They are used to construct the cells and tissues that form our bodies,
provide sources of energy to power metabolism (as well as provide
a mechanism for storing energy between meals), and are used
to form the countless enzymes that drive our metabolism. Unlike
the micronutrients (vitamins and minerals) which are needed in
small amounts and are generally reused, macronutrients undergo
a constant flux in our body, necessitating a consistent intake to
provide enough energy for our survival and enough building blocks
for the growth, maintenance, and repair of our bodies.
Each of the macronutrients is actually a complex of smaller
building blocks with important nutritional roles. Proteins are
constructed of amino acids, carbohydrates of sugars or monosaccharides,
and fats of fatty acids. As macronutrients are absorbed
from a meal, they are broken down into their individual
constituents, which are used for the various purposes in metabolism.
The great adaptability of our body chemistry gives
us the ability to take these individual building blocks (amino
acids, sugars, and fatty acids) and reassemble them or even
interconvert them to satisfy our metabolic needs. This explains
how an Innuit can consume a diet predominantly of fish protein
and fat, yet still have enough carbohydrate (glucose) in
their blood to fuel the demand of their brains. Or how someone
can adopt a completely fat-free diet, yet still become obese
through the conversion of excessive dietary carbohydrates into
body fat. Because of their interconversion in the body, macronutrients
themselves are not nutritionally essential, although
some of their building blocks may be (nine of the amino acids
and two of the fatty acids are considered essential). Evidence
also suggests that although macronutrients can be readily interconverted
in the body, each may have additional benefits
when present in the diet as a particular percentage of total dietary
intake.
To gain an appreciation of the function of each of the
macronutrients and ultimately understand how much we may
need, it is really necessary to discuss each individually. In this
first part of the series, we will examine the roles and requirements
for dietary protein.
PROTEIN
Out of all the macronutrients, protein has the most diverse
set of roles in metabolism. It forms the connective tissues
that hold our organs together; it is a significant portion of our
bones, and the predominant component of muscle fibers.
Moreover, protein forms the thousands of enzymes which carry
out critical chemical reactions, the various transporters that move nutrients in and out of cells and throughout the body, the
antibodies of our immune system, and many of the hormones
that direct our growth, energy utilization, and homeostasis.
Due to its ubiquitous nature in the function of all living
things, dietary protein occurs in most whole food sources. Muscle
meats are the most concentrated sources (muscle fibers are
constructed of protein filaments); milk and eggs are also good
sources. Legumes, exotic grains, and many vegetables are good
protein sources, such that a balanced vegetarian or vegan diet
can provide adequate protein for the body’s needs.
Once consumed, dietary protein is broken down by the
action of stomach acid and several digestive enzymes secreted
from the stomach and pancreas (called proteases). From here,
the resulting individual amino acids or small protein fragments
(called peptides) are absorbed from the small intestine,
and distributed throughout the body to satisfy various roles.
We generally don’t absorb enzymes or other proteins intact.
ROLES OF DIETARY PROTEIN IN NORMAL METABOLISM
Dietary protein has several fates in human metabolism:
New Protein Synthesis. The amino acids liberated from dietary
protein can be used to make other proteins in the body. While
organisms can make amino acids from other sources (such as
fats or carbohydrates), making new proteins from dietary amino
acids is the quickest and most energetically economical way. This
is especially important for sustaining periods of rapid growth,
such as during childhood development or intense weight training.
Perhaps more importantly, dietary protein is the only source
of essential amino acids for metabolism. Of the twenty different
amino acids used to make proteins, humans have lost the ability
to produce nine of them on their own (methionine, lysine, valine,
tryptophan, phenylalanine, isoleucine, leucine, threonine,
and histidine). Therefore, a minimal amount of dietary protein
is required to supply enough of these essentials to maintain protein
synthesis in the body.
Precursors to other biomolecules. Several of the essential
amino acids from dietary protein are used to construct important
“non-protein” chemicals for the body. For example,
the hormones seratonin and melatonin, and vitamin B3 (niacin)
are derived from the essential amino acid tryptophan;
thyroxine (thyroid hormone), adrenaline, and the endorphins
(natural analgesics) depend on intake of the essential amino
acid phenylalanine. Thus, our ability to produce these hormones
and neurotransmitters is heavily dependent on the
presence of essential amino acids in the diet.
Dietary protein is also the predominant sources of the
elements nitrogen and sulfur, both essential to metabolism.
Nitrogen from dietary amino acids is redistributed in the
body to make other amino acids, nucleotides (the building
blocks of DNA, as well as the energy molecule ATP), and glycosaminoglycans
(components of connective tissues, such
as chondroitin, keratan, and hyaluronic acid). Sulfur is also
used in the construction of glycosaminoglycans, as well as
several important antioxidants (such as glutathione and alpha
lipoic acid).
Fuel Source. Dietary protein can serve as an energy source.
Following a meal, about 50 percent of the amino acids that
have been released from dietary protein are metabolized by
the liver into energy (ATP). Unlike carbohydrates or fats, excess
amino acids from the diet are not stored in the body; if they are
not immediately used to make new protein or energy, they are
converted to carbohydrates or fats for storage. The liver can
also metabolize most amino acids into glucose, to provide energy
to the brain and other tissues. Proteins are less efficient at
raising blood glucose than are carbohydrates; while they provide
the same number of calories on a gram-for-gram basis (4
calories/gram), they only raise blood glucose 50–80 percent as
much as an equivalent amount of carbohydrates.
Skeletal muscles depend heavily on amino acids for energy.
