Tuberculosis

Contents

Nutrition and Susceptibility Nutritional Management

Nutrition and Susceptibility

J P Cegielski, Centers for Disease Control and Prevention, Atlanta, GA, USA D N McMurray, Texas A&M University, College Station, TX, USA

Published by Elsevier Ltd.

The oral traditions of medicine and public health state that malnutrition is an important risk factor for the development of tuberculosis (TB). Malnutrition profoundly affects cell-mediated immunity (CMI), and CMI is the principal host defense against TB. It makes biological sense. Although most health professionals readily accept this principle, much of this belief is based on uncontrolled observations such as disaster situations or on backwards logic from the cachexia common among TB patients. In fact, the evidence in humans is surprisingly thin from the perspective of scientific rigor. Also, few data, if any, quantify the extent of the relative or attributable risk of TB due to malnutrition. Moreover, data from experimental animals, until recently, were based on animal models that largely were not relevant to human TB infection and disease. Malnutrition may account for a greater population-attributable risk of TB than HIV infection, and certainly a much more correctable one.

Malnourished individuals have an increased likelihood of primary or latent infection progressing to active disease. In populations with substantial latent TB infection, the occurrence of malnutrition may be an important determinant of the incidence of TB. The potential public health impact of malnutrition on the global incidence of TB was summarized in the US Surgeon General's 1988 Report on Nutrition and Health, which emphasized that malnutrition was the leading cause of acquired, correctable immune system dysfunction throughout the world. The United Nations Food and Agriculture Organization (FAO) estimated that 841 million people in developing countries, or 20% of the world's 1990-1992 population, were undernourished. Modest decreases in resistance affecting such large numbers of people may result in substantial increases in TB incidence at a population level. Population groups at highest risk for poor nutrition are also at high risk for TB, with poverty being the common denominator.

In general, there are three streams of evidence relating the risk of TB to malnutrition: observations in humans, experimental work in animal models, and inferences from related work in microbiology and immunology. In humans, direct evidence for the risk of TB due to malnutrition is sparse, and the data have not been reviewed critically in more than three decades. In vitro studies have generated a substantial body of evidence documenting the negative effects of malnutrition on cell-mediated immune function and on the immunology of TB. Although one can reason from the in vitro evidence, it cannot replace in vivo data. This article summarizes the evidence from observations in human populations and from experimental animal models with relevance to human TB.

TB risk is of two kinds: the risk of becoming infected with Mycobacterium tuberculosis and the risk of the infection progressing to TB disease. This article focuses on the risk of infection progressing to disease because that is where CMI comes into play. There is no evidence of a direct relationship between malnutrition and the risk of initial infection. Although both TB and malnutrition are linked with poverty, the data reviewed here suggest no independent association between malnutrition and primary or latent TB infection.

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