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Letters to the Editor |
Harvard School of Public Health, Department of Nutrition, 665 Huntington Avenue, Boston, MA 02115, E-mail: dosulliv{at}sph.harvard.edu
Dear Sir:
I appreciate Gibney's call to consider seriously the placement of potatoes at the apex of the dietary pyramid along with sweets, to be eaten sparingly. Of course, the humble potato deserves honor for keeping famine at bay in Ireland and in the United States during economic depressions, and it is a staple in some countries today. However, this does not necessarily mean that consumption of potatoes as a major energy source is optimal for health and longevity in contemporary Western societies. Nor does this history prove that potatoes have intrinsic positive health benefits, as is implied by their inclusion as a vegetable in the US dietary pyramid. Although the topic cannot be fully considered in the context of a letter, 2 general points deserve consideration.
First, as a major energy source, potatoes displace other foods with higher nutritional values, in particular, vegetables and whole grains. In a major review, Steinmetz and Potter (1) found consistent evidence that foods usually considered to be vegetables were associated with lower risks of cancer; in striking contrast, direct and inverse associations were seen with equal frequency for potatoes, as would be expected by chance. Similarly, in the extensive review of diet and cancer conducted by the World Cancer Research Fund (2), no evidence of benefit was seen for potato consumption, again in contrast with the findings for vegetables. Moreover, a large and consistent body of epidemiologic data shows that higher consumption of whole grains and cereal fiber (which will be low if potatoes are the staple carbohydrate) is associated with reduced risks of coronary artery disease (3).
Second, apart from preventing energy deficiency (hardly an issue in Western countries, where obesity is the dominant nutritional problem), potatoes have adverse metabolic effects on health. Carbohydrate ingestion per se is not harmful (virtually all diets will have carbohydrates as the major energy source), but some carbohydrate-containing foods are more healthful than others. Potatoes, along with white bread, have a nearly maximal glycemic index (4), higher than that of table sugar. Thus, these foods raise insulin concentrations and C-peptide excretion to a greater extent than do foods that contain identical amounts of carbohydrates but with lower glycemic indexes (5). Hyperinsulinemia is independently predictive of coronary artery disease risk (6) and is associated with hypertriglyceridemia and low HDL-cholesterol concentrations. In both men and women, consumption of potatoes is associated with higher risk of type 2 diabetes (7, 8). In contrast, consumption of cereal fiber and whole-grain foods is associated with reduced risk.
Gibney asserts that 30 min of brisk walking can totally negate the adverse metabolic effects of high-carbohydrate diets, but offers no evidence. Certainly, physical activity has many benefits, including reducing insulin resistance, and should lessen the adverse effects of a high glycemic load (9). However, adverse effects of high glycemic loads are likely to be present even with 30 min of walking, which is after all, modest compared with the physical activity of traditional agriculturalists. Specifically, to address the issue of these metabolic effects in the context of low insulin resistance, West et al (10) studied 8–9-y-old boys in 12 countries, including developing nations. Even in this group, the percentage of energy from carbohydrate intake was directly correlated with serum triacylglycerol concentrations and inversely correlated with serum HDL-cholesterol concentrations. Contrary to Gibney's implication, advocating the consumption of whole grains and vegetables is not a defense of inactivity; both good diets and regular exercise are essential for optimal health. In summary, abundant metabolic and epidemiologic evidence support the conclusion that those who consume potatoes (or white bread and white rice) as their staple would be healthier if they replaced, to the extent feasible, this source of carbohydrate with whole grains and vegetables.
REFERENCES
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