The essential amino acids leucine, isoleucine, and valine
(called the branched chain amino acids or BCAAs) are preferentially
taken up by muscle cells after a meal to be burned as
fuel. Healthy individuals will metabolize upwards of 10 grams/
day of BCAAs in their muscles if they are available in the diet.
SPECIFIC HEALTH BENEFITS OF DIETARY PROTEIN
In addition to their standard roles in metabolism, dietary protein
has been associated with specific health effects, including:
Weight Loss and Satiety. High protein diets have been associated
with better glycemic control, and have been shown to
promote greater fat reduction than high carbohydrate or high
fat diets that provide the same number of calories. Dietary proteins
are more difficult to convert into energy than the carbohydrates
or fat; diets high in protein have been reported to have a
greater thermogenic effect and expend more energy than lower
protein diets. Randomized, controlled trials comparing low and
high-protein diets studies have shown that diets higher in
protein are more effective at preserving lean muscle, reducing
body fat, and maintaining lower insulin levels after a meal.
There is convincing evidence that proteins are more satiating
than the other macronutrients, and that the satiating ability
of proteins may be related to their amino acid composition
and how quickly they are digested. Much of this effect has been
attributed to the rapid appearance of the essential branched
chain amino acids in the blood; leucine, one of the BCAAs,
has been shown to influence the metabolic pathways in the
brain that regulate food intake, at least in animal models. Evidence
also suggests that higher protein intake at one meal
may significantly decrease appetite at the next meal, although
the studies in this area are not consistent.
Promoting Healthy Levels of Blood Lipids.
Dietary protein, particularly dietary soy protein, has been studied for its ability
to lower cholesterol levels by either increasing the removal
of low-density lipoprotein cholesterol (LDL or “bad” cholesterol)
from the blood, or as a replacement for other high fat/
high cholesterol protein sources. Over 60 controlled trials of
soy protein consumption in humans have been performed,
many in hypercholesterolemic patients. Taken together, these
studies revealed that an average intake of 47 g/day of soy
protein resulted in significant improvements in blood lipid/lipoprotein parameters, with average reductions in total cholesterol of 9 percent
and LDL cholesterol of 12.9 percent. These
data were the foundation for the FDA approved
health claim for soy protein in
the prevention of cardiovascular disease.
Promoting Healthy Blood Pressure. Several
human trials and epidemiological
studies have indicated an inverse associate
between dietary protein intake and
blood pressure. Forty-six human studies
of protein intake and blood pressure (20
clinical trials, 15 observational studies and
13 biomarker studies) have demonstrated
a clear, beneficial effect for plant protein on
reductions in blood pressure. The reductions
averaged up to a 1.4 mm Hg reduction in
systolic blood pressure and a 1 mm Hg reduction
in diastolic blood pressure for every
11 g of plant protein consumed per day,
based on observational studies. The mechanism
by which protein may reduce blood
pressure is unclear; it may be helping to rid
the body of sodium, it may increase insulin
sensitivity, or it may increase the blood
concentration of the amino acid arginine,
the precursor to the blood-pressure lowering
hormone nitric oxide.
HOW MUCH DIETARY PROTEIN SHOULD I BE GETTING?
As with most macronutrients, the required
amount of dietary protein depends
on individual needs. Discussion of the
research regarding the merits of diets differing
in the relative ratios of protein, carbohydrate,
and fats will be the subject of
a future article, but some general considerations
on dietary protein content bear
mentioning here.
The Food and Nutrition Board of the
National Institute of Medicine has established
a dietary reference intake of 56 g/day
for adult men, 46 g/day for adult women,
based on metabolic studies. These figures
are based on a reference (“average”) body
weight; a more individualized assessment
of daily protein requirements for a healthy
individual would be 0.8 g/kg (about 0.36
g/lb) body weight. Under circumstances
of increased metabolic demand, higher
protein intakes may be warranted. In
pregnant and lactating women, for example,
daily protein requirements increase
to approximately 1.1 g/kg of body weight,
or 71 g/day for the “average” woman. The
dietary protein requirements of athletic individuals
who wish to increase their lean
body mass has been the subject of considerable
debate, but is generally believed to
exceed the reference values. Studies have
suggested protein requirements of 1.1 g/
kg per day for endurance athletes and 1.3
g/kg per day in strength-trained athletes.
There are some circumstances where
reduced protein intake may be warranted.
Since excess dietary protein is not stored
in the body, it must be immediately used
up (to make new proteins), converted into
energy, or converted to carbohydrates or
fat for storage. In the latter two cases, amino
acids are broken down, and the nitrogen
they carry is eliminated from the body
(as urea). The breakdown of amino acids
and excretion of nitrogen are fundamental
functions of the liver and kidneys, but for
individuals with kidney or liver disease, excessive
protein consumption can be problematic.
Low-protein diets (below 0.8 g/kg
per day) may be beneficial in these cases.
Dietary protein requirements should also
be carefully considered in individuals with
hyperuricemia, a condition of excessive
uric acid in the blood. Hyperuricemia affects
an estimated 21 percent of Americans,
and is a primary risk factor for gout,
a type of arthritis typified by a rapid onset
of inflammation, usually in the joints of
the extremities. Elevated blood uric acid is
also a risk for kidney and cardiovascular
diseases, and diabetes. While dietary protein
itself does not elevate blood uric acid,
compounds found in some sources of animal
protein (called purines) do increase
hyperuricemia risk. Therefore, individuals
with elevated blood uric acid should limit
their intake of protein from meats (other
animal proteins, such from milk or eggs,
as well as vegetable protein, do not appear
to be associated with hyperuricemia
risk and may actually reduce it).
To read the series on Macronutrients:
