Picture source https://www.anhinternational.org/campaigns/test-take-vitamin-d/
Forms of vitamin D
There are two major forms of vitamin D from two different sources.
- Vitamin D3 (cholecalciferol) is our main source (80-90%) and it is produced in the skin following sunlight exposure but it is also found in foods of animal origin. Technically this makes vitamin D a pro hormone.
- Vitamin D2 (ergocalciferol) is found in foods of vegetable origin and supplements
A whole host of factors affect how much vitamin D our bodies can make including:
- how much skin we expose to the sun
- the height and angle of the sun in the sky
- the time of year
- the duration of exposure
- the health (and age) of our liver and kidneys
- how well we digest and absorb our food especially fats
- excess weight seems to reduce our vitamin D levels
- levels of inflammation in the body
Sources of Vitamin D in the diet
In the UK our main dietary sources of vitamin D are food of animal origin, foods fortified with vitamin D and supplementation. Naturally rich food sources include egg yolk and oily fish such as salmon, mackerel, herring and sardines.
| Food sources | International Units (IU) |
| Salmon 140g | 408 |
| Mackerel 140g | 476 |
| Sardines canned 140g | 184 |
| Mushrooms 80g (enriched) | 128 |
| 1 egg | 64 |
| Tuna 140g | 60 |
| Beef mince 100g | 24 |
| Lamb 90g | 20 |
| Butter 10g | 4 |
Vitamin D content is taken from British Nutrition Foundation ‘Vital vitamin D’ resource sourced from McCance and Widdowson’s The Composition of Foods: Seventh Summary
Role in the body
Vitamin D plays an important role in musculoskeletal health and neuromuscular function because of its’ role in regulating calcium and phosphorus balance in the body. It also helps regulate magnesium absorption. Vitamin D is therefore vital for bone mineralisation, growth and health. It has other roles especially in immune regulation, insulin sensitivity and heart health.
Vitamin D ideal blood levels
Dr Damien Downing, president of the British Society for Ecological Medicine and vitamin D expert, recommends a vitamin D blood level of at least 75 nmol/L for immune support and levels over 100 nmol/L to lower your risk of cancer and autoimmune disease.
How much to supplement
A general guideline for adults over the age of 18 is around 25 mcg (1000IU) but it very much depends on your baseline blood level. It’s always best to do a test as they are quick, easy and economical blood spot kits. A recent study has shown that 20mcg of vitamin D per day was effective at bringing middle-aged and older adults to vitamin D sufficiency over a 4-week period. It’s also best to retest and discontinue once blood levels of 100 nmol/L are achieved. Fat soluble vitamins can be stored in the body and over 125 nmol/L is considered to increase the risk of adverse events.
Magnesium and K2 may also be required to optimise conversion to the active form and hence optimise absorption.
References
Aislinn F. McCourt, A.F. et al (2023) Serum 25-hydroxyvitamin D response to vitamin D supplementation using different lipid delivery systems in middle-aged and older adults: a randomised controlled trial. British Journal of Nutrition 130 pp.1548–1557. doi:10.1017/S0007114523000636
Bikle, D.D. (2009) Vitamin D and immune function: understanding common pathways; Curr Osteoporos Rep; Jul; 7(2); pp.58-63. doi: 10.1007/s11914-009-0011-6.
SACN (2016) Vitamin D and Health; The Scientific Advisory Committee on nutrition; Available from https://www.gov.uk/government/groups/scientific-advisory-committee-on-nutrition. (SACN)