Influences of diet and nutrition on clinical pharmacokinetics

Enzyme induction and inhibition

The rate and extent of drug metabolism significantly influences drug effect. Enzyme induction by increasing the metabolism of drugs may result in important drug interactions. Other implications of enzyme induction include alterations in the metabolism of endogenous substrates, vitamins and activity of extrahepatic enzyme systems. Similarly a wide range of drugs may produce clinically significant drug interactions following enzyme inhibition. Assessment of enzyme induction and inhibition in man involves diverse methods including the use of model drugs.

Dietary and pharmacological control of estradiol metabolism in humans

Clinical research has demonstrated that increased or decreased estradiol 2-hydroxylation can easily be achieved with a number of experimental approaches. In contrast, estradiol 16 alpha-hydroxylation, which may have potentially deleterious effects in estrogen-dependent tissues, cannot be readily altered. Predictable hormonal consequences have thus far been found in response to the modification of 2-hydroxylation. This approach offers promise as a method for specifically altering the risk for diseases associated with either too little estrogen (osteoporosis) or too much estrogen (breast and uterine cancer).

Effect of dietary protein on renal tubular clearance of drugs in humans

Diet is one of many factors that influence the pharmacokinetics of drugs. The level of protein intake has been found to significantly influence drug metabolism and glomerular filtration, both of which play an important role in the clearance of drugs. Recently, a marked change, resulting from restricted dietary protein intake, has been reported in the handling of several drugs which are reabsorbed and/or secreted by the renal tubules. In studies of healthy volunteers on protein-restricted diets the renal clearance and fractional excretion of model compounds have been altered, falling to 30% of values obtained on normal diets in the case of the weak acids oxipurinol and uric acid; the fractional excretion of the weak base cimetidine has been increased by 30%. These studies have also found that the change in the renal clearance of both acids is sustained with prolonged dietary protein-calorie restriction, and that, for oxipurinol, the magnitude of the change is directly related to the quantity of protein in the diet, the change is related specifically to the protein content in the diet (and not the total calories), the onset of change is rapid, and on a low-protein diet the renal clearance undergoes marked diurnal variation. The mechanism for the alteration in tubular function is not clear, but may be related to renal haemodynamic changes or competition for transport associated with protein intake. Regardless of the mechanism, these results have important implications for pharmacokinetic research and clinical practice.