In this work, based on a large population-based sample of cardiovascular disease free people, we revealed a statistically significant dose-response relationship between physical activity and HDL-cholesterol and apolipoprotein A1 levels in women, but not in men, after adjusting for age, body mass index and smoking habits of the participants (Figure 1). Inverse linear associations were also observed between physical activity and triglycerides, LDL cholesterol, total cholesterol, and apolipoprotein B levels. However, the benefits from physical activity on these lipids were, mainly, explained by the observed differences in body mass and smoking habits among the physical activity groups.
Numerous studies documented the relation of increased physical activity with an improved cardiovascular risk factor profile in adults. In particular these studies have shown the overall benefit of physical activity in reducing the risk of developing coronary heart disease, especially due to the reduction of arterial blood pressure levels [8–11], inflammation and coagulation markers [7, 12]. One of the main findings of the present work is the dose-response relationship between physical activity and HDL cholesterol levels, in women. Particularly, women at medium and high physical activity levels had 6% and 9% higher HDL-cholesterol levels, respectively, as compared to sedentary. Similar results were observed in apolipoprotein A1 levels. It is notable that the previous associations were independent from the effect of body mass index, smoking habits and age of the participants. The Pawtucket Heart Study reported that physical activity was significantly associated with lower blood pressure as well as body mass index and higher HDL-cholesterol levels [13]. Moreover, among 3000 adult Japanese men frequency of physical activity was independently and positively related to HDL-cholesterol [14]. Similarly, a pooled analysis among three European cohorts consisting of a total of 402 elderly men demonstrated a significant relation of physical activity with HDL-cholesterol [15]. The same findings were observed in several studies that included only women [16–18]. Our insignificant findings regarding physical activity and HDL or apolipoprotein A1 levels among men, could be attribute to the differences on body mass composition, and smoking habits between physical activity groups. The present study cannot provide causal inference about the effect of the gender interaction with physical activity on HDL cholesterol and apolipoprotein A1 levels. Potential differences on eating habits between men and women, that they were not evaluated in the present analysis, could the answer in the stated hypothesis.
We also evaluated the effect of physical activity on several other lipids levels. Particularly, we observed that compared to sedentary physically active men had a reduction that varied from 3% to 11%, in all lipids measurements. In women, we also observed a reduction starting from 5% to 18%, in all lipids, with the most significant reduction in triglycerides (18%). However, we revealed that the benefits from physical activity on total cholesterol, triglycerides, LDL cholesterol and apolipoprotein B levels were mainly explained by the reduction in body mass index and the decreased smoking habits among physically active men and women. Our findings are in accordance with several other observational studies that reported a non significant effect of physical activity on lipids levels, by the exception of HDL cholesterol and triglycerides [14–19].
Regarding the intensity of physical activity that is needed for a considerable lowering on lipids levels several investigators report that positive changes in HDL cholesterol levels observed in those who were highly exercised, i.e. at least 12 – 15 miles per week [19, 20]. Although, it is difficult to compare results among studies, because different methods were used to classify physical activity levels, our study revealed that even medium physical activity levels (4 – 7 kcal/min, > 3 times per week) was associated with a significant reduction in HDL cholesterol levels, among women. Three or more times per week, along with relatively moderate intensity and duration, are within the ability and tolerance of most middle aged and older individuals.
It is widely accepted that exercise affects blood cholesterol and other lipids in a positive way, by regulating the metabolism of all lipids in the blood. Several studies, including the present, demonstrate that exercise raises HDL-cholesterol in the blood. Another major impact of exercise on lipids is that it lowers triglyceride levels [19]. However, exercises exert a minor impact on the LDL-cholesterol and to a lesser extend, total cholesterol. In fact dietary interventions have much better luck in lowering blood cholesterol levels [19]. The latter could be viewed as disappointing concerning the effect of physical activity. However, some investigators suggest that the way to avoid a decline in HDL-cholesterol that occurs with dieting is to combine diet with exercise. It has been shown that those who decrease their fat intake through dieting, but maintain an active exercise program, note an increase in HDL-cholesterol levels [1, 19]. Thus, if we use the above example, one can easily realize the benefits of reducing cholesterol through dieting while HDL-cholesterol are maintained. As a result, those who exercise regularly have a more favourable lipid profile than those who do not exercise. This may translated to a reduction in the risk for developing atherosclerosis [1]. How much exercise is required to increase HDL-cholesterol levels and improve the overall lipid profile? Our study revealed that even medium exercise is adequate to increase HDL-cholesterol levels, in women.
Our study is a cross section survey. Thus, we cannot provide evidences for causality between physical activity and lipids levels. The self-reported history of weekly physical activity is also a limitation. This work is also limited is assessing the effects of an intensity – duration interaction on lipid and lipoprotein metabolism. In addition, the influence of diet and genetic factors cannot be addressed by our data. All these issues need to be addressed by future interventional exercise studies. Nevertheless, physical activity was associated with reduced lipids levels, both in men and women, but these benefits did not reach statistical significance when adjusted for the effect of exercise on body mass index of the participants and the confounding effect of smoking and dietary habits. On the other hand, substantial increases in HDL cholesterol and apolipoprotein A1 concentrations were observed in women who exercised even at medium levels.