Biomedical Engineering Reference
In-Depth Information
Modifying nutritional regimens through determination of the respiratory
quotient (RQ), the ratio of moles of carbon dioxide produced to moles of
oxygen consumed (or
VV
) over a certain time interval.
/
CO
O
2
2
One of the formulas for predicting energy expenditure is the Harris-Benedict
equation, established in 1919 by American physiologists and nutritionists J. A.
Harris and F. G. Benedict ; this equation took into account gender, age, height, and
weight:
Male (calories/day): (66.473
+
13.752*weight)
+
(5.003*height)
−
(6.775*age)
Female (calories/day): (655.095
+
9.563*weight)
+
(1.850*height)
−
(4.676*age)
where weight is in kilograms, height is in centimeters, and age is in years. Harris-
Benedict is correct 80-90% of the time in healthy, normal volunteers. However,
these formulas are skewed towards young and nonobese persons. There is a large
variation between individuals, when comparing their measured energy expenditure
to the calculated amount. These equations have limited clinical value when tailoring
nutrition programs for specific individuals for weight loss purposes or acute as well
as chronic illness feeding regimens.
In 1948, British physiologist John B. De V. Weir derived an equation to relate
O
2
consumption and CO
2
production to energy released in the body. Experimen-
tally, Weir determined the amounts of calories released from carbohydrates, fats,
and proteins via metabolism with one liter of oxygen (Table 4.3). Based on the
three fractions, Weir wrote three equations for oxygen consumption, CO
2
pro-
duced, and total energy released. Using these equations and neglecting the effect of
protein, he arrived at representation for the overall energy (
E
E
) released per liter
O
2
(Kcal/LO
2
):
V
CO
E
[Kcal/L]
=
3.941
+
1.106
2
(4.30)
E
V
O
2
Weir showed that the error from neglecting the effect of protein (i.e., assuming
U
N
=
0) is negligible and shows little variation between individuals.
E
E
can therefore
be estimated from
C
V
and alone. More recent studies have calculated
different RQ and energy release equivalent values for each of the three substrates.
To account for energy released by proteins, (4.30) was modified as
V
and
2
V
U
CO
E
[Kcal/L]
=
3.941
+
1.106
−
2.170
N
2
(4.31)
E
V
V
O
O
2
2
Table 4.3
Weir's Data
Carbohydrate
Protein Fat
R.Q.
1
0.802
0.718
Kcal released/L O
2
5.047
4.463
4.735
