The popular notion that carbohydrates (search) make you hungry while fat keeps you full is being challenged by newly published research.
In an animal study, rats fed a fat-rich diet produced more of an appetite-stimulating hormone than carbohydrate-fed rats, leading them to get hungrier quicker after eating.
The findings offer clinical support for the idea that different foods affect fullness differently. But studies in rodents don't necessarily tell us anything about hunger in humans, a hunger expert tells WebMD, and the researchers did not examine the impact of protein on hunger. Recent research in humans has suggested that protein helps keep you feeling full longer.
The strongest clinical evidence we have suggests that eating protein is associated with more feeling of fullness, California endocrinologist Francine Kaufman, MD, tells WebMD. "That is certainly the basis of protein-based diets, but this study does not address this. It does suggest, however, that the other main assumption of these diets -- the idea that carbohydrates are evil and should be avoided -- is wrong."
The Hunger Hormone
The rat study, reported in the November issue of the journal Endocrinology, examined the impact of fat or carbohydrates on the production of the hormone ghrelin (search). Produced in the stomach, ghrelin has been shown to trigger appetite and is believed to play a key role in regulating body weight. Levels of the hormone generally rise before meals and quickly decline after eating.
Ghrelin levels were high in the rats following 14 hours of fasting, but they dropped when the animals were fed either fat- or carbohydrate-based food. Among the fat-fed rats, however, hormone levels rose again within 45 minutes of eating. Ghrelin levels remained low in the carbohydrate-fed rats.
Researcher Andreu Palou says the next step is to determine how different types of carbohydrates and fats affect ghrelin levels. Almost all of the currently popular weight loss diets restrict simple carbohydrates such as highly refined flours and sugars, which break down quickly and spike insulin (search) levels. Complex carbohydrates that have lots of fiber are promoted by some weight loss plans but are limited by others, like the popular Atkins diet (search).
"The rats in this study got hungry quicker when they ate diets that were high in fat vs. those high in carbohydrates," Palou tells WebMD. "But this is very basic research, and we don't yet know the role of specific types of fats and carbohydrates."
One of Many Hunger Regulators
Whatever the studies show, Kaufman says ghrelin is undoubtedly just one of many factors that drive human hunger.
Feeling full is an incredibly complex phenomenon and there are clearly a number of regulators, she says. "The study of hunger in humans is still in its infancy, but it is hard to imagine that there will be one magic bullet to control it. If we were able to modulate ghrelin in some way, that probably wouldn't be enough."
Kaufman has written a book on obesity and diabetes, scheduled for publication early in the spring. In it, she places much of the blame for the obesity epidemic on environmental factors.
"We are living in a society where nutritionally poor food is everywhere and there is little opportunity for activity," she says. "Whatever may be happening (within the body), the environment has got to change to make it easier for people to eat better and get more exercise."
SOURCES: Sanchez, J. Endocrinology, November 2004; vol 145: pp 5049-5055. Andreu Palou, PhD, professor of biochemistry and molecular biology, University of the Illes Balears, Mallorca, Spain. Francine Kaufman, MD, professor of pediatrics, Keck School of Medicine, University of Southern California; head of the division of diabetes and endocrinology, Children's Hospital of Los Angeles.