The lungs are the primary excretory organ in weight loss, according to the researchers.
Although it is often pushed to the side during the festive rush, during
the post-Christmas fallout, weight loss will once more be at the top of
many of our agendas. But where does excess weight go when we lose it?
The answer provided by a new paper may be a surprise.
According to researchers from the University of New South Wales in
Australia, when weight is lost, the majority of it is breathed out as
carbon dioxide. Their paper is published in the Christmas issue of
The BMJ.
Prof. Andrew Brown and Ruben Meerman reported widespread misconception
regarding how weight is lost, finding physicians, dietitians and
personal trainers all equally guilty of not knowing. Most believed that
fat is converted to energy or heat, "which violates the law of
conservation of mass," they write.
Others believed that fat could be excreted within feces or converted to
muscle. These responses may well have provoked gasps from Brown and
Meerman, who have now formulated a calculation to illustrate how we
actually "lose weight."
Excess dietary
carbohydrates
and protein are converted to a type of fat called triglyceride. When
people attempt to lose weight, they are attempting to metabolize these
triglycerides while keeping their fat-free mass intact, explain the
authors.
Triglycerides are comprised of three types of atom: carbon, hydrogen and
oxygen. Triglyceride molecules can be broken down only by unlocking
these atoms, through a process known as oxidation.
Tracking the atoms
The researchers chose to follow the path of these atoms when leaving the
body. They found that when 10 kg of fat were oxidized, 8.4 kg were
converted and excreted as carbon dioxide (CO
2) via the lungs, and 1.6 kg became water (H
20).
In order for 10 kg of human fat to be oxidized, the researchers
calculated that 29 kg of oxygen must be inhaled. Oxidation then produces
a total of 28 kg of CO
2 and 11 kg of H
20.
"None of this biochemistry is new," say the authors, "but for
unknown reasons it seems nobody has thought of performing these
calculations before. The quantities make perfect sense but we were
surprised by the numbers that popped out."
The results suggest that the lungs are the main excretory organ for weight loss, with the H
20 produced by oxidation departing the body in urine, feces, breath and other bodily fluids.
On average, a person weighing 70 kg will exhale around 200 ml of CO
2 in 12 breaths each minute. The authors calculate that each breath contains 33 mg of CO
2,
with 8.9 mg comprised of carbon. A total of 17,280 breaths during the
day will get rid of at least 200 g of carbon, with roughly a third of
this weight loss occurring during 8 hours of sleep.
The carbon that is lost through exhalation is only replaced through the
consumption of food and beverages such as fruit juice, milk and soft
drinks. "Keeping the weight off simply requires that you put less back
in by eating than you've exhaled by breathing," state the authors.
'Eat less, move more'
The amount of carbon that is lost can be increased with exercise. By
substituting 1 hour of rest for 1 hour of moderate exercise such as
jogging, the metabolic rate is increased sevenfold, removing an
additional 40 g of carbon from the body, increasing the daily total by
around 20% to 240 g.
However, this can easily be offset with unhealthy eating. A single 100 g
muffin, for example, provides around 20% of an average person's total
daily energy requirement. "Physical activity as a weight loss strategy
is, therefore, easily foiled by relatively small quantities of excess
food," write the authors. The solution is a traditional one - "eat less,
move more."
"We recommend these concepts be included in secondary school
science curriculums and university biochemistry courses to correct
widespread misconceptions about weight loss," they conclude.
Although the findings of the paper may alter how weight loss is
understood, strategies for losing weight should remain unchanged. Eating
less and moving more is a surefire way to combat the overindulgences of
the festive season.
