http://www.sciencemag.org/cgi/content/full/297/5588/1788
"IOM Panel Weighs In on Diet and Health".
Jennifer Couzin
Consumers trying to make sense of divergent diet-book claims aren't
likely to find easy answers in a new, 1000-page tome on diet and health
issued by the Institute of Medicine (IOM). But they will find a review
of the risks and benefits of consuming the disputed "macronutrients": carbohydrates,
fats, and proteins. Although the report makes specific recommendations,
it also laments gaps and contradictions in nutrition research,
suggesting that even the experts are struggling to sort out the
information.
After nearly 3 years of spirited debate, the 21 scientists on the IOM panel agreed on a bottom line: 20% to 35% of one's calories should come from fat, 45% to 65% from carbohydrates, and 10% to 35% from protein. A similar panel in 1989 suggested hard numbers within these ranges: no more than 30% from fat, no less than 50% from carbohydrates, and the rest from protein.
The flexible 2002 standards might reflect a newfound humility. "Twenty-five years ago, guidelines were presented with absolute certainty, [for example,] 'Thou shalt not eat eggs,' " says Walter Willett, an epidemiologist at Harvard's School of Public Health in Boston, who was not on the panel. "I think [this report] is a healthy acknowledgment that we don't know absolute truths."
The panel set out to determine the impact of macronutrients on chronic diseases such as diabetes. The assignment proved enormously complex. Fat, for example, is an umbrella that covers the omega-3 fatty acids and monounsaturated fat, which are considered healthy; the trans-fatty acids, which are considered unhealthy; and the saturated fats, about which there is no consensus.
The report also comes during a raging public debate on diet. To remain neutral, the panel members "put on blinders" to the policy implications of their work, according to panel chair Joanne Lupton, professor of nutrition at Texas A&M University in College Station. Popular diets, such as the heavily criticized Atkins diet, advocate nearly eliminating carbohydrates and relying on fat and protein. At the same time, the evidence favoring a low-fat diet has been questioned (Science, 30 March 2001, p. 2536).
Smorgasbord. The public has been offered a bewildering array of recommendations on healthy diets.
IOM panelists tried to limit the scope of their review by focusing on diets for healthy individuals, not those seeking to lose weight. But the duel over fat and carbohydrates edged its way into discussions anyway, as the panelists examined scientific studies dating back to the 1930s. "It's a very, very difficult decision as to whether high carb ... and lower fat is better," says Sheila Innis, a panel member and expert in pediatric nutrition at the University of British Columbia in Vancouver. On the one hand, Innis notes, "there are populations that do very well with high-fat diets," such as the Greeks. Their so-called Mediterranean diet, though, is composed largely of the healthy fats found in fish and olive oil--not the kind consumed by most Canadians and Americans, the report's intended audience. In the end, says Innis, the panel leaned toward carbohydrates because, in the context of a North American diet, they were deemed safer.
Although the IOM report aims to stay out of the big battle, it is one of the first government-funded efforts to parse out the underlying science. And it stakes out some clear positions. For example, it suggests roughly doubling average fiber intake, to 38 grams per day for men and 25 grams for women.
Certain recommendations--such as urging every adult to get an hour's daily exercise, twice the amount recommended in the past--seem to ignore the real-life lifestyles of North Americans. "I couldn't possibly do an hour of exercise a day," says Marion Nestle, chair of the department of nutrition and food studies at New York University. Nestle, who was not on the panel, complains that the report is too complex for "an already confused public." What causes obesity "isn't rocket science ... eating too much [does]."
Indeed, panel member Ronald Krauss, who studies diet and heart disease at Lawrence Berkeley National Laboratory in California, agrees that the report might not respond to the question people generally ask: "How much of this should I eat?" Unfortunately, science isn't able to deliver such detailed diet advice quite yet.
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