Metabolic Rate Equations Overestimate Caloric Needs for Blacks

Miriam E Tucker

October 31, 2017

WASHINGTON, DC — Commonly used resting metabolic rate equations overestimate an individual's caloric needs, particularly for African Americans, new research shows.

The findings were presented October 30, 2017 here at Obesity Week 2017 by James Reneau, BS, a third-year medical student at the Medical College of Wisconsin, Milwaukee.

Mr Reneau and colleagues compared resting metabolic rate measured by the gold standard — indirect calorimetry, which is impractical in the clinic — with those estimated by three commonly used clinical formulas that incorporate height, weight, gender, and age, but not race.

These equations — all decades old — are widely used in clinical settings such as by dieticians and in weight-loss clinics for estimating resting metabolic rate, and from that, daily caloric needs and the necessary daily deficit in order to lose weight are calculated.

But, he noted, "It was our observation that many patients will fail to lose weight despite strictly adhering to dietary restrictions based on these equations."

Among 79 white and 36 African American subjects, the Mifflin (Am J Clin Nutr. 2009;51:241-247), Harris-Benedict (Proc Natl Acad Sci. 1918;4:370–373), and Cunningham (Am J Clin Nutr. 1980;33:2372-2374) all overestimated caloric needs overall, but to a much greater degree among African Americans, amounting to the equivalent extra calories of about a can of sugared soda every day or a pound of fat every 12 to 15 days. "It was quite a big difference," Mr Reneau commented during his presentation.

The study also suggested that the addition of variables such as respiratory quotient, lean body mass, total fat mass, race, and hip circumference could improve the accuracy of future equations for estimating resting metabolic rate.

Asked to comment, obesity expert Steven B Heymsfield, MD, professor of medicine at the Pennington Biomedical Research Center, Louisiana State University, Baton Rouge, said "This study shows that African Americans have lower metabolic rates than whites for the same weight, height, and age.…I think it's very important to know that because then we're overestimating how many calories blacks should be eating."

Moreover, Dr Heymsfield added, "There's been a lot of research over the years looking at these kinds of prediction equations in whites," but much less in blacks. "We need more research in black populations, with very carefully measured metabolic rates."

In the meantime, he said, clinicians could advise black patients to consume 10% fewer calories than they would normally recommend based on the current equations, taking into account the usual amount of physical activity per day.

Large Overestimation of Calorie Needs in Black Subjects

Mr Reneau and colleagues measured resting metabolic rate in the 115 healthy fasted subjects using indirect calorimetry and used dual-energy X-ray absorptiometry to assess lean body mass, total fat mass, and estimated visceral adipose tissue.

The 36 black and 79 white subjects differed significantly by percent female (53% of blacks were women vs 75% of whites, respectively), age (35 vs 50 years), and waist circumference (101 vs 113 cm) (< .05 for all), but there was no significant difference between the races in body mass index (33 vs 36 kg/m2).

The white group also had significantly higher total fat mass, percent fat mass, and respiratory quotient (< .05 for all).

Measured resting metabolic rates by indirect calorimetry were 1663 kcal/day for whites and 1554 kcal/day for African Americans. For the white subjects, the three equations overpredicted those values by 40 to 120 kcal/day (5%-10%), with only the Harris-Benedict reaching statistical significance (< .0001).

However, the equations overestimated resting metabolic rates among the black group to a much greater extent, by 12% to 18%, ranging from 190 kcal/day for the Mifflin to 282 kcal/day for the Harris-Benedict, an equation that was published in 1918 and still in use today.

Overall, the difference between the measured and predicted resting metabolic rates were highly statistically significant (< .0001).

Clinical Variables Could Improve Prediction

In linear regression analysis, negative predictors of resting metabolic rate included age, race, height, and respiratory quotient, while lean and fat mass were positive predictors. There were no significant predictors for resting metabolic rate that differed between white and African American subjects, Mr Reneau noted.

In a second analysis using only significant (< .05) variables that are routinely measured in clinical practice, age, race, sex, and hip circumference were negative predictors while weight was a positive predictor.

For example, a weight increase of 1 kg equated to a 15.6-kcal increase in resting metabolic rate, while aging 1 year corresponded to a 5.2-kcal decrease. African Americans had a 148-kcal lower metabolic rate than did whites, and females had 118-kcal lower resting metabolic rates than males.

Dr Heymsfield told Medscape Medical News that although previous studies had shown in general that the equations tend to overestimate resting metabolic rate, "the effect in blacks is real. It's been seen in other studies but not so clearly as in this one."

As to why, he said the most likely mechanism is differences in body structure, with black individuals tending to have "more muscle and bone and less metabolically active organs for same height, weight and age. We all differ. There are probably evolutionary differences.…I think we have to have equations for predicting energy requirements that are race-specific."

 Mr Reneau and Dr Heymsfield have no relevant financial relationships. Dr Heymsfield was formerly global director of scientific affairs at Merck.

Obesity Week 2017. October 30, 2017; Washington, DC. Abstract T-OR-2003

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