 Citrulline is an amino acid produced in the intestine and in the liver, but the liver does not contribute significantly to circulating citrulline concentrations. The intestine is thus the only organ that normally releases significant amounts of citrulline into the blood stream. The investigators have designed a study looking at the value of measuring plasma citrulline concentration in patients with tropical enteropathy of mixed HIV status. The focus will be on the ability of the intestine to sustain the individual concerned from a nutritional standpoint. The investigators hypothesise that plasma citrulline concentration is a marker of small bowel absorptive integrity and an appropriate surrogate for HIV related enteropathy. 