Berkeley - A new study released today by the
National Academies' Institute of Medicine provides the most
detailed criteria yet for deciding what levels of fat, carbohydrate
and protein Americans should aim for in their diet, according
to two University of California, Berkeley, experts on the
panel that developed the report.
Amidst the scientific data supporting specific recommendations,
the 1,000-page study recommends doubling the amount of physical
activity previously advised - from a minimum of 30 minutes
per day to 60 minutes per day - balancing energy intake
and expenditure, and minimizing intake of saturated fats,
cholesterol and trans fatty acids.
One of the most significant changes is a move away from
one-size-fits-all recommendations for the intake of calories
and macronutrients, that is, carbohydrates, fats and proteins,
said panel member George Brooks, professor of integrative
biology at UC Berkeley and an exercise physiologist. Instead,
the panel members urge people to balance caloric intake
with physical activity. The more active you are, the more
calories you should consume, and the more flexible your
daily diet can be in terms of fat and carbohydrate.
Overweight people need to reduce their overall intake,
increase physical activity level and be more strict about
dietary fat, since fat has more than twice the calories
per gram as carbohydrate and protein.
"We're going to change the whole paradigm," Brooks said.
"Instead of recommendations based on age, height and weight
alone, we're basically saying, 'You tell me what exercise
you do, and I'll tell you what you can eat.'"
The panel recommended an hour of moderately intense physical
activity, such as brisk walking, each day - twice the recommended
minimum of the 1996 Surgeon General's Report on Physical
Activity and Health, which advised at least a half-hour
daily of vigorous exercise.
The panel of 21 scientists and physicians was convened
at the request of various U.S. agencies and Health Canada.
The organizations wanted the National Academy of Sciences
(NAS) to evaluate the literature on macronutrients in the
diet and suggest any changes or updates to the nations'
dietary recommendations. The panel was one of seven plus
two subcommittees overseen by the Food and Nutrition Board
of NAS, formed to look at all aspects of diet to develop
dietary reference intakes.
Panel members spent two and a half years reviewing nutritional
studies conducted in the past decade, and arrived at a consensus
about what the public at large, as well as children and
those with special needs, such as diabetics, should consume
on a daily basis. Its recommendations will become the basis
for setting policy in both countries, and will guide the
Food and Drug Administration (FDA) and other organizations
in their efforts to educate the public about healthy eating
habits. The U.S. Department of Agriculture, for example,
could use the data to make specific food recommendations
or revise the well-known Food Pyramid.
Panel member Ronald M. Krauss, adjunct professor of nutritional
sciences at UC Berkeley and a staff senior scientist at
Lawrence Berkeley National Laboratory, cautioned that the
study is not intended to replace current dietary guidelines.
"This is like an encyclopedia of what was already known
about the macronutrients in our diet," said Krauss, who
studies how genes determine the body's response to dietary
fats and cholesterol. "Though there are recommendations
in the report, I would not consider them recommendations
for an overall diet. They are recommendations for considering
the effects of specific nutrients. Our task was to put numbers
out there that could help future dietary guideline committees."
One panel recommendation, that the public wean itself away
from trans fatty acids, was released July 10 in order to
help the FDA decide whether to require that food labels
include trans fatty acid content. The panel could find no
evidence that trans fatty acids - hydrogenated oils found
primarily in processed and fried foods - are essential to
the diet. Since, even in small amounts, they increase cholesterol
levels and thus the risk of heart disease, the study suggested
that consumption be minimized. This change has already spurred
McDonald's USA to halve the amount of trans fat in the oil
it uses to cook French fries and Chicken McNuggets.
Regarding the panel's recommendations about physical activity,
Brooks acknowledged that doctors have always recommended
exercise. Until recently, however, studies correlating dietary
intake with physical activity have not shown a clear benefit
from exercise, primarily because they have relied upon questionnaires
and self reporting.
In the past 10 years, thanks to techniques such as stable,
non-radioactive isotope tracer studies, scientists have
been able to measure energy expenditure directly. As a result,
they discovered that people have tended to overestimate
their activity level and underestimate their consumption
of food - in particular, fats and alcohol.
"For the first time, from stable, non-radioactive isotope
tracer studies, we know how active people are when they
have a healthy body weight, what proportions of carbohydrate,
fat, and protein they use as energy sources, so we can link
physical activity with dietary recommendations," Brooks
said. "If people find themselves gaining weight, they need
to do something. An hour a day keeps the disease away."
People must work this activity into their daily lives,
he said, whether by taking the stairs instead of an elevator,
or walking to the store instead of driving. He emphasized
that the one hour is cumulative, so it can include 15 minutes
of brisk walking in the morning, 10 minutes of stair climbing
throughout the day, combined with recreational biking or
other walking in the afternoon or evening.
"Previously, the Surgeon General's report talked about
how much activity you need to minimize progression of chronic
diseases such as coronary heart disease, Type II diabetes
and some forms of cancer, whereas the National Research
Council issued RDAs, which focused on what you should consume,"
Brooks noted. "Now, it's fair to say, this is the first
time physical activity has been an intrinsic part of dietary
recommendations. This is a product of our change in lifestyle."
There are two key concepts, he said. Because both physical
activity and body fatness affect chances of developing chronic
diseases, people need sufficient physical activity to reduce
the morbidity and mortality of chronic diseases. Beyond
that, we need enough physical activity to balance energy
intake and expenditure to control body weight. If you are
active about 60 minutes most days, maintain a stable weight
and have a body mass index (ratio of weight to square of
height, measured in kilograms per square meter) between
20 and 25, "you're doing great," he said.
Another significant change from previous recommendations
is the panel's establishment of a range of acceptable carbohydrate,
fat and protein intakes. Previous recommendations were mostly
maximums or minimums.
"If you are in energy balance, a person doesn't need to
worry on a daily basis, am I getting the right percentage
of carbohydrate or fat. There's a broad range there," Brooks
For example, instead of recommending fewer than 35 percent
of daily calories in the form of fat, the panel lists an
acceptable range of 20 to 35 percent for total dietary fat,
assuming energy balance in the overall diet. The acceptable
range of carbohydrates is 45-65, while protein should comprise
between 10 and 35 percent of daily calories.
Krauss worries that the report will be misconstrued as
promulgating hard and fast levels of macronutrients in the
diet. But that was not the goal of the panel, he said. It
was, instead, to look at fats, carbohydrates and protein
from a perspective more appropriate to micronutrients like
vitamins and minerals. This approach was not always appropriate,
"The panel worked very hard to apply criteria developed
for micronutrients, present in generally small amounts in
diets, to this larger issue of the major food components
in the diet," he said. "But most macronutrients are not
essential in the way that many micronutrients are. Most
of us in the U.S. don't need all the carbohydrate we are
eating, we don't need all the fat we are eating, even all
the protein we are eating. A major role for these nutrients
merely is to provide energy.
"Within these constraints, however, it doesn't make a whole
lot of difference how you divide those extra calories between
protein, carbohydrates and fat as long as your total energy
intake is balanced by expenditure."
"The key is energy balance and nutrient balance," Brooks
agreed. "Once you have that, you have a lot of latitude."