Grapefruit: no link to breast cancer

Q:Is it true that women should avoid grapefruit because of a possible link to breast cancer?

A: Research has raised questions, but does not currently support women completely avoiding this nutritious fruit. We already know that one or more compounds found in grapefruit can inhibit a liver enzyme that metabolizes certain medications, resulting in higher blood levels of those drugs.

Scientists believe these compounds may act in the same way to decrease estrogen metabolism and raise its blood levels. One major population study in 2007 found that postmenopausal women who ate the most grapefruit — about half a grapefruit every other day — had a 30 percent higher risk of breast cancer than those who ate none. The link remained after adjusting for weight, exercise, hormone replacement therapy and family history. However, the researchers emphasized that confirmation by other studies is needed, and in 2009 another major, and much larger, study found no link between the same level of grapefruit consumption and breast cancer risk or estrogen levels.

Grapefruit is a good source of vitamin C and several antioxidant phytochemicals. One of them, naringenin, has been shown to inhibit growth of breast and several other cancer cells in laboratory studies. So for now, unless a woman’s doctor tells her otherwise or if she is taking medications that necessitate avoidance of grapefruit, she can reasonably include grapefruit as one fruit among the recommended wide variety of healthy fruits.

Q:Does the increased calorie burning following exercise really last all day?

A: No. Reports that metabolism stays elevated for extended periods after exercise are untrue. Although metabolism speeds up briefly following moderate or vigorous activity, research shows that most people’s metabolic rate returns to normal within 15 to 20 minutes after moderate activity and 30 to 45 minutes following vigorous activity. Beyond what is used during the activity itself, extra calories burned by a briefly increased metabolic rate add no more than 10 to 25 calories to the day’s total. That’s equal to the calories in five to 12 grapes. Even in people with low physical fitness who show some differences in metabolic response to activity, post-exercise calorie burning is still unlikely to add up beyond this. In addition to planned exercise, focus on increasing activity throughout your day to increase the calories you burn, and remember that the biggest impact on the balance between calories in and calories out usually comes from adjustments in what and how much you eat.

Karen Collins is a registered dietician with the American Institute for Cancer Research, a cancer charity that fosters research on the relationship of nutrition, physical activity and weight management to cancer risk, interprets the scientific literature and educates the public about the results.