PHILADELPHIA — Food can be one of those unexpected flash points of late life.

Grandma may say she’s never hungry or that the only things that taste good are salty foods such as french fries. Grandpa may lose control over his sweet tooth, living on Tastykakes and ice cream.

The rest of the family worries that poor nutrition will make their elders’ already tenuous health even worse and hasten death. So, in frustration and fear, they chide or tempt loved ones to change their habits. Often, they learn what stubborn means.

“It is extremely distressing,” said Louisa Miceli, a nurse with the Visiting Nurse Association of Greater Philadelphia who has heard about eating problems in many a home.

Because metabolism slows and activity declines, it’s normal for elderly people to want less food. What families may not know is that, as people get older, the way food tastes changes — and not in a good way. While our taste buds are one of the few things that hold up fairly well as we age, our sense of smell is what contributes most of what we think of as flavor — herbs and spices, chocolate, vanilla, strawberry, coffee, wine. And that sense falls apart.

According to research by Richard Doty, director of the University of Pennsylvania’s Smell and Taste Center, ability to smell peaks by age 40. It’s all downhill from there, with the slope growing sharply steeper after 60. Sixty percent of people between 65 and 80 have major olfactory impairment. More than 80 percent do after 80.

Men are more impaired than women, and smokers fare worse than nonsmokers. Alzheimer’s and Parkinson’s diseases are also associated with problems with sense of smell. Some medications cause trouble, too.

Older people can also have distortions of taste that make everything — even water — taste salty or give foods a sour or bitter taste, Doty said. In cacosmia, one of the more alarming problems, foods take on a fecal flavor.

In younger people, the sense of smell can be impaired by viruses, head injury or chemotherapy.

“People don’t appreciate how important these senses are until it happens to them or one of their loved ones,” Doty said.

Not being able to smell well can be a serious problem, said Ronald DeVere, a neurologist who directs the Smell and Taste Center in Austin, Texas. People may not smell smoke, leaking gas or spoiled food. If they lose interest in food, they may lose weight, a risk factor for other health problems in the elderly.

Chris Loss, a professor in the department of culinary science at the Culinary Institute of America, said the school recently began working on a project to improve food for cancer patients who have had radiation or chemotherapy. He expects the impending wave of aging baby boomers to inspire the food industry to do more research on the special needs of the elderly.

DeVere, who has treated several chefs with smell loss in his practice, said he has tried to educate the culinary industry, without much success. “Most people don’t think it’s a big deal,” he said.

What you’re able to “taste” when your sense of smell goes are only the five basic flavors: salty, sweet, bitter, sour, and umami, a savory meaty taste. But subtle differences are gone. This explains the attraction of salt and sugar to the elderly.

“They pour salt and they pour sweet,” Devere said. He said many older people don’t even realize they have a problem. Others complain that food is tasteless.