Free radicals are chemicals that react with other chemicals in oxidation reactions. If this oxidation process occurs in the body, damage to cells and tissues becomes evident. Antioxidants are chemicals that can quench free radicals, meaning that they sacrifice themselves to prevent tissue damage by allowing themselves to be oxidised. There are a number of important antioxidants that are synthesised in the body including glutathione and coenzyme Q10.
Plant Foods And Antioxidants
Plants also synthesise antioxidants. These chemicals protect the plant tissues from free radical damage as they do in humans. However, when we eat the plants the antioxidants are absorbed to our blood where they boost our own endogenous antioxidant defences against free radicals. A number of important antioxidants that are found in plants are essential nutrients in human nutrition including vitamin C and vitamin E.
Is Vitamin A An Antioxidant?
Vitamin is is a group of chemicals which all have the same activity as retinol. Included in this category are the carotenoids because they are converted in the body to retinol. However, carotenoids also have important antioxidant effects of their own outside of their role in vitamin A production. The carotenoids are lipid soluble and as such accumulate in the fatty parts of the body such as adipose tissue and the cell membranes.
Carotenoids Protect Cell Membranes?
Because they are lipid soluble, carotenoids accumulate in cell membranes, which are made of lipids, cholesterol and phosphate. Here they can protect the unsaturated fatty acids within the membrane from oxidation. Unsaturated fatty acids are particularly susceptible to oxidation from free radicals because they have double bonds in their structure which is vulnerable to free radical induced oxidation. Such oxidation creates lipid peroxides which are damaging to health.
Phospholipids carry cholesterol and fatty acids around the body. Structurally they have a lipid core surrounded by a plasma membrane and protein coat. As with cells, the plasma membrane in phospholipids can contain unsaturated fatty acids and these are susceptible to oxidation. Lipid soluble antioxidants such as the carotenoids (and vitamin E) accumulate in phospholipids and protect the fatty acids from oxidation.
Evidence suggests that oxidation of phospholipids may increase the likelihood that the particles will damage artery walls. Lipid soluble antioxidants such as the carotenoids therefore play an important protective effect against cardiovascular disease by protecting phospholipids from oxidation. In addition, carotenoids may protect the endothelial walls themselves by accumulating in the cell membranes and thus prevent free radicals damaging the arteries directly.
Plant Food Sources
Finding carotenoids in the diet is pretty easy. They are responsible for most of the yellow, red and orange colours in vegetables. It makes sense therefore to eat a variety of different coloured vegetables to maintain high tissue concentrations of carotenoids. Because different carotenoids have slightly different effects the larger range of carotenoid containing foods that can be eaten, the higher the chance of affording a wider antioxidant protection.