Mitochondria - the structures within cells that convert food into energy - slow down or die off after age 40. Subsequently, you can’t burn calories like you did before and can start gaining weight.
To combat the loss of mitochondria, cut 100 calories from your daily intake. Easy ways to trim 100 calories include removing skin from chicken breasts, using skim instead of whole milk in coffee, swapping mustard for mayo on sandwiches, and eating plain hamburgers rather than cheeseburgers. Studies show that eliminating 100 calories a day helps to keep weight off long term.