Both your nutritional needs (the food and water) and your metabolism (how fast your body converts food to energy) change at this age. Your metabolism gets slower. Women lose about half a pound of muscle per year starting around the age of 40. That makes losing weight even more difficult. Some of the changes women experience are due to decreased hormones, reduced activity level, and medical conditions.
I subscribed to this in MY early 20's way back in the magazines infancy. Back when the cover photo's were black and white and most of the cover "models" were female athletes. It's changed since then, catering a little more to the "Cosmo" crowd. Which is fine, just not for me. It just doesn't feel like it applies as much to someone in their thirties married with kids as someone in their twenties who apparently has the money and lack of self-control to spend $90 on a designer t-shirt (really).
It's a cliché, to be sure, but a balanced diet is the key to good nutrition and good health. Following that diet, however, isn't always that easy. One challenge is that women often feel too busy to eat healthfully, and it's often easier to pick up fast food than to prepare a healthy meal at home. But fast food is usually high in fat and calories and low in other nutrients, which can seriously affect your health. At the other extreme, a multimillion dollar industry is focused on telling women that being fit means being thin and that dieting is part of good nutrition.