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Sixty Minutes of Physical Activity for Women Required Just to Maintain Weight?

Apr 2, 2010

Sixty minutes daily of physical activity seven days a week is recommended for women just to maintain weight….

The amount of physical activity needed to prevent long-term weight gain is unclear. In 2008, federal guidelines recommended at least 150 minutes per week (7.5 metabolic equivalent [MET] hours per week) of moderate-intensity activity for “substantial health benefits.”

The study was done to examine the association of different amounts of physical activity with long-term weight changes among women consuming a usual diet.

A prospective cohort study involved 34,079 healthy US women (mean age, 54.2 years) from 1992-2007. At baseline and months 36, 72, 96, 120, 144, and 156, women reported their physical activity and bodyweight. Women were classified as expending less than 7.5, 7.5 to less than 21, and 21 or more MET hours per week of activity at each time. Repeated-measures regression prospectively examined physical activity and weight change over intervals averaging 3 years.   The main outcome measure was a change in weight.

Women gained a mean of 2.6 kg throughout the study. A multivariate analysis comparing women expending 21 or more MET hours per week with those expending from 7.5 to less than 21 MET hours per week showed that the latter group gained a mean (SD) 0.11 kg (0.04 kg; P = .003) over a mean interval of 3 years, and those expending less than 7.5 MET hours per week gained 0.12 kg (0.04; P = .002). There was a significant interaction with body mass index (BMI), such that there was an inverse dose-response relation between activity levels and weight gain among women with a BMI of less than 25 (P for trend < .001) but no relation among women with a BMI from 25 to 29.9 (P for trend = .56) or with a BMI of 30.0 or higher (P for trend = .50). A total of 4540 women (13.3%) with a BMI lower than 25 at study start successfully maintained their weight by gaining less than 2.3 kg throughout. Their mean activity level over the study was 21.5 MET hours per week (approx 60 minutes a day of moderate-intensity activity).

This study involved only exercise habits, not diet which is a crucial part of both losing weight and preventing weight gain. The study claims these women were following a ‘usual diet,’ but what does that mean? Maintaining a balance of calories can be accomplished with both exercise and diet. Using only exercise without changing how you eat is better than nothing, but if you’re eating an unhealthy diet, you’ll absolutely have to exercise more to offset the extra calories and prevent weight gain.

The study concluded that the women averaged 60 minutes of moderate exercise a day, but this broad generalization doesn’t take into account different intensity levels. They actually based this amount of exercise on how many METs (Metabolic Equivalent Tasks) the women expended each week. A MET is a ratio that compares your metabolic rate while at rest to other activities. So that means when you’re at rest, you expend 1 MET every hour. Taking a stroll might burn twice as much as being at rest, expending 2 METs an hour. Jogging at a 12-minute-per-mile pace would expend 8 METs an hour. That means that if one of these women jogged for an hour at a 12-minute per mile pace 3 times a week, she would easily surpass the 21.5 METs of the successful women in the study, all with just 3 days of exercise. The harder you exercise, the more METs you expend and the more calories you burn, which can change how often and how long you need to exercise.

From the results it was concluded that, among women consuming a usual diet, physicalactivity was associated with less weight gain only among women whose BMI was lower than 25. Women successful in maintaining normal weight and gaining fewer than 2.3 kg over 13 years averaged approximately 60 minutes a day of moderate-intensity activity throughout the study.

The truth is, no one can tell you exactly how much exercise you need. It depends on so many things — how active you are, your diet, age, gender, genetics, metabolism and proper planet alignment. Headlines like these can scare people who already struggle to stay active, but the kind of broad generalizations that come from these kinds of studies are just that: broad generalizations that will never fit every person.

JAMA Vol. 303 No. 12, March 24/31, 2010; 303(12):1173-1179.