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NUTRITION Eating Less Fat,More
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But a contradiction has arisen. Adhering to the food pyramid's principles, Americans have decreased their fat intake compared to the 1960s, yet rates of obesity have noticeably increased. Implying that the standard food pyramid may need some adjusting, preliminary research from HMS and Children's Hospital suggests that a decrease in dietary fat may have inadvertently made Americans hungrier.
"Over the past three decades, fat intake has de-creased from 42 percent of our calories to 34 percent. At the same time, the rate of obesity has doubled," says David Ludwig, HMS instructor in pediatrics at Children's. Ludwig says Americans have increased their consumption of refined carbohydrates: processed grains such as cereal, bread, and pasta. He says this food group has substituted for the decrease in our dietary fat, and it is this type of carbohydrate that may be making us fatter.
Glycemic Index Is Key
To examine how the various food groups affect appetite, Ludwig
and his colleagues from Children's and Tufts University studied
the responses of 12 obese boys to three different meals. All the
meals had the same number of calories but varied in their food group
composition. Published in the March 3 Pediatrics, the study
found that the appetites of the boys varied depending on the composition
of the meal.
"Over the past three decades, fat intake has decreased from 42 percent of our calories to 34 percent. [But] the rate of obesity has doubled."
--David Ludwig
In selecting the three meals, the researchers emphasized the foods' glycemic index, that is, their ability to induce a rise in blood glucose. Rapidly digested into glucose and subsequently absorbed into the blood stream, refined carbohydrates have a high glycemic index. In contrast, most fruits and vegetables, also carbohydrates, have a low glycemic index. The body digests them slowly, thereby raising blood sugar only moderately. Protein and fat do not raise blood glucose directly and therefore do not have a glycemic index; however, these foods, as well as fiber, slow down digestion and were included in greater amounts in the low glycemic index meal. The researchers found that both the low and medium glycemic index meals proved more effective at staving off hunger than the high glycemic index meal.
"This is the first study that provides evidence that carbohydrate ingestion can play a role in appetite. If it's right, it may revise the food pyramid and really change the way people think about food," says Joseph Majzoub, HMS associate professor of pediatrics at Children's and an author on the Pediatrics paper.
The researchers found that when fed the high glycemic index meal for breakfast and lunch, the boys requested more food for an afternoon snack than when they had eaten either the medium or low glycemic index meals.
Fuel Falls, Hunger Climbs
To help figure out the physiological basis to the boys' responses,
the researchers measured the amount of glucose, fatty acids, and
several hormones present in the blood. Predictably, the high glycemic
index meal, with its high levels of circulating glucose, caused
a surge in insulin, a hormone promoting the storage of nutrients
in fat cells, liver, and muscle. Glucagon, a hormone counterbalancing
insulin by signaling for release of stored glucose into the blood,
fell very low. Levels of fatty acids, another source of energy for
the body, were also low.
Several hours after a high glycemic index meal, the researchers found that insulin's instructions had greatly depleted the blood of glucose and fatty acids. "The body effectively runs low on fuel, leading to excessive hunger and overeating," says Ludwig.
Evolution may explain why the human body does not do well on a diet loaded with refined carbohydrates, Ludwig speculates. He says that before the advent of agriculture 10,000 years ago, our ancestors ate a diet rich in fruits and vegetables, with moderate amounts of protein and fat. "Humans living in the Paleolithic period, before agriculture, ate grain products rarely, if at all."
Popular regimens like the Zone and Atkins diets have also supported the notion that more carbohydrates make people fatter. Ludwig started his studies on glycemic index because he wanted to see whether the diets' successes had a scientific basis.
Ludwig and his colleagues have already begun testing their theories with larger sample sizes and over longer periods of time. Director of the Optimal Weight for Life (OWL) program at Children's, Ludwig has used low glycemic index diets on about 200 children over the last two years. "So far, the results appear promising," he says.
In addition, Ludwig and his colleagues are also looking at how diet influences cardiovascular risk. Evaluating 3,000 young adults, the study will examine how fiber and fat consumption affect variables such as weight gain, blood pressure, and serum lipids like cholesterol and triglycerides.
Though preliminary, the Pediatrics study suggests that America may want to start altering the way it thinks about food.
--Judy Silber
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