Despite repeated calls for us to be more active in order to counter rising rates of overweight and obesity, the evidence suggests that activity has, generally speaking, very modest effects on body weight. I wrote quite recently about this here and here.
Generally, two main reasons for why exercise does not promote significant weight loss are cited: the calorie burn during exercise is generally quite small, and people who exercise more tend to eat more too. The result? It’s generally difficult to get into caloric deficit through exercise alone. A recent piece of research appears to have unearthed another reason why exercise has generally little impact where weight loss is concerned.
In this study, 34 overweight and obese sedentary women (average age 31) were put on an exercise programme for a period of 8 weeks. The women were instructed to exercise for a total of 150 minutes a week at a heart rate of 135-145 beats per minute (about 75 per cent of maximum heart rate for these women). The women were assessed for a number of things including weight, body mass index and fat mass.
Overall, average weight loss was 0.15 kg (1/3 of a pound). Fat loss was an average of 0.0 kg. Basically, for the group as a whole, the exercise turned out to be thoroughly ineffective from a weight and fat loss perspective.
The authors of this study noted, however, that while average fat loss was negligible, it varied from a loss of 3.2 kg to a gain of 2.6 kg (yes, that’s right, some women gain quite considerable quantities of weight on their exercise regime). So, the authors of the study then decided to do further analysis by splitting up the group into those who lost more weight than expected (the ‘responders’) and those who lost less weight than expected (the ‘non-responders’).
Further analysis, including measurements of resting metabolic rate and total energy expenditure revealed that the lack of response to exercise was mostly explained by a reduction in activity generally. In other words, some women upped their formal exercise, but then just slowed down when they were not exercising (the ‘responders’ actually increased their activity outside of the formal exercise programme).
This study was hampered somewhat by not having a proper control group (i.e. a group not exercising). However, it again provides evidence to support the notion that, generally speaking, aerobic exercise is not very effective for the purposes of weight/fat loss. It should be said, however, that in this study did find that exercise appeared to promote fat loss in some women. And, it should perhaps be noted, women in both responder and non-responder groups saw a reduction in waist circumference, which suggests that even in those not losing fat, some healthy fat redistribution is going on.
This research also provides another potential explanation for why aerobic exercise has generally been found to be ineffective for the purposes of weight loss: in addition to relatively small calorie burn and a tendency to cause individuals to eat more, upping exercise may lead some to become more sedentary in their daily lives.
1. Manthou E, et al. Behavioral Compensatory Adjustments to Exercise Training in Overweight Women Medicine & Science in Sports & Exercise 2010;42(6):1121-1128