The simplest and most direct relationship between food and immunity is that of protein. Without sufficient protein, the immune response is compromised and the potential for infection and inflammation increases. The immune system relies on protein-based protectors such as antibodies, lymphocytes (such as T-cells), leukocytes and a host of helper cells and compounds. But carbohydrates and lipids also have their places.
Obesity, a form of malnutrition, is a risk factor for a variety of chronic diseases, including cardiovascular disease, cancer and type 2 diabetes. One mechanism that links these diseases is inflammation. While malnutrition leads to suppression of the immune system, excess caloric intake leads to obesity, increasing susceptibility to inflammation. And obesity leads to chronic long-term inflammation. The excess accumulation of body fat – adipose tissue – is key to this destructive metabolic state.
More than merely a store of energy, adipose tissue is a dynamic group of cells involved in the regulation of a variety of processes both physiological and pathological, including immunity and inflammation. Adipose tissue frees into circulation chemicals that control food intake, energy balance, insulin action, lipid and glucose metabolism and even the growth and distribution of blood vessels needed to accommodate the increase in tissue.
Fatty acids omega-6 and omega-3 are precursors of compounds called eicosanoids that play an important role in immune response. Omega-3 produces eicosanoids that reduce biomarkers of inflammation: C-reactive protein (CRP), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNFα). Trans fatty acids create an increase of these markers. Some studies show increased intake of fruits and vegetables is associated with a decrease in CRP.
In a landmark women’s health study published in 2008 in the American Journal of Clinical Nutrition, it was shown that higher blood levels of beta-carotene – the plant form of vitamin A – and alpha-carotene, a related antioxidant, were associated with low levels of CRP. In obesity, these fat-soluble carotenoids tend to be associated with adipose tissue and are at low levels in circulation.
Vitamin C is important to the immune system. Some studies show high levels of plasma vitamin C associated with low levels of CRP in obese subjects. In some studies, supplementation with vitamin C reduced CRP levels in plasma. In the women’s health study, higher magnesium (Mg) intake also was associated with lower plasma levels of CRP, IL-6, and TNFα. This effect appeared to be independent of the fruits vegetables and high-fiber foods that tend to be rich in magnesium. Other essential nutrients that function in antioxidant activity, such as vitamin E, zinc and selenium, may also aid in immune function.
One class of phytochemicals called flavonoids is found in fruits and vegetables, as well as herbs, chocolate, tea and dark-colored berries. Many of these are powerful antioxidants. In observational studies, for example the landmark “Nurses’ Health Study,” foods rich in flavonoids were associated with low inflammatory markers.
Phytoestrogens, found in beans and many seeds, are known anti-inflammatory agents, however though some studies demonstrate reduction in inflammatory markers, these results are not consistent. Still, among the many established benefits of beans and seeds, especially soy, are some that at least indirectly aid in immune function.
Perhaps one of the most interesting directions in food and immunity studies has been the science of probiotics, “friendly” bacteria known to be beneficial for a healthy gastrointestinal tract. Probiotics, and the prebiotic, non-digestible soluble fibers that feed them, are associated with overall intestinal health. And intestinal health is a key component of immune defense.
The relationship between diet and immunity is highly complex, as is the relationship between obesity and immunity. The recognition of adipose tissue as an active immune organ could increase our understanding of the relationship between nutrition and the immune system. The promise of emerging ingredients and the research involving them spells promise for formulators creating foods that target immune function.