Effets du taux de perte de poids suite à un programme de perte de poids de 5 semaines sur les changements de la composition corporelle et du profil métabolique chez des femmes post-menopausées, obèses et sédentaires
Obesity, worldwide epidemic, affects one third of all Canadian women over 50. Menopause is a critical period for these women in particular when accompanied by overweight. In fact, hormonal changes and waist enlargement are associated with metabolic deteriorations and multiply risks of heart diseases considerably. Therefore, it seems important to make extra efforts in primary and secondary prevention in order to decrease the negative impacts of obesity in postmenopausal women. Some late studies demonstrate that a loss of 10 to 15% of initial body weight is efficient to improve the metabolic profile of obese individuals. However, postmenopausal women have been under-represented in weight loss studies. This is a surprising fact considering that this particular group (1) has a higher risk of cardiovascular diseases compared to younger women and (2) can also benefit significantly from a weight loss program aimed at improving the overall health profile. Moreover, when dieting without exercising, almost 25% of the weight loss will be of lean body mass, which can contribute to a decrease in resting metabolic rate. General guidelines for the treatment of obesity in the adult population recommend a weight loss of 1 to 2 lbs per week. Except for the fact that there is no extra benefit for health at higher rates of weight loss, there is no other rational explanation related to those recommendations. Moreover, it suggests a combination of caloric restriction and exercise program, the latter not always feasible, particularly with obese elderly with functional limitations. Despite the abundant literature on weight loss, no study has specifically investigated the impact of the rate of weight loss by diet alone on the body composition and the resting metabolic rate, which are major contributing factors of long-term weight loss maintenance. The goal of the project was to study the effects of the rate of weight loss during a hypocaloric diet, without exercise, on changes in body composition (fat mass and lean body mass) and the metabolic profile (resting arterial blood pressure, lipid profile and resting metabolic rate) in sedentary obese postmenopausal women. For this purpose, 20 obese (baseline % body fat: 46.4 « 5.1 %), postmenopausal (no menses for 12 consecutive months) and sedentary women (< 2 bouts of moderate physical activity/week), aged between 51 and 74 years, (61.8 « 5.9 years) were recruited to participate in a weight loss program. The general goal of the study was to induce, if possible, a weekly loss of weight equal to 1% of their initial weight. Subjects were afterward characterized according to their rate of weight loss; 9 subjects were in the low rate group (-0.44 to -0.72 kg/week; n = 9) and 11 subjects were in the high rate group (-0.74 to -1.38 kg/week; n = 11). Measurements of the fat mass and the lean body mass (by DXA), the resting metabolic rate (by indirect calorimetry) and the fasting lipid profile (total cholesterol, triglycerids and LDL-cholesterol) were taken before and after 5 weeks of weight loss. As expected, the high rate group showed a greater weekly loss of body weight compared to the low rate group (-0.96 « 0.24 kg/week vs -0.62 « 0.10 kg/week; P = 0.0002). Despite a significant difference in the total weight loss after 5 weeks (high rate : -4.8 « 1.2 kg vs low rate : -3.1 « 0.5 kg; P < 0.01), both groups showed similar decreases in fat mass (high rate : -0.63 « 0.15 kg/week vs low rate : -0.56 « 0.18 kg/week; P = NS) during the 5 weeks of the program. The difference in the weekly total weight loss between both groups was explained by the significantly higher rates of lean body mass loss in the high rate group compared to the low rate group (-0.33 « 0.23 kg/week vs -0.05 « 0.19 kg/week; P = 0.02). After adjustments for age and lean body mass at baseline, a significant and positive correlation was observed between the rate of body weight loss (kg/week) and changes in lean body mass (kg/week) (r = 0.71; P = 0.002). No association was observed between the rate of weight loss and changes in fat mass after adjustments for age and fat mass at baseline (r = 0.20; P = NS). Significative improvements of diastolic blood pressure, plasma total cholesterol and triglycerids were observed in both groups (P < 0.05). Furthermore, only the high rate weight loss group demonstrated a significative diminution of systolic arterial blood pressure (P < 0.05) and LDL-cholesterol. However, no difference was observed between both groups for all these variables. Finally, results regarding changes in resting metabolic rate showed no differences between groups after the 5-week program, as well as no correlation with changes in lean body mass and fat mass. The results of this present study demonstrate that a hypocaloric diet with a rate of weight loss > or = 0.74 kg/week (or 1.6 lb/week) does not seem to produce extra benefits on the metabolic profile or to be related to a greater loss of fat mass after 5 weeks. In fact, the greater decreases of body weight in the high rate group may be explained in part by a loss of lean body mass. Based on our observations, it seems that current guidelines which recommend an energy deficit leading to weight losses up to 0.91 kg per week (2 lbs per week) may not be optimal for obese postmenopausal women when no exercise is added to the weight loss program. Two alternatives appear from our results: first, a more conservative approach with a maximal rate of weight loss of 0.6 to 0.7 kg per week (or 1.3 to 1.5 lb per week) seems to be optimal to better preserve lean body mass when no exercise is incorporated in the weight loss program. Secondly, in the case of a caloric restriction aiming at a weight loss of more than 0.7 kg/week, the addition of an exercise program should be considered in order to minimize the loss of muscular mass. Other studies are therefore necessary to validate these hypotheses.