BRISK walking is the most effective way to stay slim, experts say. A study found it was better than expensive fitness sessions, cycling or swimming. Speedy strollers are about 4½lb lighter and women are almost a dress size smaller than gym-goers, said experts at the London School of Economics. The researchers looked at physical activity levels of more than 50,000 adults in the annual Health Survey for England between 1999 and 2012.They compared types of moderate intensity exercise that raises the heart rate and causes sweating. The team found women and men over 50 who had a regular brisk walk for more than 30 minutes had lower body mass index and smaller waists. Lead researcher Dr Grace Lordan said: “Brisk walking is a lifetime activity rather than a fad. Many go to the gym and then drop off. But walking fits in with busy lifestyles.”For men under 50 most exercise had a similar effect, the report said. The NHS recommends at least 150 minutes of moderate exercise a week, but four in five fail to do it. That costs the health service at least £1billion a year, mainly due to obesity.