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Letter to the Editor |
Department of Nutrition, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115
Dear Sir:
Hardman's effort to expand the discussion of interactions between diet and exercise on blood lipids is appreciated. As I had commented, and Hardman agrees, there is no specific evidence to support Gibney's assertion that the HDL-lowering, triacylglycerol-raising effects of high-carbohydrate diets are totally negated by 30 min of brisk walking daily (1, 2). Hardman noted correctly that there are some data that bear indirectly on this issue. However, it is first important to clarify the question. Was Gibney implying that the effects of diet and exercise (30 min of walking) are simply additive (but in opposite directions), or that diet has no effect in combination with exercise (an interaction)? Gibney was unclear, but the latter seems to have been implied because, in an additive model, there would still be an effect of diet in persons who exercise, even if it were counterbalanced by the effect of exercise. In other words, the additive model would predict that the most favorable HDL and triacylglycerol concentrations would be seen in persons who exercised and did not consume high-carbohydrate, low-fat diets.
Hardman cited 2 earlier studies by Wood et al (2, 3) but, surprisingly, omitted more recent work of those authors (1) that is more informative because it included 4 groups (exercise alone, diet alone, both exercise and diet, and neither diet nor exercise). This study thus allowed the independent and interactive effects of diet and exercise to be estimated. As in the earlier studies by Wood et al, the exercise program included both brisk walking and jogging, which is probably somewhat more aerobic than is brisk walking alone. Also, the dietary intervention [National Cholesterol Education Program (NECP) Step II diet] was not purely a reduction in percentage of energy from fat, because the fatty acid composition was changed and the intervention included a modest weight reduction. Probably because of the reduction in weight, fat reduction and weight loss had a very small net effect on triacylglycerol in women and reduced triacylglycerol in men. Nevertheless, the relative effects of diet on blood lipids in the exercise and nonexercise groups can be evaluated. The findings related to triacylglycerol and HDL cholesterol are shown in Table 1
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Gibney and Hardman should not be satisfied that exercise merely compensates for the adverse effects of low-fat diets on blood lipids, because most people would not be happy to learn that, despite a major change in diet, regular aerobic exercise, and modest weight loss, they had not measurably altered their CHD risk. From numerous metabolic studies (4) we know that replacement of saturated and trans fats with nonhydrogenated unsaturated fats reduces LDL cholesterol without increasing triacylglycerol or reducing HDL cholesterol. Although further investigations of exercise in combination with diet are needed, the effects of this dietary change and exercise on blood lipids would both be in the beneficial direction, so the combination should provide the greatest benefit.
REFERENCES
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