Immune Functions And Resistance To Infections

 

Immune suppression caused by lack of nutrients has been called nutritionally acquired immunodeficiency syndrome, or NAIDS. Lack of zinc, of which only about 1 mg is needed every day to replace losses, leads to virtual disappearance of the thymus, a key immunological organ. Thus micronutrient deficiencies often increase risk and severity of infectious diseases. Since infections also impair nutritional status, then we have a synergistic two-way relationship or a vicious circle (see Figure 1). Nevertheless, even if a specific nutritional deficiency impairs immunity, administration – especially if unphysiological doses or routes are used – of that nutrient may favor the pathogen more than the host, and hence increase the risk and severity of infectious diseases. For example, parenteral and even oral administration of iron may stimulate growth of bacteria or parasites and lead to increased morbidity. Vitamin A, for example, is essential to a range of immune functions, and large randomized trials have found that periodic administration of vitamin A capsules to children under 5 years reduces mortality by 23–30%. Vitamin A supplementation, given in large doses two or three times per year, is therefore recommended by the WHO. Given the considerable effect of vitamin A on mortality, the adequacy of dosing only a couple of times per year, often in combination with national immunization days, and the low price per capsule, makes vitamin A supplementation one of the most cost-effective health interventions.

Zinc supplementation may prevent diarrhea and pneumonia in populations with an inadequate zinc intake, and has also been shown to reduce the duration and severity of diarrhea when given therapeutically. In contrast to vitamin A, zinc has to be taken frequently if not daily, since there are no body stores. Thus, zinc supplementation may be feasible in the treatment of children hospitalized or attending health facilities due to diarrhea but is not a feasible intervention in the prevention of diarrhea and pneumonia, even though these diseases account for a large proportion of the 10 million child deaths each year.
Nutrition And HIV Infection
The biological two-way relationship between nutrition and HIV infection is complex.
HIV Infection Impairs Nutritional Status
HIV infection has direct effects on nutritional status, in addition to the effects on food security described later. HIV infection increases nutritional requirements by reducing absorption and increasing utilization and loss of nutrients, and resting energy expenditure. Yet, the intake of nutrients and energy may be reduced, due to loss of appetite. Depending on the dietary intake of nutrients and energy, micronutrient status may be impaired and fat and particularly lean body mass lost even at early stages of the infection. With more advanced HIV infection, when opportunistic and other infections occur, resting energy expenditure is further increased, whereas food intake may be further reduced, due to painful sores and infections in the mouth and esophagus.

If the additional energy requirements are not met, then weight will be lost, unless physical activity is reduced, which may impair household food security (see Figure 1).

If the additional nutrient requirements are not met, then specific deficiencies will eventually occur, or existing deficiencies will become exacerbated, which will impair important body functions, including maintenance of lean body mass and immune functions. An exception is iron. Although absorption of iron may be impaired, even early HIV infection results in anemia of infection, which is due to suppression of the production of red blood cells in the bone marrow. But red blood cells will continue to age, and eventually be engulfed by white blood cells and taken to the stores, where iron will accumulate as it is now longer incorporated into new red blood cells.

Energy requirements are estimated to be 10% higher during asymptomatic HIV infection, to maintain body weight and physical activity, and they are 30% higher during symptomatic HIV infection (WHO, 2003). In contrast, protein requirements do not seem to be increased. The requirements for most vitamins and minerals are increased, and it is recommended that patients with HIV eat a healthy diet. In practice, many patients are advised to take a daily supplement containing one recommended dietary allowance (RDA) of the essential vitamins and minerals, except iron.

But the extent to which the requirements are increased by HIV infection is still not clear, as it will be different for different micronutrients and depend on the stage of HIV infection. Nevertheless, recommendations should not only be based on which intakes are necessary to avoid deficiency, but on which intakes give optimal health, and prevent progression of HIV infection.