Vitamin D deficiency and tiredness: what the evidence supports
Deficiency is genuinely common here — a meta-analysis of studies across Arab countries put the pooled prevalence of deficient levels at 51.4%[3] — but tiredness on its own cannot diagnose it. Only a 25-hydroxyvitamin D blood test can[2]. Correcting a real deficiency is worth doing; expecting it to cause weight loss is not, because randomised trials of supplementation found no effect on body weight[4].
On this page
What the test measures, and where the line is
Vitamin D status is judged by the concentration of 25-hydroxyvitamin D in blood. The Institute of Medicine's reference intakes treat a level of 50 nmol/L (20 ng/mL) as sufficient for bone health in nearly the whole population, with levels below 30 nmol/L (12 ng/mL) as deficient and the band between the two as inadequate for some people[1][2].
That is worth knowing because it explains the wildly different prevalence numbers you will see quoted. A study using a cut-off of 30 ng/mL will report far more deficiency than one using 12 ng/mL, on identical blood. When a headline says most of a country is deficient, the first question is which line was drawn.
Sunny countries, deficient people
The paradox is real and it is documented. A systematic review and meta-analysis of studies from Arab countries pooled 21 studies covering more than 25,000 patients and found 51.4% with deficient levels, with a further 24% classed as insufficient[3]. A study of healthy Egyptian adolescents found 94.8% deficient and 4.2% insufficient[5].
The explanation is that sunlight only helps if it reaches skin. In a climate where the sun is avoided at the hours it is strongest, where much of the day is spent indoors, where clothing covers most of the body and sunscreen covers the rest, the ultraviolet dose that actually reaches the skin can be small. Darker skin also synthesises less vitamin D from the same exposure[2]. Latitude is not the constraint here; behaviour is.
What tiredness can and cannot tell you
Deficiency severe and prolonged enough to cause osteomalacia produces bone pain and muscle weakness[2]. That is a real, describable syndrome, and it is not the same thing as feeling tired in the afternoon.
Fatigue is the least specific symptom in medicine. Iron deficiency, thyroid disease, depression, sleep apnoea, poor sleep, an aggressive calorie deficit and simply being under-slept all produce it, and several of them are far more common than symptomatic vitamin D deficiency. This is why the honest answer to 'is my tiredness vitamin D?' is a blood test rather than a supplement bought on the assumption. If the level comes back low, treating it is straightforward. If it comes back normal, you have ruled out one cause and can go looking for the real one instead of taking a pill for a year.
Vitamin D and body weight
Two facts are often confused here. People with obesity do tend to have lower blood concentrations of 25-hydroxyvitamin D, partly because the vitamin is fat-soluble and is sequestered in adipose tissue rather than circulating[2]. That is a consequence, and it does not run the other way as a treatment: a meta-analysis of randomised controlled trials found vitamin D supplementation had no effect on body weight[4].
So a low reading in someone carrying extra weight is worth correcting for its own sake, and is not a lever on the scale. If you are trying to interpret a body-weight number in the first place, the guide on what BMI tells you and what it doesn't is the more useful place to start.
Where it actually comes from
Few foods contain much vitamin D naturally. The richest sources are the flesh of oily fish and fish liver oils, with smaller amounts in egg yolks, beef liver and some mushrooms; in many countries the practical supply comes from fortified milk and cereals[2]. Across much of the Middle East fortification is patchy, so the dietary contribution is often small.
Sun exposure is the other route, and it resists precise prescription: the amount needed depends on skin tone, the season, the time of day, how much skin is uncovered and air quality[2]. Since dermatology advice on skin cancer pulls in the other direction, this is one of the places where a supplement is the tidier answer for many people.
What to do
Test before treating, and treat with a number rather than a guess. The recommended dietary allowance for adults aged 19 to 70 is 600 IU per day, rising to 800 IU above 70, and the tolerable upper intake level for adults is 4,000 IU per day[1]. Doses well above that are sometimes prescribed to correct a documented deficiency, and that is a decision for the doctor who ordered the test — excess vitamin D causes hypercalcaemia and is toxic, which is not a risk sun exposure carries[2].
Three practical notes. Take it with a meal containing some fat, because absorption of a fat-soluble vitamin is better with food. Do not stack supplements that each contain it without adding up the total. And if you were tested, retest after the course rather than assuming the dose worked — the whole advantage of this problem over most nutrition questions is that it has a number attached to it.
None of this is medical advice for your case. If you are pregnant, have kidney disease, a parathyroid condition, or take medication that affects calcium, the dose is a clinical decision and not a shelf decision.
Sources
- Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The National Academies Press, 2011. ↩
- National Institutes of Health, Office of Dietary Supplements. Vitamin D — Fact Sheet for Health Professionals. ↩
- Prevalence of vitamin D deficiency in the Arab countries: a systematic review and meta-analysis. Discover Public Health 2025. ↩
- Pathak K, Soares MJ, Calton EK, Zhao Y, Hallett J. Vitamin D supplementation and body weight status: a systematic review and meta-analysis of randomized controlled trials. Obesity Reviews 2014;15(6):528–537. ↩
- Vitamin D status and healthy Egyptian adolescents: where do we stand? Medicine (Baltimore) 2021;100(29):e26661. ↩