The influence of fat on the immune function has only recently come under serious study. It is known that polyunsaturated fatty acids, particularly linoleic acid, are required for optimal functioning of the immune system, but there is an optimum level which should not be exceeded. In recent years there has been a tremendous increase in the consumption of polyunsaturated fats to combat heart disease, but this has brought to the fore a host of problems not previously envisaged. High levels of fats, particularly polyunsaturated fats, impact negatively on the immune system and decrease its ability to cope with cancer tumours, allergies, infections by microbial organisms and both thymic-dependent and thymic-independent antigens.28
Immune responses can thus be enhanced or depressed, depending on the concentration and extent of unsaturation of dietary lipids. It has been found that high-fat diets consistently depress resistance to malaria and tuberculosis in rats, and respiratory infections in chickens, but the same seems to be true for humans. Lower respiratory tract infections in infants, for example, are significantly more common in obese infants than in non-obese infants, and in one third of obese infants, adolescents and adults studied there was impairment of cell-mediated immune responses.29
The mechanism whereby fats interfere with the body’s ability to combat the growth of cancerous tumours has also been investigated. A subpopulation of T-lymphocytes, known as natural killer cells, specifically react to destroy tumour cells before they can proliferate. Recently it has been found that diets high in polyunsaturates, particularly those rich in n-6 fatty acids (e.g. linolenic acid), impact negatively on the ability of these killer cells to seek out and destroy cancer cells.30 The three types of blood cell associated with the immune response are the granulocytes, monocytes, and lymphocytes. The neutrophils are the most abundant granulocyte and they destroy antigens by simply engulfing them. Macrophages of monocytic origin, are also phagocytes but they carry out other functions as well. They secrete substances known as lymphokines and prostaglandins that affect B- and T-cell activity in many ways.
Examples of lymphokines are interferon and interleukon 1, of which interferon stimulates T-cell proliferation and interleukon 1 stimulates a broad range of cells, including the natural killer cells, neutrophils, and B-and T-lymphocytes. T-cells do not produce antibodies, but B-lymphocytes produce antibodies which combine with antigens, rendering them inactive and enabling phagocytes to engulf the invaders.
Figure 3.7. The fatty acid profiles of foods high in monounsaturated fats. (From reference 25)
Prostaglandins, thromboxanes, and leucotrienes are eicosanoids which are produced from the essential fatty acids, linolenic and linoleic acid. Generally, prostaglandins function as vasoconstrictors, thromboxanes affect platelet aggregation, and leucotrines contract smooth muscle cells. Those prostaglandins that have a relaxing, anti-inflammatory and anti-clotting effect are generally formed from alpha-linolenic acid (Triene prostaglandins) whilst those with the opposite effect are manufactured from linoleic acid (Monoene prostaglandins) and arachidonic acid (Diene prostaglandins). More than one hundred different prostaglandins have been identified, and they promote or inhibit basic bodily functions such as fever, blood clotting, vasodilation and constriction, stress, allergy response, membrane permeability, eye pressure, inflammation, steroid production, appetite, fat metabolism and the functioning of the immune system.31 When prostaglandins occur in a balanced relationship they tend to relax arteries and reduce blood pressure as well as slow down tumour formation and decrease platelet aggregation, thus lowering the risk of thrombus formation. If the balance of prostaglandins is, however, disturbed then the opposite effects are achieved. It is interesting to note that tumour cells produce large amounts of the prostaglandin PGE2 and cancer patients can produce four times the normal amount of this prostaglandin and has an immunorepressive effect and leukotrine B4 is a potent chemotactic and chemokinetic agent.6 For a summary of the effects of eicosanoids see figure 3.8.
A reduction in the amounts of polyunsaturated fats in the diet, inclusive of the essential fatty acids, can provide a substantial anticarcinogenic effect.32,33 A whole-food diet, which includes grains, legumes, seeds and nuts will provide the ideal blend of fatty acids and total fat composition to ensure the optimal functioning of the immune system.