How do drugs affect food?

Nutrients can change the effect of medications; and, conversely, medications affect nutrition. Food products can increase, delay or reduce absorption of the drug. Food products depress the absorption of many antibiotics. They can change the metabolism of drugs; for example, protein-rich foods can accelerate the metabolism of certain drugs, stimulating cytochrome P-450. Consumption of grapefruit can inhibit cytochrome P-450, slowing the metabolism of the same drugs. Nutrients that affect the bacterial flora can significantly affect the overall metabolism of certain medications. Some certain foods affect the effects of medications. For example, tyramine, a cheese component and a powerful vasoconstrictor, can cause hypertensive crisis in some patients who take monoamine oxidase inhibitors and eat cheese.

Deficiency of nutrients can affect the absorption and metabolism of the drug. Severe energy and protein deficiency reduces the concentration of tissue enzymes and can adversely affect the action of drugs, disrupting the absorption or aggregation of proteins, causing liver dysfunction. Changes in the digestive tract can disrupt absorption and inhibit the action of the drug. Deficiency of Ca, Mg or zinc adversely affects the metabolism of drugs. Deficiency of vitamin C inhibits the activity of enzymes that metabolize medicines, especially in the elderly.

Most drugs affect appetite, absorption of nutrients and tissue metabolism. Some drugs (for example, metoclopramide) increase peristalsis of the digestive tract, reducing the absorption of nutrients. Other drugs (eg, opiates, anticholinergic drugs) depress the peristalsis of the gastrointestinal tract.

Some drugs affect the metabolism of minerals. For example, diuretics (primarily thiazides) and glucocorticoids decrease the K content in the body, increasing the susceptibility to digoxin-induced cardiac arrhythmias. Repeated use of laxatives also reduces the K content in the body. Cortisol, deoxycorticosterone and aldosterone increase the Na content and water retention in the body, at least temporarily; water retention is observed significantly less with the use of prednisolone and some other analogues of glucocorticoids. Combined estrogen-progesterone oral contraceptives also increase Na concentration and water retention in the body. Preparations of sulfonylurea and lithium can inhibit the accumulation and realization of iodine by the thyroid gland. Oral contraceptives can reduce the level of zinc in the plasma and increase the level of copper.

Influence of medicines on nutrition

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

Certain drugs affect the absorption and metabolism of vitamins. Ethanol inhibits the utilization of thiamine, breaks the compound of isoniazid with nicotinic acid and the metabolism of pyridoxine. Ethanol and oral contraceptives block the absorption of folate. In most patients taking phenytoin, phenobarbital, primidone or phenothiazines, folate deficiency develops, probably caused by damage to microsomal hepatic enzymes that metabolize these drugs. Folate supplements may reduce the effectiveness of anticonvulsants, but substances that make up yeast preparations appear to increase folate levels without compromising efficacy. Anticonvulsants can cause vitamin D deficiency. Reduction in absorption of vitamin B ₁₂ can be observed with aminosalicylic acid, slowly absorbed iodide K, colchicine, trifluopyrozine, ethanol and oral contraceptives. Oral contraceptives with a high content of progestogens can cause depression, probably because of a deficiency of tryptophan, which develops during the metabolism of drugs.

The metabolism of nutrients can also affect other dietary substances. For example, the absorption of iron that is not part of the heme is affected by many nutrients that help reduce or, conversely, increase absorption.

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