You may think it is the sweetness of holiday treats that prompts you to shovel in the thousands of tongue-tickling calories that show up as extra padding by year's end. But recent studies of the human appetite show that while sugar may lead one into temptation, fat is what pushes the indulgence to caloric excess.
Fat is the nutrient most commonly overconsumed and least likely to be compensated for by eating less at subsequent meals or by exercising the calories away. And while holiday treats like cookies, pies, cakes, eggnog and chocolates are certainly sweet, their main source of calories by far is fat.
"High-fat foods disturb appetite control while people are in contact with food and afterward -- for the rest of the day and even the next day," said Dr. John Blundell, a psychobiologist at the University of Leeds in England, who has done extensive studies on how the composition of foods affects appetite, consumption and weight gain.
"There is no compensation for an energy-dense meal," and a meal high in fat is the most energy-dense a person can get, he said last week at a conference on reducing dietary fat.
Blundell's studies have shown that high-fat meals "compromise satiation," that is, the processes or signals that bring, or should bring, a meal or a snack to an end.
He and his colleagues call it the "fat paradox." They found that although fat in foods sets off the release of potent biochemical signals that suppress hunger, these signals often do not register soon enough in the brain when people eat fatty foods.
Furthermore, he said, the body is designed to hold on to consumed fat unless it receives a consistently rich supply of it. While a meal high in carbohydrates causes a near-immediate increase in oxidation, the process of turning those carbohydrates into energy, Blundell said that a high-fat meal does not induce an increase in the oxidation of fat. "Rather," he said, "it takes three to seven days of high-fat eating to increase fat oxidation."
Thus, he explained, after single episodes of high-fat eating, instead of burning the excess fat for energy, the body stores it. The result is weight gain.
"The brakes on fat consumption were never designed to cope with the amount of fat currently in the Western diet," Blundell said.
For example, Blundell cited a study at the University of Leeds in which obese adults were offered dinners and told to eat until they were full. Those who ate a low-fat, high-carbohydrate meal consumed 680 calories, but those given a high-fat meal consumed twice that much: 1,350 calories.
At subsequent meals that day and the next, the high-fat diners failed to decrease their consumption significantly to make up for the caloric overload.
Blundell says this sort of overeating happens not only because foods high in fat taste good, smell good and feel good in the mouth but because high-fat foods are energy-dense; a lot of fat calories can get packed into a relatively small quantity of food.
For example, a 5-ounce prime rib (minus the bone) supplies 578 calories, 78 percent of which come from 50 grams of fat. But a 5-ounce baked potato supplies only 132 calories and less than .2 of a gram of fat, yet the quantity of food is the same. It would take more than four baked potatoes -- a total of 22 ounces -- to reach the caloric content of the 5-ounce steak.
Blundell said the body's brakes on protein consumption were far more effective, making it "very difficult" to overeat protein. "The body can't store protein, so it has to process it," he explained. "But since most high-protein foods are also high in fat, by the time the satiety signal for protein kicks in, you've already overconsumed calories from fat."
Carbohydrates, too, have their limits, though the signals to stop carbohydrate consumption are somewhat slower to register than those for protein. The body can store only about a pound of
carbohydrates in the liver and muscles, so when extra carbohydrates are consumed, they are normally oxidized, releasing heat energy.
"But there is no limit to how much fat we can store," he said. "We have a very good adaptive mechanism for dealing with excess dietary fat. We just put it on hold in the body. Furthermore, having a large store of fat tissue does not inhibit the drive to eat fat."
To be sure, Blundell said, "there is more than one pathway" from nutrition to obesity. "People can overeat on carbohydrates, although this is much more difficult," he said.
Diets high in carbohydrates and diets high in protein suppress the oxidation of fats for energy, he said, because carbohydrates and protein are metabolized first. This process promotes the storage of fat in the body if extra calories are consumed as fat.
Although it is widely thought that fat and sugar consumption operate like a seesaw -- when one goes up, the other goes down -- Blundell has shown that when sugar grams increase, fat grams do too, suggesting that foods rich in sugars and fat, like cookies, ice cream, chocolate, pies and cakes, can become a major source of extra fat calories.
In a survey that questioned 1,500 men and 1,500 women from 16 through 65 years old, the Leeds researchers found that high-fat consumers also ate more grams of carbohydrates than low-fat consumers ate. High-fat consumers were more likely to skip breakfast and to consume fewer meals.
However, this study, conducted among a random sample of the population, showed high-fat diets do not necessarily lead to excess weight. While there were 20 times as many obese people among the high-fat consumers, there were also a lot of normal-weight and underweight high-fat eaters.
"There is no biological imperative," Blundell said. "Not everyone eating a high-fat diet becomes obese." However, among the high-fat consumers, older people were more likely to be overweight. As few as 50 extra calories a day, or a gain of a 10th of a pound a week, would add up to the weight gain observed in the study population over an 11-year age range.
Pub Date: 12/24/96