New tools produce better results
To determine energy requirements, you first need to measure energy expenditure
In the past, energy requirements were based on imprecise food intake data. Some subjects under-reported their consumption of foods considered bad and overestimated foods considered good. Others tried to provide the answers researchers were looking for. And since the person tested didn't always have an optimal weight or activity level, it didn't make sense to use that information to make recommendations.
By the time of the 1981 consultation, nutritionists agreed to use a new approach based on energy expenditure, not intake. The standard method is now to base the calculation on basal metabolic rate, or BMR, which is the rate at which the body consumes energy at rest. This figure is combined with an estimate of physical activity levels to determine the overall energy expenditure. But there are limitations.
Experts realized these limitations 20 years ago and
called for more investment in research and the development
of new technology. One outcome is a refined tool for
estimating energy requirements, the doubly labelled water
method, so-called because it uses water enriched with two
isotopes that act as markers.
Instead of measuring oxygen consumption to show energy expenditure as with previous methods, this technique tests the rate of carbon dioxide production. Subjects ingest water containing the two isotopes, one that is expelled in exhaled carbon dioxide and body fluids, the other primarily in body fluids. A few weeks later, scientists analyse the saliva or urine. By determining the difference in how quickly the two markers leave the body, the technicians are able to obtain a very accurate measure of carbon dioxide production, and thus energy expenditure. The method has several advantages: it doesn't require the subject to sit still nor does it require an estimate of physical activity. And since it measures carbon dioxide production over a period of weeks, it more accurately determines average energy expenditure. Unfortunately, due its high cost and the need for a certain level of technical training, its suitability in the developing world is open to debate.
"It will be a long time before this is a common method in developing countries," predicts Senior Nutrition Officer Barbara Burlingame. Still, its place in nutritionists' arsenal of tools is guaranteed --considering that the test has greatly added to the understanding of energy expenditure in children and that so much of the developing world is under 15.
17 January 2002
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