The source of this advice is a study of 34,000 American women, average age 54, over 13 years. The women gained an average of 5.7 pounds over the course of the study, but according to Shari Roan of the LA Times, "Only those women who were normal weight at the start of the study and engaged in moderate-intensity activity an average of 60 minutes per day, seven days a week, maintained a normal body weight, defined as a body mass index of less than 25." Says study author I-Min Lee, "We wanted to see in regular folks — people not on any particular diet — what level of physical activity do you need to prevent weight gain over time. It's a large amount of activity. If you're not willing to do a high amount of activity, you need to curtail your calories a lot."
The study and coverage thereof raise a lot of questions. What does it mean to "curtail your calories a lot," for starters? And is it significant that one of the main predictors for being of "normal" weight at the end of the study was being of "normal" weight to start out with? The question is especially pressing given that LiveScience's overview of the study remarks that "Women who are overweight or obese do not appear to reap the same benefits in terms of weight-gain prevention." LiveScience even quotes Lee saying, "Among heavier women, there was no relationship between how much [activity] a women did and what weight she gained." Large amounts of exercise may have prevented weight gain in normal-weight women, but the message of the study could just as easily be that no amount of exercise seemed to affect the weight of women with higher BMIs.
Questions about the study aside, however, it's clear that exercise has many physical and psychological benefits aside from weight control. But recommendations like "an hour a day, seven days a week" are going to fall on deaf ears, or even turn people off of exercise out of resentment, unless we develop ways of actually making them realistic. In an otherwise obnoxious comment, KristyHowardClark asks the important question, "What about the ladies who work full time and have only the week end to work out?" And what about the ladies who have multiple jobs, or family members to take care of, or live in unsafe neighborhoods with few opportunities for exercise? The problem with activity recommendations is that they're always aimed at individuals, who then supposedly have unhealthy lifestyles if they don't fulfill them. But our entire nation is set up in such a way as to make it difficult or impossible to exercise — we have to work long hours to support ourselves, we don't have subsidized childcare, our cities and towns aren't walk- or bike-friendly. Changing these things might actually help women get close to an hour a day of activity, but issuing that fiat without such changes is just going to add guilt — and maybe anger — to the many problems we already have to deal with.