Fine-Tuning a Woman's True Caloric Needs

How many calories does it really take to see healthy, moderately active women through an average day? A recent study by scientists at the Agricultural Research Service's Children's Nutrition Research Center in Houston, Texas, set out to determine the actual energy requirements of healthy underweight, normal-weight and overweight women of reproductive age. Led by Nancy F. Butte, an energy metabolism expert, the study's ultimate objective will be to define the precise energy requirements of pregnant and lactating women, to best ensure good health for mothers and infants.

Researchers used what's called the doubly labeled water method to measure total energy expenditure (TEE) in 116 women living in urban areas. This method is a way to measure energy expenditure and energy requirements of free-living individuals. Doubly labeled water, or heavy water, is a safe, non-radioactive form of water enriched with heavier forms of hydrogen and oxygen that are used to track end products of metabolism, water and carbon dioxide.

Volunteers followed their usual diet and activity regimens. Thirteen of them had low body mass index (BMI), 70 had normal ones and 33 had high BMIs. The BMI is the ratio of weight to height squared and is used to gauge body fat in adults. A multi-component model was used to measure body fat.

Researchers measured the volunteers' 24-hour basal metabolic rate (BMR) and energy expenditure in a special room called a respiration calorimeter. They calculated physical activity levels by dividing volunteers' total energy expenditure, or TEE value, by their BMR values.

The study showed that current suggested daily caloric intakes for healthy women of childbearing age living in industrialized societies need to be revised, based on women's BMIs. Low-, normal- and high-BMI groups used varying amounts of energy, ranging from about 2,100 to 2,700 calories--8.9 to 11.5 megajoules--per day. A joule is a measure of work or energy. The findings were published in the March issue of the American Journal of Clinical Nutrition.

ARS is the U.S. Department of Agriculture's chief scientific research agency.