How do medications affect nutrition?

Nutrients can alter the effect of drugs; and drugs can affect nutrition. Foods can increase, delay, or decrease the absorption of a drug. Foods inhibit the absorption of many antibiotics. They can alter the metabolism of drugs; for example, protein-rich foods can accelerate the metabolism of certain drugs by stimulating cytochrome P-450. Grapefruit consumption can inhibit cytochrome P-450, slowing the metabolism of the same drugs. Nutrients that affect bacterial flora can significantly affect the overall metabolism of certain drugs. Certain foods affect the action of drugs. For example, tyramine, a component of cheese and a potent vasoconstrictor, can cause hypertensive crisis in some patients taking monoamine oxidase inhibitors and eating cheese.

Nutrient deficiencies may affect drug absorption and metabolism. Severe energy and protein deficiency reduces tissue enzyme concentrations and may adversely affect drug action by impairing protein absorption or aggregation, causing liver dysfunction. Gastrointestinal tract changes may impair absorption and inhibit drug action. Ca, Mg, or zinc deficiency negatively affects drug metabolism. Vitamin C deficiency inhibits drug-metabolizing enzyme activity, especially in the elderly.

Most medications affect appetite, nutrient absorption, and tissue metabolism. Some medications (eg, metoclopramide) increase gastrointestinal motility, decreasing nutrient absorption. Other medications (eg, opiates, anticholinergics) inhibit gastrointestinal motility.

Some drugs affect mineral metabolism. For example, diuretics (primarily thiazides) and glucocorticoids deplete body K, increasing susceptibility to digoxin-induced cardiac arrhythmias. Repeated use of laxatives also depletes body K. Cortisol, deoxycorticosterone, and aldosterone increase Na and water retention, at least temporarily; water retention is significantly less with prednisolone and some other glucocorticoid analogs. Combined estrogen-progesterone oral contraceptives also increase Na and water retention. Sulfonylureas and lithium may inhibit thyroid iodine uptake and utilization. Oral contraceptives may decrease plasma zinc levels and increase copper levels.

The influence of medications on nutrition

Effect	Medicine
Increased appetite	Alcohol, antihistamines, glucocorticoids, dronabinol, insulin, megestrol acetate, mirtazapine, psychotropic drugs, sulfonylurea, thyroid hormones
Decreased appetite	Antibiotics, bulk reagents (methylcellulose, guar gum), cyclophosphamide, digoxin, glucagon, methindol, morphine, fluoxetine
Decreased fat absorption	Orlistat
Increased blood glucose levels	Octreotide, opiates, phenothiazines, phenytoin, probenecid, thiazide diuretics, glucocorticoids, warfarin
Lowering blood glucose levels	Aspirin, barbiturates, beta blockers, monoamine oxidase inhibitors (MAO), oral antihyperglycemic drugs, phenacetin, phenylbutazone, sulfonamides
Reduction of plasma fat levels	Aspirin and 5-aminosalicylic acid, L-asparaginase, chlortetracycline, colchicine, dextrans, glucagon, nicotinic acid, phenindione, statins, sulfinpyrazone, trifluperidol
Increased plasma fat levels	Adrenal glucocorticoids, chlorpromazine, ethanol, growth hormone, oral contraceptives (combined estrogen-progesterone), thiouracil, vitamin D
Decreased protein metabolism	Chloramphenicol, tetracycline

Certain drugs interfere with the absorption and metabolism of vitamins. Ethanol inhibits thiamine utilization, interferes with the conjugation of isoniazid with nicotinic acid, and interferes with the metabolism of pyridoxine. Ethanol and oral contraceptives block folate absorption. Most patients taking phenytoin, phenobarbital, primidone, or phenothiazines develop folate deficiency, probably due to damage to the liver microsomal enzymes that metabolize these drugs. Folate supplements may reduce the effectiveness of anticonvulsants, but substances in yeast preparations appear to increase folate levels without reducing efficacy. Anticonvulsants may cause vitamin D deficiency. Decreased absorption of vitamin B ₁₂ may occur with aminosalicylic acid, slowly absorbed K iodide, colchicine, trifluopyrosine, ethanol, and oral contraceptives. Oral contraceptives with high progestogen content may cause depression, probably due to a tryptophan deficiency that develops during drug metabolism.

Nutrient metabolism can also be affected by other dietary substances. For example, the absorption of non-heme iron is affected by many nutrients, which can decrease or increase absorption.

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