The U.S. Department of Agriculture reports that grapefruits are the second most popular citrus fruits consumed by Americans. They are rich sources of vitamins A and C, potassium, folic acid and fiber. A study published in the March 2006 issue of "Journal of Medicinal Food" suggests that grapefruit even helps with weight loss and blood sugar control in people with metabolic syndrome. However, grapefruit interferes with the absorption and metabolism of many medications. Toprol does not appear to be affected by grapefruit consumption.
Cytochrome Enzymes
Your intestine is an important barrier between your bloodstream and a potentially toxic environment. The cells in your liver and gastrointestinal tract are endowed with iron-containing enzymes called cytochromes that help to detoxify substances in your diet before they are absorbed and introduced to your circulation. Hundreds of cytochromes have been identified, and their substrates -- the substances they metabolize -- have been identified in many cases.
Cytochrome Inhibition
An August 2006 "American Family Physician" review states that grapefruit interferes with a specific intestinal cytochrome designated CYP 3A4. By blocking the activity of CYP 3A4, grapefruit slows the degradation of at least 30 medications, according to The People's Pharmacy. Several beta-blockers, which are used to treat high blood pressure, abnormal heart rhythms and other medical problems, are among the drugs affected by grapefruit. In many instances, these drugs' effects are magnified by concurrent use of grapefruit. Toprol, or metoprolol, is a beta-blocker whose metabolism appears to be unaffected by grapefruit.
Different Cytochromes
Metoprolol is prescribed for the treatment of high blood pressure, congestive heart failure, coronary artery disease, cardiac arrhythmias, migraine headaches and some forms of anxiety. Although it, too, is broken down by the cytochrome system of enzymes, metoprolol is metabolized by CYP 2D6, according to researchers at Global Metabolism and Pharmacokinetics, a division of drug maker Sanofi-Aventis. Unlike CYP 3A4, CYP 2D6 is apparently not affected by grapefruit.
Considerations
Grapefruit products inhibit the activity of a specific intestinal cytochrome enzyme called CYP 3A4. If you eat grapefruit or drink grapefruit juice, drugs that are metabolized by CYP 3A4 typically reach your bloodstream in higher amounts than they normally would, potentially leading to increased side effects. Although some beta-blockers are metabolized by CYP 3A4, Toprol is not. Therefore, Toprol's absorption and metabolism are essentially unchanged by grapefruit consumption. If you take a prescription medication, you should check with your doctor or pharmacist before consuming grapefruit or grapefruit juice.
References
- U.S. Department of Agriculture Economic Research Service
- "Journal of Medicinal Food"; The Effects of Grapefruit on Weight and Insulin Resistance: Relationship to the Metabolic Syndrome; K. Fujioka et al.; March 2006
- "American Family Physician"; Management of Grapefruit-Drug Interactions; A.L. Stump et al.; August 2006
- The People's Pharmacy: Graedon's Guide to Grapefruit Interactions
- "British Journal of Clinical Pharmacology"; Pharmacokinetic Assessment of a Five-Probe Cocktail for CYPs 1A2, 2C9, 2C19, 2D6 and 3A; S. Turpault et al.; December 2009
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