely affected limbs due to the effect of vitamin D deficiency — an entirely preventable condition. 2

Here, we will explain why vitamin D deficiency is so widespread, who is most at risk, how obesity affects D biology, and how to choose the right form and dose.

Which groups are more vulnerable and why?

Vitamin D deficiency is seen in certain at-risk groups, with large variations, depending on where you live, diet, age, sex, race, BMI and socioeconomic status.

The further that you live north of the equator, the greater the risk of low vitamin D. The sun sits too low in the sky for half the year to stimulate vitamin D synthesis. This worsens the further north you go, so we see a higher prevalence of low vitamin D in Northern Siberia, compared to the UK compared to Greece. 3

We have been brainwashed into believing that all sunshine is bad and that our skin should never be exposed directly to sunshine. This is wrong! Many people even in the summer, spend long hours indoors, commute when the sun is setting, and rarely expose their skin to the sun. This means that for 12 months a year, there is minimal production of vitamin D in the skin. 4

Couple this to a diet low in vitamin D foods such as a vegan diet, lacking oily fish and fish oils, egg yolk, some cheeses, butter, fortified foods such as milk, and yoghurts, and vitamin D deficiency will quickly supervene. Studies consistently show lower blood vitamin D levels in vegans compared to non-vegans. 5

Skin tone plays a major biological role. The darker the skin, the smaller the vitamin D production for the same amount of sunlight, as higher melanin reduces ultraviolet penetration of the skin. Ageing skin also manufactures less Vitamin D, making the elderly more prone to vitamin D deficiencies. 6

When all these factors overlap, deficiency becomes extremely common, and elderly sedentary adults, with limited sun exposure and poor diets are particularly at risk.

Vitamin D and Obesity: The Real, Biological Story

The link between vitamin D and obesity is not a behavioural issue, and it does not entirely reflect time spent outdoors. Instead, the connection more complex. 7

Vitamin D is fat-soluble, and stored in body fat. In people with obesity, the same total amount of vitamin D becomes diluted across a larger body volume, which lowers the concentration in the bloodstream. More vitamin D is also drawn into fat tissue, reducing the amount available in circulation. Vitamin D release from the fat stores is impaired due to insulin resistance, altered enzyme expression and reduced responsiveness to catecholamines.

These mechanisms make low vitamin D levels more likely in people with obesity.

Who Should Consider Taking Vitamin D?

Because so many risk factors overlap — latitude, lifestyle, skin tone, age, diet, and body composition — public health bodies recommend vitamin D supplementation for broad groups of people. In the UK, everyone is advised to take vitamin D from autumn to spring, when sunlight is not strong enough to maintain healthy levels. And this is a safe and reasonable approach. 8

Those at high risk of vitamin D deficiency should consider taking supplements all year round. This includes people with darker skin, older adults, indoor workers, and those who rarely expose their skin to sunlight. Vegans, or anyone with a diet low in oily fish and fortified foods, also benefit from supplementation. And finally, those who are obese require higher doses to correct deficiency because of the biological distribution and the fat trapping aspect of stored vitamin D. 9

Which Form of Vitamin D Should You Take — and What Dose Works?

There are two main supplemental forms of vitamin D: vitamin D2 and vitamin D3.

Vitamin D2 (ergocalciferol) is derived from plants and vitamin D3 (cholecalciferol), derived from animals. Vitamin D3 is the form your body makes from sunlight and is more effective in raising blood levels than D2. The preferred version (unless you are vegan) is to take vitamin D in the form of D3, as it is more potent.

Some products combine vitamin D3 with vitamin K2, but K2 is not required to correct deficiency. The combination supports bone metabolism and is optional rather than essential. Combined D3 and K2 supplements have shown greater improvements in bone mineral density than D3 alone in people with low bone density.

The dose varies depending on your risk factors. For most adults, 1,000 IU (25 µg) daily is a reasonable starting point. Those with darker skin, older adults, vegans, and indoor workers often benefit from 1,000–2,000 IU per day. People with obesity may need 2,000–4,000 IU. 10

As vitamin D is a fat-soluble vitamin, it is absorbed best when eaten with a meal containing fat — such as eggs, olive oil, nuts, avocado, dairy, meat, or fish.

If you are high risk (see above), take up to a maximum of 4,000 IU per day and you would probably benefit from taking vitamin D all year round.

If you are low risk, take 1,000 IU per day. Take vitamin D supplements in the winter, and if you expose your skin to sunlight during the summer, you will be able to stop during the summer.

Is It Safe?

Vitamin D toxicity is extremely rare and usually associated with very high doses (above 10,000 IU daily for months). Doses up to 4,000 IU per day are well tolerated and generally considered safe for adults. Potential adverse events include raised calcium, and in some cases, reduced bone density in women. 11

What about the sun?

During the summer months, in the UK, you can make lots of vitamin D and perhaps obviate the need for winter supplements. Winter sun exposure is insufficient to make adequate levels of vitamin D. Short, regular exposure to midday sun is usually sufficient for adequate vitamin D production, but duration should be tailored to skin type and risk of sunburn. Expose arms, hands and face to sunlight for about 5-15 minutes, 2-3 per week, between 10.00 am and 15.00. Darker skin requires 2-6 times longer. Further north, at higher latitudes, longer exposure times will be needed. 12

Obviously, there is a risk of sunburn and the window between making enough vitamin D and sunburn is small, especially for those Celts, with fair skin. 13

What will it mean for you?

We once recommended vitamin D to a friend who had become uncharacteristically down in the dumps, during a long dark winter. Carrying a bit of excess baggage, and not someone who spent much time outdoors, he was a prime candidate for low vitamin D and low mood because of it. He took his supplement, as we suggested and returned to being the jolly friend that we all had known, a salutary lesson that simple measures can put you back on track — you just need to know. He became an evangelist of vitamin D supplements, doling them out to the grumpy old gits in the pub at regular intervals— and the laughter soon returned, long before the winter had shed its dull shroud.

Restoring vitamin D levels will improve mood significantly in those who are deficient, and I saw this many times, during winter consultations, when I recommended vitamin D. The same patients on returning in the summer posed the invariable question — Dr Hope-Ross, “How did you know?”

With your vitamin D levels in a healthy range, you’re far more likely to get through winter without serious colds and infections, as you’ve strengthened your immune system. You are less likely to become frail, as you age, reducing the risk of weakness, falls and bone loss. All this adds up to a genuine improvement in how you feel day to day — and how well your body functions in the background.

The Bottom Line

Vitamin D deficiency is widespread in the UK because the sunlight is insufficient for half the year, and modern life keeps us indoors for much of the remainder. Age, skin tone, diet, and body composition all contribute to a pattern where millions of people drift into low vitamin D levels without realising it. 14

Supplementation is a safe and effective way to correct deficiency. If in doubt, check your vitamin D levels, you have a 50/50 chance of getting a nasty surprise. If vitamin D hasn’t been on your radar for a while, now is a good time to take a moment to think about it – and act.