This passage is excerpted from Rob Dunn, “Science Reveals Why Calorie Counts Are All Wrong.” © 2015 by Scientific American.
Food is energy for the body. Digestive enzymes in the
mouth, stomach and intestines break up complex food
molecules into simpler structures, such as sugars and amino
acids that travel through the bloodstream to all our tissues. Our
cells use the energy stored in the chemical bonds of these
simpler molecules to carry on business as usual. We calculate
the available energy in all foods with a unit known as the food
calorie, or kilocalorie—the amount of energy required to heat
one kilogram of water by one degree Celsius. Fats provide
approximately nine calories per gram, whereas carbohydrates
and proteins deliver just four. Fiber offers a piddling two
calories because enzymes in the human digestive tract have
great difficulty chopping it up into smaller molecules.
Every calorie count on every food label you have ever seen
is based on these estimates or on modest derivations thereof.
Yet these approximations assume that the 19th-century
laboratory experiments on which they are based accurately
reflect how much energy different people with different bodies
derive from many different kinds of food. New research has
revealed that this assumption is, at best, far too simplistic. To
accurately calculate the total calories that someone gets out of a
given food, you would have to take into account a dizzying
array of factors, including whether that food has evolved to
survive digestion; how boiling, baking, microwaving or
flambéing a food changes its structure and chemistry; how
much energy the body expends to break down different kinds
of food; and the extent to which the billions of bacteria in the
gut aid human digestion and, conversely, steal some calories
for themselves.
Nutrition scientists are beginning to learn enough to
hypothetically improve calorie labels, but digestion turns out to
be such a fantastically complex and messy affair that we will
probably never derive a formula for an infallible calorie count.
Consider how vegetables vary in their digestibility. We eat
the stems, leaves and roots of hundreds of different plants. The
walls of plant cells in the stems and leaves of some species are
much tougher than those in other species. Even within a single
plant, the durability of cell walls can differ. Older leaves tend
to have sturdier cell walls than young ones. Generally
speaking, the weaker or more degraded the cell walls in the
plant material we eat, the more calories we get from it.
Cooking easily ruptures cells in, say, spinach and zucchini, but
cassava (Manihot esculenta) or Chinese water chestnut
(Eleocharis dulcis) is much more resistant. When cell walls
hold strong, foods hoard their precious calories and pass
through our body intact (think corn).
Some plant parts have evolved adaptations either to make
themselves more appetizing to animals or to evade digestion
altogether. Fruits and nuts first evolved in the Cretaceous
(between 145 and 65 million years ago), not long after
mammals were beginning to run between the legs of dinosaurs.
Evolution favored fruits that were both tasty and easy to digest
to better attract animals that could help plants scatter seeds. It
also favored nuts and seeds that were hard to digest, however.
After all, seeds and nuts need to survive the guts of birds, bats,
rodents and monkeys to spread the genes they contain.
Even foods that have not evolved to survive digestion differ
markedly in their digestibility. Proteins may require as much as
five times more energy to digest as fats because our enzymes
must unravel the tightly wound strings of amino acids from
which proteins are built. Yet food labels do not account for this
expenditure. Some foods such as honey are so readily used that
our digestive system is hardly put to use. They break down in
our stomach and slip quickly across the walls of our intestines
into the bloodstream: game over.
Finally, some foods prompt the immune system to identify
and deal with any hitchhiking pathogens. No one has seriously
evaluated just how many calories this process involves, but it is
probably quite a few. A somewhat raw piece of meat can
harbor lots of potentially dangerous microbes. Even if our
immune system does not attack any of the pathogens in our
food, it still uses up energy to take the first step of
distinguishing friend from foe. This is not to mention the
potentially enormous calorie loss if a pathogen in uncooked
meat leads to illness.
Food is energy for the body. Digestive enzymes in the
mouth, stomach and intestines break up complex food
molecules into simpler structures, such as sugars and amino
acids that travel through the bloodstream to all our tissues. Our
cells use the energy stored in the chemical bonds of these
simpler molecules to carry on business as usual. We calculate
the available energy in all foods with a unit known as the food
calorie, or kilocalorie—the amount of energy required to heat
one kilogram of water by one degree Celsius. Fats provide
approximately nine calories per gram, whereas carbohydrates
and proteins deliver just four. Fiber offers a piddling two
calories because enzymes in the human digestive tract have
great difficulty chopping it up into smaller molecules.
Every calorie count on every food label you have ever seen
is based on these estimates or on modest derivations thereof.
Yet these approximations assume that the 19th-century
laboratory experiments on which they are based accurately
reflect how much energy different people with different bodies
derive from many different kinds of food. New research has
revealed that this assumption is, at best, far too simplistic. To
accurately calculate the total calories that someone gets out of a
given food, you would have to take into account a dizzying
array of factors, including whether that food has evolved to
survive digestion; how boiling, baking, microwaving or
flambéing a food changes its structure and chemistry; how
much energy the body expends to break down different kinds
of food; and the extent to which the billions of bacteria in the
gut aid human digestion and, conversely, steal some calories
for themselves.
Nutrition scientists are beginning to learn enough to
hypothetically improve calorie labels, but digestion turns out to
be such a fantastically complex and messy affair that we will
probably never derive a formula for an infallible calorie count.
Consider how vegetables vary in their digestibility. We eat
the stems, leaves and roots of hundreds of different plants. The
walls of plant cells in the stems and leaves of some species are
much tougher than those in other species. Even within a single
plant, the durability of cell walls can differ. Older leaves tend
to have sturdier cell walls than young ones. Generally
speaking, the weaker or more degraded the cell walls in the
plant material we eat, the more calories we get from it.
Cooking easily ruptures cells in, say, spinach and zucchini, but
cassava (Manihot esculenta) or Chinese water chestnut
(Eleocharis dulcis) is much more resistant. When cell walls
hold strong, foods hoard their precious calories and pass
through our body intact (think corn).
Some plant parts have evolved adaptations either to make
themselves more appetizing to animals or to evade digestion
altogether. Fruits and nuts first evolved in the Cretaceous
(between 145 and 65 million years ago), not long after
mammals were beginning to run between the legs of dinosaurs.
Evolution favored fruits that were both tasty and easy to digest
to better attract animals that could help plants scatter seeds. It
also favored nuts and seeds that were hard to digest, however.
After all, seeds and nuts need to survive the guts of birds, bats,
rodents and monkeys to spread the genes they contain.
Even foods that have not evolved to survive digestion differ
markedly in their digestibility. Proteins may require as much as
five times more energy to digest as fats because our enzymes
must unravel the tightly wound strings of amino acids from
which proteins are built. Yet food labels do not account for this
expenditure. Some foods such as honey are so readily used that
our digestive system is hardly put to use. They break down in
our stomach and slip quickly across the walls of our intestines
into the bloodstream: game over.
Finally, some foods prompt the immune system to identify
and deal with any hitchhiking pathogens. No one has seriously
evaluated just how many calories this process involves, but it is
probably quite a few. A somewhat raw piece of meat can
harbor lots of potentially dangerous microbes. Even if our
immune system does not attack any of the pathogens in our
food, it still uses up energy to take the first step of
distinguishing friend from foe. This is not to mention the
potentially enormous calorie loss if a pathogen in uncooked
meat leads to illness.