The sunshine vitamin, vitamin D, is receiving continued research attention this year. Two recently published studies (1,2) add to the mounting evidence for the importance of this nutrient. As I have previously discussed vitamin D is linked to far more than just bone health. A lack of this crucial nutrient has now been linked to heart problems, cancer, brain disorders, sleep problems, headaches, weight problems and immune problems to name but a few.
The first study I would like to review (1) looks at vitamin D in relation to muscle strength in young girls. The authors of the paper note that in the UK vitamin D deficiency in infants, toddlers and adolescents is becoming more and more common. Reduced muscle strength seems to be associated with a lack of vitamin D in the body but has not been widely researched or studied as yet. The authors wanted to look at the relationship between blood serum vitamin D levels and muscle power in 12-14 year old post-menarchal girls, girls who had already started their period/menstruation. The study took place in a secondary school and involved 99 girls.
The body height, weight and blood serum levels of vitamin D were among the measurements that the researchers took. Muscle power was assessed via a series of jumping tests. The researchers found that there was a significant positive association between vitamin D levels and muscle power. The greater the vitamin D level the more muscle power was exhibited by the girls. The authors of the study conclude that vitamin D is significantly associated with muscle power and force in adolescent girls.
A lack of vitamin D seems to hinder the ability of the muscles to contract. The problem goes deeper than simply affecting muscles. The problem may cause issues for bone health since muscles actually help to build bone mass and strength via their force on bones. The authors of the study (1) stress the need for more research since poor bone health and osteoporosis in older women is becoming a serious issue in the UK and bone strength is mainly built in early life, before the age of 30. A bone healthy diet and regular exercise in early life (and throughout life) are important in order to build and maintain strong bones.
The second study (2) looked at the association of vitamin D levels in the body and prevalence of colds/flu. Previous studies have found that vitamin D is very important for the immune system. The authors of this study wanted to look at the link between blood serum vitamin D levels and the risk of colds/flu. The researchers looked at data from 18883 individuals aged 12 and above.
Lowered vitamin D levels (2) were significantly associated with recurrent bouts of colds and flu. Participants with the lowest vitamin D levels were about 40% more likely to report having a cold/flu than those with the highest vitamin D levels. In addition to this the, association seemed to be stronger in individuals suffering from asthma (or other respiratory tract problems such as chronic obstructive pulmonary disease). Asthma patients with the lowest vitamin D levels were five times more likely to have had a recent respiratory infection. This study only showed an association and the authors called for further randomised studies in order to solidify the evidence, but the findings certainly support a role for vitamin D in the prevention of common respiratory infections such as colds and flu.
The study authors are said to be planning clinical trials to test the effectiveness of vitamin D to boost immunity and fight respiratory infection, with a focus on individuals with asthma and other respiratory diseases, as well as children and older adults – groups that are at a higher risk for more severe illness.
Colds and flu are common in the autumn and winter months, this is also a time when sunlight is scarce in the UK, people wrap up and there is less skin exposure to light. This means there is less opportunity for vitamin D to be manufactured in the body. This is purely circumstantial associative evidence but certainly makes theoretical sense.
Foods rich in vitamin D are limited, with oily fish such as mackerel, salmon and sardines being the primary natural source. Eggs also contain limited amounts of the vitamin and some breakfast cereals are fortified with the vitamin. The exposure of our skin to sunlight is definitely the main source of vitamin D. Recently, however, there has been mass debate about whether UV exposure in the UK is sufficient to keep vitamin D levels optimal for health. Most individuals in the UK are falling short of recommended levels. If you rarely get out into the sunlght you may wish to consider a vitamin D supplement which provides around 12mcg of the nutrient per day (around 500iu). Children will require less, many specific child supplements are available and it is also a good idea to check with your GP or health professional before providing your children with supplemens. Vitamin D supplements may be particularly useful during the autumn and winter months.
(1)Ward KA, Das G, Berry JL, Roberts SA, Rawer R, Adams JE, Mughal Z. 2009. Vitamin D status and muscle function in post-menarchal adolescent girls. J Clin Endocrinol Metab. 94(2):559-563.
(2)Ginde AA et al. 2009. Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the third national health and nutrition examination survey. Arch Intern Med. 169:384-390
Written by Ani Kowal