Thanks to sedentary lifestyles and fast-food habits, there is a worldwide epidemic of overweightness and obesity, which not only increases the risk of physical illness but also contributes to depression and dementia.

As people age, they can develop a condition called central obesity, which refers to excess accumulation of fat cells around the waistline. These highly active cells promote an inflammatory reaction that attacks healthy brain cells.

When scientists examine the abnormal amyloid plaques found in the brains of Alzheimer’s patients, they routinely see evidence of inflammation.

Portion control combined with regular exercise can help people limit central obesity. In fact, cardiovascular conditioning is known to reduce inflammation throughout the body and the brain.

Eating several small meals throughout the day (breakfast, lunch, dinner, plus midmorning and midafternoon snacks) can stave off the hunger pangs that trigger binge eating and lead to central obesity.

Combining healthy carbohydrates and proteins in every meal and snack also helps avoid weight gain. That’s because carbohydrates provide immediate energy while proteins offer a sustained sense of fullness.

Experts agree that proteins are helpful for curbing appetite, but the mechanism by which they do it is not clear. One hypothesis is that protein lowers levels of appetite-stimulating hormones in the brain.

Consuming protein may also lead to fewer spikes of insulin, so that blood sugar levels remain constant. When blood sugar drops, appetite increases.

Proteins consist of 20 amino acid building blocks — nine of which are essential, meaning that our bodies can’t make them so we must consume them in our diet. Sources of these essential amino acids include fish, red meat, poultry, yogurt, eggs, milk, cheese, nuts, and soybeans.

A high proportion of muscle compared with fat helps people maintain normal body weight because muscle cells require much more caloric energy than fat cells. Lean muscle mass also helps older people avoid sarcopenia — age-associated muscle tissue loss that is linked to shorter life expectancy.

Of course, the hardest part of any diet is getting started. Scientists have used functional MRI scanning to pinpoint the areas of the brain that control your ability to resist or give in to unhealthy food urges.

When you overeat, the frontal lobe’s ventromedial region (in the middle of the forehead, just above the eyes) becomes activated.

When you resist the urge to overeat and make healthy food choices, the frontal lobe’s dorsolateral region (just behind the temples) takes over.

Once you develop healthier habits, the neural circuits controlling your urges become stronger — and that makes it easier to resist temptation in the future.