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Loughborough University, Sports Science and Recreation Management, Department of Physical Education, Loughborough, Leicestershire LE11 3TU, United Kingdom
Dear Sir:
In letters published in the October 1999 issue of the Journal, Gibney (1) and Willett (2) both referred to the putative role of physical activity in counteracting the decrease in plasma HDL cholesterol and the increase in plasma triacylglycerol associated with high-carbohydrate diets. However, neither of these distinguished scientists cited evidence in support of their (conflicting) views, and this issue deserves a more objective airing.
The potential of physical activity to maintain favorable blood lipid profiles in the face of high-carbohydrate diets is clear: endurance-trained athletes have low plasma triacylglycerol concentrations and high HDL-cholesterol concentrations (3) despite consumption of diets high in carbohydrate—both in absolute terms and when expressed as a proportion of energy intake (4). From a public health standpoint, it is of course important to examine the efficacy of lower levels of exercise, and I am not aware of specific evidence for Gibney's assertion that "These [HDL-lowering, triacylglycerol-raising] effects of high-carbohydrate diets are totally negated by moderate physical activity on the order of 30 min of accumulated brisk walking." On the other hand, several observations suggest that it may not be wide of the mark. Sedentary persons who become physically active increase their carbohydrat intake but still experience an increase in HDL-cholesterol concentrations. For example, in middle-aged men who began jogging [building up over 2 y to an average of 19.2 km (12 miles)/wk, probably <20 min/d], HDL cholesterol increased by 0.1 mmol/L (>8%); over the same period, carbohydrate intake increased by 70 g/d and from 38.7% to 43.8% of energy intake (5). More compelling evidence comes from Wood's randomized, controlled intervention study of overweight men and women, which determined the effects of moderate exercise on changes in plasma lipoproteins with a low-fat, high-carbohydrate diet (National Cholesterol Education Program Step I) (6). In men, decreases in plasma triacylglycerol and the increases in plasma HDL cholesterol were significantly greater in those who walked or jogged an average of 2.1 km (1.3 miles)/d—say 15–20 min of activity—than in the diet-only group. In women, only the diet-plus-exercise group showed a significant decrease in plasma triacylglycerol and a significant increase in HDL cholesterol compared with sedentary control subjects; these women reported walking or jogging an average of 1.6 km (1 mile)/d, <20 min of activity.
Evidence specifically for walking, mentioned in the recent Surgeon General's Report (7) as one way to achieve the target of 30 min daily of moderate exercise, is lacking and sorely needed. We showed recently that this amount of walking, accumulated in 10-min bouts, decreases daylong postprandial plasma triacylglycerol concentrations in subjects consuming meals representative of the typical diet in Northern Ireland (8). However, whether this is sufficient to compensate for the increase in lipemia expected with a high-carbohydrate diet is not known.
At present, therefore, there is no clear-cut answer to the question, "Can moderate amounts and intensities of physical activity counteract the potentially adverse changes to plasma lipoprotein lipids attributable to a low-fat, high-carbohydrate diet?" The answer will, of course, depend on both the level of physical activity engaged in and the extent of dietary change.
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