Diet therapy

Vitamin D for growth and immunity

All vitamins are important for health but recent research around Vitamin D is highlighting just how important it is for growth and the maintenance of our immune system.

What is vitamin D?

Often called ‘The Sunshine Vitamin’, Vitamin D3 (cholecalciferol) is a fat-soluble vitamin which is found naturally in animal containing foods such as fish liver oil, tuna, salmon, eggs, beef and lambs fry. 1

It can also be added in the plant form vitamin D2 (ergocalciferol) to fortify foods such as margarine and fat spreads; some reduced-fat diary products, milk, dried milk as well as plant based dairy substitutes such as soy drinks, soy yoghurt and liquid meal replacements. 2

Our body can also make vitamin D3 from sunlight.  When the ultraviolet B, or UVB, rays in sunlight hit the skin, it activates 7-dehydrocholesterol to produce pre-vitamin D3 which is then carried in the blood stream to the liver. Here it is modified (hydroxylated)  into 25-hydroxy-vitamin D (25(OH)D) and then onto the kidneys where it is converted into the biologically active form 1,25(OH)2 vitamin D3.
Obviously vitamin D produced in this way is subject to considerable seasonal variation, with cloud cover and pollution all reducing UVB rays, particularly in winter.

How much do we need?

The minimum required intake of Vitamin D3/day 1

Children: Birth to 18 years 5 µg
Adults: 19-50 years 5 µg
51-70 years 10 µg
>70 years 15 µg


As vitamin D is made up of dietary as well as cutaneous production standards of sufficiency are set using serum 25(OH)D and nutritional adequacy determined as follows:

Deficiency      <25nmol/L
Insufficient     25-49.9nmol/L
Adequate        >50nmol/L

Note 1:

1 µg cholecalciferol is equal to 0.2 µg 25(OH)D.
Vitamin D is also sometimes expressed in International Units where 1 IU equals 0.025 µg cholecalciferol or 0.005 µg 25(OH)D.

Some researchers are now calling for levels of adequacy to be raised to 25(OH)D >75-100 nmol/L  particularly for high risk athletes and the elderly.1 Also better absorption appears to come when Vitamin D is administered in daily or weekly doses rather than one bolus /mega dose.3

A blood test can quickly determine Vitamin D status.  If considering supplementation be guided by your doctor regarding the amount and dosage as Vitamin D is a fat soluble vitamin that can be accumulated by the body and can be toxic if consumed in excessive amounts. (NB: Toxicity is less likely to come from sunlight than from an excess of supplementation).

What does Vitamin D do?

As Vitamin D3 can be formed in the skin, yet go on to affect other organs such as the bones, gut and kidneys, it can also be called a hormone and part of the endocrine system. Hormones signal to the body what to do and when to do it for example when to build up calcium in bone and when to break it down or remodel it. 2

Bones, joints and muscles

Vitamin D is essential for the absorption of calcium, magnesium and phosphorus which are important for bone growth, density, remodeling and strength and without adequate amounts, bone loss or injury will occur.

A lack of Vitamin D can cause Rickets a disease in young people that reduces the calcification (hardening) of the growing ends (epiphyses) of bones, causing bone weakness, bowed legs, deformity of joints and stooped posture.

Long bones can be particularly at risk of under mineralization during the pubertal growth spurt when young athletes are still trying to achieve peak bone density so a good vitamin D status during this period is important for growth and to prevent painful stress fractures. 2

There is limited evidence regarding vitamin D and improved muscle strength in adolescents, however poor vitamin D status (<20ng/mL) was found to reduce forearm muscle strength when a group of 301 Chinese female adolescent athletes were compared with individuals with adequate levels.4

It is thought that when individuals with low vitamin D status receive supplementation, muscle strength may be gained due to an  increase in the size and amount of type 2 (fast twitch) muscle fibres. 2

Essential body processes

On a daily basis 80% of vitamin D in the body is involved in the autocrine pathway responsible  for regulating electrolyte metabolism, gene expression; protein synthesis; cell turnover; immune and inflammatory responses 2 making this an essential vitamin to maintain body processes and growth.

