Vitamin A: the key to a tolerant immune system?
There has been plenty press of late about the necessity of adequate vitamin D and how chronically low levels are implicated in anything from insulin resistance to depression. Additionally, it is very unusual to test someone’s vitamin D levels and to find them to be adequate – a level above 45 ng/mL or more. Yet, with all the attention we’ve been giving vitamin D (and rightly so), we have forgotten about its essential co-partner in immunological and bone health: vitamin A.
Vitamin A and immunity
Vitamin A is essential to immune tolerance in the digestive system, particularly that to do with the gut lining. And immune tolerance is the basis of good health. An intolerant immune system will lead to a wide range of illnesses, and the gut is where many people first lose immune tolerance. Vitamin A (retinoic acid) is key to our ability to consume a wide range of antigens (food) and yet not react adversely.
Vitamin A is fat soluble and its molecules are known as retinol, retinal and retinoic acid. Retinol is the form in which vitamin A is stored, whilst retinal is crucial for vision. Retinoic acid functions like a hormone, binding to two receptors (RAR and RXR) and impacting over 500 different genes. Vitamin A is required for innate and adaptive immunity and is an immune enhancer that potentiates the antibody response, maintains and restores the integrity and function of all mucosal surfaces.
Vitamin A cannot be synthesized by the human body; it must be absorbed by the intestines from the diet. The most concentrated source of vitamin A is in liver (meat, poultry and fish), but eggs, cheese and yoghurt area also options if there is no history of either allergy or intolerance. Some carotenoids can be converted to vitamin A. For example, beta-carotene and lycopene from butternut, tomatoes or green leafy vegetables. However, the converstion of carotenoids to vitamin A is not efficient and research has shown about 50% of women to be slow converters. In addition, up to 50% of beta-carotene is highly dependent on fat consumption at the same time, and cooked carotenoids are better absorbed than raw. Poor protein status or zinc deficiency also affect beta-carotene uptake, and its conversion to retinol (vitamin A).
Vitamin A and secretory IgA
Vitamin A has been well known for its protective role against infections. Indeed, it is often given to children with chronic diarrhoea. An important part of the protective roles might be through its ability to enhance antibody responses, especially IgA antibody responses in mucosal tissues.
IgA is secreted into the gut lining and provides protection against harmful pathogens. It thus helps maintain a healthy flora (good bacteria in the gut). Retinoic acid, derived from vitamin A in the diet, exerts a positive impact on the precursors for IgA-producing plasma cells. With adequate vitamin A our gut is less likely to be chronically inflamed by inappropriate T-cell conversion leading to a myriad of inflammatory diseases – from eczema to depression.
It is possible to have secretory IgA levels tested through saliva and stool (although stool seems to be less indicative of overall secretory IgA status).
Vitamin A toxicity
Unfortunately, vitamin A has been vilified for many years so most people are scared to eat food stuffs high in vitamin A or take any supplement that contains vitamin A. Yet, it is crucial to health. However, as with all nutrients, vitamin A must be balanced. One way of balancing it is to take it with adequate levels of vitamin D. A good example is Cod Liver Oil. This cod oil is now back in favour as it contains not only EPA & DHA fish oils which regulate inflammation, but also contains both vitamin A and vitamin D.
Both vitamin A and vitamin D are crucial nutrients. They are sophisticated and capable of wonderful things, but having too much or too little of either one interferes with the other’s capacity to do its job properly.