PHILADELPHIA (MCT) — When you’re cold, you’re cold. And when you’re older, you’re colder.

No surprise to anyone with an older relative who cranks up the thermostat during the chilly days of winter. But after half a century of research, scientists are still teasing out some of the subtle differences in how the young and old respond to Jack Frost.

In a new paper, Pennsylvania State University researchers describe what may be one of the odder sacrifices human beings have made in the name of science. Eighty-eight volunteers stripped down to their shorts — and sports bras for the women — sat in a climate-controlled room, one at a time, while the temperature was lowered.

Their core temperatures were recorded with a flexible thermometer that was snaked through the nostrils down the esophagus – a procedure that requires the participant to swallow and try not to gag.

The key to staying warm? For the young, a thicker layer of fat under the skin was the best defense. For the old, most of the variance among subjects could not be explained. But a higher percentage of body fat helped ward off the cold somewhat, as did having less muscle in the arms and legs — an unexpected finding.

Not that the authors recommend getting fatter or letting your muscles go to waste, of course. Just put on a heavier coat.

And if you’re old, beware: Your core body temperature can drop without you realizing it — a key reason that older people are more susceptible to hypothermia, said David W. DeGroot, lead author of the study in the Journal of Applied Physiology.

“In terms of how cold it felt, there was no difference between them,” DeGroot said, “even though the older people were in fact cooling down, and the younger people were maintaining.”

Fat has been shown to help before. But rather than analyze just one body trait and its effect on temperature, the authors looked at 10 that had cropped up in previous studies: weight, body surface area, hormone levels, subcutaneous fat and percentage of body fat, among others.

Some factors were interdependent; this comprehensive approach was valuable in determining which ones were truly important, said Ellen Glickman, a professor of exercise physiology at Kent State University who was not involved with the study.

While the findings elucidate the differences within the young and old, they don’t change the accepted view of what causes the difference between the two groups.

Scientists have known for decades that older bodies are not as good at one of the key mechanisms for regulating temperature: constricting the blood vessels near the skin.

In warm weather, the vessels dilate so as to increase blood flow and radiate excess heat from the body. In the cold, the vessels constrict to achieve the opposite effect.

This explains why folks with more muscle mass aren’t as good at staying warm, said Pennsylvania State University’s W. Larry Kenney, who collaborated with DeGroot and another researcher from Loughborough University in England. Muscle insulates the body somewhat, but it also has blood vessels.

So if your body isn’t so good at constricting those blood vessels in the skin, then having more muscle circulates more blood to where its heat can escape.

“It’s like wearing fleece without a windbreaker on top,” Kenney said. “That fleece-underneath layer is no longer insulating because wind and air is going right through it.”

The volunteers started out at 80 degrees and the temperature was lowered until they started shivering, typically in the mid-60s.

The study left out certain factors known to affect temperature. None of the participants smoked, a habit that can impair the ability to stay warm. And none was on medications that might have interfered. Certain blood-pressure medications, for example, work by dilating the blood vessels.

And the old saw about the body losing most of its heat through the head? DeGroot, who is pursuing his Ph.D. in physiology, loves debunking that one.

The only reason someone would lose more heat from the head is because the rest of the body is covered with clothes, he said. If you’re naked, there isn’t much difference between the various body parts.

So, yes — go put on a hat. But keep the pants, jacket and gloves on, too.