Olympic skier Lindsey Vonn challenged celebrities at the 2013 Met Gala, saying they were half her weight but skinny fat, pointing out they lived on Diet Coke and lettuce, and lacked muscle.

"Strong is beautiful," she rightly insisted.

Skinny fat can happen if you're not overweight but lose muscle mass and add flab, so your body's healthy proportion of muscle to fat flips.

While it can happen at any age, it is a heightened risk for older people. All of us lose muscle mass as we age. We need to take steps to preserve muscle tone with resistance exercise, and maintain a healthy weight.

A new study in the journal Clinical Interventions in Aging reports that for those over 65, losing muscle mass and replacing it with high fat mass — even if they're not overweight — is a slippery slope to dementia and Alzheimer's disease.

In fact, being thin and flabby is more damaging to cognitive abilities, including memory, speed of processing and decision-making, than being obese with muscle strength. (However, being greatly overweight and flabby is a double whammy.)

The smartest way to avoid losing muscle tone at any age is strength-building exercises for 20-30 minutes two to three times a week, using stretch bands or hand weights, and aerobic exercise five days a week.

Also essential: A diet with heart-healthy plant proteins, lean animal protein from salmon and skinless poultry (studies show that older people should increase their protein intake), and lots of water to keep you hydrated and your muscles supple.