Respiratory function and immunity

A higher prevalence of pneumonia and respiratory infection has been observed in children with rickets and low vitamin D status. 2, 3

Although there is no separate data on children or athletes a recent meta-analysis of 23 studies covering 11,901 participants with Covid-19 found most of these patients were suffering from vitamin D deficiency/insufficiency. 5

It was estimated that when vitamin D levels are low there is a 3.3% chance of getting infected with SARS-CoV-2  and the chance of developing Covid 19 was found to be 5 times higher in people with vitamin D deficiency 5

Groups at risk of vitamin D deficiency

Dark skinned people

Reaching an adequate intake of vitamin D can be problematic for people living at latitudes >35 degrees where UVB radiation is absent during winter, or for those who have naturally dark skin, such as people from Africa, India and the Middle East.6

Athletes who train indoors

Vitamin D deficiency can affect outdoor athletes who avoid the sun by training early in the day or evening or athletes who wear protective clothing (e.g ice hockey) or who spend long periods of time training indoors. 7

People on energy or nutrient restricted diets

Also at risk are athletes and people who might be following energy restricted or fad diets to maintain low body weights such as gymnasts, dancers and jockeys.7 People exclusively following vegan and vegetarian diets or those who are trying very low energy diets e.g. intermittent fasting, need to take particular care and have regular blood tests to check adequacy.

Excessive users of sunscreen

The excessive use of sunscreen can reduce Vitamin D absorption from the sun. However it is still important to prevent skin cancer so stay ‘sun smart’ if exercising outdoors – that is, wear a hat and use sunscreen in sensible amounts and if possible avoid training during the hottest times of the day.

The elderly or malnourished

Elderly people, particularly if hospitalized or in residential care; people (of all ages ) who are fussy eaters ;or recovering from surgery;  those with limited access to sunlight and food intake are definitely at risk of vitamin D deficiency.

Multiple studies of the elderly have linked low vitamin D to reduced reaction times, poor balance and an increased risk of falling. Also studies show vitamin D supplementation can bring improvements in strength, walking distance and a reduction in general discomfort. 1  Making vitamin D an essential nutrient in later life.

Vitamin D for better health

While there is limited research linking vitamin D supplementation with performance enhancement at the moment it is never the less very important for the prevention of chronic and acute disease such as cancer, cardiovascular disease, type 2 diabetes, autoimmune disease and infectious disease 2 so be sure to discuss Vitamin D with your doctor and sports dietitian if you have concerns.


  1. Nutrient Reference Values for Australia and New Zealand’16 January 2020,
  2. Organ, D; Pritchett, K. Vitamin D and the athlete: risks, recommendations and benefits. Nutrients 2013 June; 5 (6) 1856-1868.
  3. Martineau, A; Jollife, D.A; Hooper, R.C., Vitamin D supplementation ot precent acute respiratory tract infection: a systematic review, meta-analysis of individual participant data. BMJ 2017, 356.
  4. Foo L.H., Zhang Q., Zhu K., Ma G., Hu X., Greenfield H., Fraser D.R. Low vitamin D status has an adverse influence on bone mass, turnover, and muscle strength in adolescent female girls.J. Nutr. 2009;139:1002–1007.
  5. Ghasemian, R; Shamshirian, A; Heydari, K; Malekan, M; Alizedeh-Navaei, R; Ebrahimzadeh, M.A; Warkiani, M.E; Jafarpour, H; Bazaz, S.R; Shahmirzadi, A.R; Khodabandeh, M; Seyfari, B; Motamedzadeh, A; Dadgostar, E; Aalinezhad,M; Sdaghat, M; Razzaghi, N; Zarandi, B; Asadi, A; Naei, V.Y; Beheshti, R; Hessami, A; Azizi, S; Mohseni, A.R; Shamshirian, D. The role of vitamin D in the age of Covid-19: A systematic review and meta-analysis. The Intern. J. of Clini. Prac. 29 July 2021
  6. Desbrow, B., Burke, L.M., Cox, G., Fallon, K., Hislop, M., Logan, R., Marino, N., Sawyer, S., Shaw, G., Star, A., Vidgen, H., Leveritt, M., ‘Sports Dietitians Australia Position Statement: Sports nutrition for the adolescent athlete’, International Journal of Sports Nutrition and Exercise Metabolism, 2014, 24(5):570–84.
  7. Nicol, L; Sports and exercise physician, Christchurch NZ, Personal communication Aug 2021

About the author View all

Lea Stening

Lea is one of New Zealand’s leading paediatric dietitians and also specialises in Sports Nutrition. She has specialised in Paediatric Nutrition for 31 years and in 1985 was the first paediatric dietitian to enter private practice in New Zealand. Lea helps families through her private consultations, public lectures, newspaper and magazine articles as well as television and radio interviews. Read more »

View all posts by Lea Stening »


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