LiftforHealth.com

The answer is simple but not easy for many to accept. The simple fact of the matter is that the middle aged and seniors don't burn as many calories as they did when they were younger.

A 2019 study described in Science Daily describes the reason for weight gain among older adults:

The scientists studied the fat cells in 54 men and women over an average period of 13 years. In that time, all subjects, regardless of whether they gained or lost weight, showed decreases in lipid turnover in the fat tissue, that is the rate at which lipid (or fat) in the fat cells is removed and stored. Those who didn't compensate for that by eating less calories gained weight by an average of 20 percent, according to the study which was done in collaboration with researchers at Uppsala University in Sweden and University of Lyon in France.

Someone who is say, 45 or maybe 60+, can't burn through calories the way they did as a teenager or young adult. But they still probably eat as much. Therefore, they gain weight.

There are no magic diets. Calories in needs to match calories burned. Americans typically eat three meals a day plus snacks. That might be OK for an active 16-year old but not for someone older. Rule number #1 should be no snacks. That should go a long way in helping younger adults.

But there comes a point where three meals a day is too much. Many older adults only need to eat twice a day. I choose to eat a fairly high protein breakfast and then don't eat again until an early dinner. After dinner, I eat nothing until the next morning when I break my fast.

If I eat lunch, I gain weight. There's nothing I can do to change that. It's a matter of simple reality. Would I rather enjoy lunch or enjoy being reasonably lean? It's a lifestyle decision.