236

Part II Nutrition Assessment, Consequences, and Implications

TYPES OF FOOD-DRUG INTERACTIONS

Pharmacodynamics is the study of the biochemical and physiological effects of a drug. This might include the binding of the drug molecule to a receptor, enzyme, or ion channel, resulting in the observable physiological response. Some important pharmacodynamic effects include heart-rate response, anticoagulant effect, anal- gesic effect, respiratory depression, cognitive function, and antihypertensive effect. “Since the effect of age on drug sensitivities varies with the drug studied and the response measured, generalizations are often difficult. Studies of drug sensitivity require measurement of concentrations of drug in plasma, as differences in pharmacokinetics with increasing age may increase or decrease differences in response to the drug (eg, anti- coagulants, cardiovascular and respiratory drugs, and psychotropics)” (15).

Pharmacokinetic interactions refer to those that

affect the movement of the drug through the body. Pharmacokinetic studies on the effect of aging on drug absorption have provided conflicting results. “While some studies have not shown significant age-related differences in absorption rates for different drugs, the absorption of vitamin B-12, iron, and calcium through active transport mechanisms is reduced, whereas the absorption of levodopa is increased.” Without consis- tent methods of assessing drug absorption, it will con- tinue to be difficult to determine the true effects of aging. The presence of chronic diseases and the inter- ference of foods can further alter drug pharmacokinet- ics (15).

THE FOUR PARAMETERS OF PHARMACOKINETICS

Absorption

Absorption is the process by which a drug/nutrient pro- ceeds from the site of administration to the blood- stream. Reduced absorption of many nutrients, such as calcium, folic acid, thiamine, and vitamin B-12, is common in the older adult. Most drugs are absorbed in the small bowel. Because of this, gastric emptying and intestinal transit time significantly impact the rate and amount of the oral absorption of drugs and certain nutrients. Other changes such as decreased salivation, decreased gastric acidity, and increased intestinal transit time can all affect absorption, as can food, food components, and nutrition supplements when the drug is administered orally. Drugs that deplete vitamin B-12, vitamin B-6, and folic acid are listed in Boxes 17.2, 17.3, and 17.4, respectively.

BOX 17.2 Drugs That Deplete Vitamin B-12

Cephalosporins Cholestyramine Cimetidine Doxycycline Esomeprazole Famotidine

Fluoroquinolones Kanamycin Lansoprazole Macrolides Metformin Minocycline Neomycin

Distribution

Distribution is the movement of the drug/nutrient from one location to another in the body. Increases in adipose tissue, decreases in total body water, and loss of skeletal muscle in the older adult can affect drug distribution. These changes are more marked in women than in men (6). The rate and extent of blood flow to an organ or tissue strongly affect the amount of drug that reaches the area. This change in body com- position can lead to alterations in the metabolism of drugs related to the volume of distribution and pro- tein-binding capacity. An example related to distribu- tion is serum albumin, the main drug-binding protein

BOX 17.3 Drugs That Deplete Vitamin B-6

Bumetanide Cephalosporins Demeclocycline Doxycycline Estrogens

Fluoroquinolones Furosemide Hydralazine

Hydrochlorothiazide Isoniazid Kanamycin Macrolides

Minocycline Neomycin

Oral contraceptives Oxytetracycline Penicillins Raloxifene Sulfonamides Tetracycline Theophylline Torsemide Trimethoprim

Nizatidine Omeprazole

Oral contraceptives Oxytetracycline Pantoprazole Penicillins Phenytoin Rabeprazole Ranitidine Sulfonamides Tetracycline Trimethoprim