Women with normal Body Mass Index need not do rigorous exercise to keep slim as a new study has claimed that an hour of moderate physical activity, like yoga and taking a stroll, daily is enough for maintaining their fitness.

In a 13-year-long study, a team of researchers led by scientists from the Women’s Hospital at Harvard Medical School found that by climbing stairs, taking a hike, taking a yoga class, or any other moderate physical activity thousands of healthy women maintain their weight without cutting calories.

“The only catch is that it only worked for women with a normal body mass index (BMI) who did these exercised for an hour daily,” the team was quoted as saying by the Scientific American. During the study, they monitored 34,079 relatively healthy middle age and older women seven times to gather information on weight and physical activity levels and saw that for women under the age of 65 and with BMIs below 25, exercise made a big difference.

“Those in that group who got less than 420 minutes of moderate activity each week gained significantly more weight than those who were able to squeeze in more than 60 minutes a day,” said lead author I-Min Lee.

However, for women who started the study either overweight or obese and who maintained average eating habits, the amount of moderate activity did not seem to affect their statistical likelihood of losing weight.

Thus it was concluded that “once overweight, it may be too late” to rely on moderate activity alone to achieve a healthier BMI, said the study published in JAMA, Journal of the American Medical Association.

Even though 150 minutes of physical activity each week is “clearly sufficient to lower the risks of chronic diseases, it is insufficient for weight gain prevention absent caloric restriction”, they added.