Vitamin D: Don't underestimate the 'sunshine vitamin'
Vitamin D's role in bone health has been questioned, so do we really need to watch our intake? Here, dietitian Orla Walsh sets the record straight
Vitamin D is having quite the moment. A recent study claimed that the 'sunshine vitamin' has no benefit for bone health. Now, while the headlines may sway some to give up taking the supplement, there are also good reasons why people are prescribed it in the first place.
When it comes to building bone, vitamin D is required to help your body absorb calcium, the main ingredient of bones. This is why vitamin D deficiency can lead to softening of the bones.
At present it is estimated that 300,000 people in Ireland have osteoporosis, a condition in which a person has porous bones which is associated with increased fracture risk. In fact, one in four men and one in two women over 50 will develop a fracture due to osteoporosis in their lifetime.
In the UK, vitamin D deficiency has emerged as a public health problem in recent years, with one study estimating that around 20% of the population have a profound vitamin D deficiency. Vitamin D deficiency in infants and children can cause rickets. Infants and young people grow so fast that their weight-bearing limbs can become bowed as bones lengthen without the necessary minerals to give them strength. Rickets can result in skeletal deformities of the head, pelvis, legs, chest and spine. It can also result in bone pain, muscle pain, short stature and increased risk of broken bones.
It was thought that vitamin D deficiency was eradicated here decades ago due to better nutrition. However, cases of rickets have doubled since the late 1990s and this is why supplementation with 200IU (50g) per day is suggested.
The above-mentioned study published in The Lancet was a review of studies assessing vitamin D supplementation in adults. Therefore, their findings cannot be applied to children. It's also important to note that it's estimated that one in five people has insufficient amounts of vitamin D. Few people within this review had low vitamin D levels at the start of each study.
This is important as supplementation in those that are deficient is most beneficial. Additionally, some studies within this review were not as reliable as others, with bias possibly affecting results. Finally, calcium and other minerals such as magnesium and phosphorus are important when considering the benefit of vitamin D on bone health. No one nutrient acts alone. Therefore studying it in isolation can only tell us so much.
Vitamin D is more than just food for bones. It acts on almost all our cells and even influences our genetics. The number of research papers revealing the role of vitamin D in the body is astounding. In fact, as the roles of vitamin D are becoming known, it's clear that appropriate levels of vitamin D within the body shouldn't be benchmarked against what vitamin D can do for bones, but rather what are the optimal vitamin D levels within the body overall. Before the additional benefits of vitamin D are discussed, it's important to understand the basics.
Is it a vitamin?
First things first - vitamin D isn't a vitamin. It's a prohormone that is synthesised by the skin when it's exposed to UVB sunlight. The body is able to produce vitamin D and it's also found in some oily fish, eggs and fortified foods.
However, because of our northerly latitude, in the months between November and March there is inadequate quality and quantity of sunlight to enable sufficient production of vitamin D. Even on sunny days in the winter, the sun's rays are the wrong type. Couple this with more time spent indoors and appropriate use of sunscreen, and our levels produced from the sun aren't providing the body with enough vitamin D. Can food make up the rest? Perhaps, but that requires the diet to be varied and contain foods fortified with vitamin D. As there isn't an abundance of food sources, picky eaters and those with allergies are unlikely to meet their needs through food.
Link to body composition
A link between low vitamin D levels and obesity has been reported. A recent report provided more detail showing an association between low vitamin D levels and the type and location of the excess fat.
If body fat levels influence vitamin D levels, do vitamin D levels influence body fat levels? A study assessed 232 obese children and adolescents over 12 months. Vitamin D supplements were given to 117 randomly selected children.
The study reported that children given vitamin D supplements weighed less, had lower body fat levels and better cholesterol levels within 12 months. Therefore, vitamin D supplementation may be a small part of a strategy to tackle childhood obesity as well as reduce the risk of health issues later in life.
Interestingly, vitamin D also appears to affect muscle mass. In the study, 881 children aged five got their muscle strength measured. In about half of the children vitamin D levels were checked. They adjusted their statistical analyses to consider height, weight and body fat percentage. This study found that girls with low vitamin D levels have a 70% increased risk of being among the lowest 10% in a test for muscle strength. Therefore, not only does vitamin D appear to be related to bone strength, but also muscle strength and body fat levels.
Link to the immune system
At this time of year, the risk of colds and flu rises for many reasons. Scientists have discovered that vitamin D helps to activate our body's defences.
The killer cells of the immune system called T cells detect and kill pathogens such as bacteria and viruses. For this to occur the cells must first be 'triggered' into action. Once triggered most of these cells 'transform' from inactive and harmless immune cells into killer cells that seek to destroy nasty pathogens. The rest of the activated T cells become helper cells that assist the immune system in remembering the pathogen and how best to fight it. Researchers have found that when a T cell is exposed to a pathogen, it extends an 'antenna' to search for vitamin D. If the T cells cannot find enough vitamin D, it won't even begin to activate.
Therefore, if the body has low vitamin D levels at this time of year, the immune system may not be able to fight all the bugs, putting a person at risk of colds and flu.
Don't forget magnesium
When it comes to vitamin D, it's important not to forget magnesium. A review found that vitamin D cannot be metabolised without sufficient magnesium within the body. Data shows that the standard diet in the United States contains only about 50% of the amount of magnesium required by the body.
It's unlikely that our diet meets our daily needs. Therefore, focus needs to be paid to both of these two important nutrients in order to reap the benefits of adequate vitamin D levels.