Andrews, October 13th, 2008.
What are proteins?
Proteins are organic molecules made up of amino acids – the building blocks
of life. These amino acids are joined together by chemical bonds and then folded
in different ways to create three-dimensional structures that are important to
our body’s functioning.
A diagram of protein structures. Image source: Madison
Technical College’s Lab Manual on Protein Structure
There are two main categories of amino acids in the body. First, we’ve got
essential amino acids – those that the body can’t manufacture, and thus we must
consume in our diets. Some amino acids are conditionally essential, which means
that our bodies can’t always make as much as we need (for example, when we’re
Next, kinda obviously, we’ve got nonessential amino acids – those that the
body can usually make for itself.
|Conditionally essential amino acids||Nonessential amino
Why is it important to get enough protein?
During digestion, the body breaks down the protein we eat into individual
amino acids, which contribute to the plasma pool of amino acids. This
pool is a storage reserve of amino acids that circulate in the blood. The amino
acid pool in the bloodstream readily trades with the amino acids and proteins in
our cells, provides a supply of amino acids as needed, and is continuously
replenished. (Think of it like a Vegas buffet of protein for the cells.)
Since our bodies need proteins and amino acids to produce important molecules
in our body – like enzymes, hormones, neurotransmitters, and antibodies –
without an adequate protein intake, our bodies can’t function well at all.
Protein helps replace worn out cells, transports various substances throughout
the body, and aids in growth and repair.
Consuming protein can also increase levels of the hormone glucagon, and
glucagon can help to control body fat.1 Glucagon is released when
blood sugar levels go down. This causes the liver to break down stored glycogen
into glucose for the body. It can also help to liberate free fatty acids from
adipose tissue – another way to get fuel for cells and make that bodyfat do
something useful with itself instead of hanging lazily around your
How much protein do you need?
How much protein you need depends on a few factors, but one of the most
important is your activity level.
The basic recommendation for protein intake
is 0.8 grams per kilogram (or around 0.36 g per pound) of body mass in
untrained, generally healthy adults. For instance, a 150 lb (68 kg) person would
consume around 54 grams a day.
However, this amount is only to prevent protein deficiency. It’s not
necessarily optimal, particularly for people such as athletes who train
regularly and hard.
For people doing high intensity training, protein needs might go up to about
1.4-2.0 g/kg (or around 0.64-0.9 g/lb) of body mass.2 Our
hypothetical 150 lb (68 kg) person would thus need about 95-135 g of protein per
These suggested protein intakes are what’s necessary for basic protein
synthesis (in other words, the creation of new proteins from individual
building blocks). The most we need to consume throughout the day for protein
synthesis probably isn’t more than 1.4 – 2.0 g/kg.
But wait – there’s more! Beyond the basics of preventing deficiency and
ensuring a baseline of protein synthesis, we may need even more protein in our
diets for optimal functioning, including good immune function, metabolism,
satiety, weight management and performance.3 In other words, we need
a small amount of protein to survive, but we need a lot more to thrive.
We can only store so much protein at one time. As the graph below shows, the
body’s protein stores fluctuate over the course of a day. Notice how the upper
limit never increases; the amount of protein in the body just cycles up and down
as we eat or fast.
The take-home here is that you can’t simply eat a 16-pound steak (a la Homer
Simpson consuming “Sirloin A Lot”) once and be done with it. The body needs its
protein stores to be continually replenished, which means that you should
consume moderate amounts of protein at regular intervals – which just happens to
be an important Precision Nutrition guideline.
Consuming more protein may help maintain an optimal body composition (in
other words, help you stay leaner and more muscular) and a strong immune system,
good athletic performance, and a healthy metabolism. It may promote satiety
(i.e. make you feel full longer) and consequently help you manage your body
weight. Indeed, physique athletes such as bodybuilders have long relied on the
rule of 1 gram of protein per pound of body weight – or 150 g per day for a 150
For extra credit
When you eat protein is just as important as how much. After resistance
exercise (RE) such as weight training, the body synthesizes proteins for up to
48 hours after training.4 Interestingly, during and immediately after
RE, protein breakdown is increased as well. In fact, for a brief period, the
rate of breakdown exceeds the rate of building. The body actually drops into a
short-term wasting or catabolic state. However, taking in enough protein during
the pre- and post-exercise period can offset catabolism. (Check out the Precision Nutrition guide for more
on nutrition timing.)
The graph below shows that as the blood concentration of essential amino
acids (EAA) increases, so too does protein synthesis.
The graph below shows how amino acid (and amino acid + carbohydrate)
consumption after exercise results in a positive muscle protein balance (in
other words, helping muscles rebuild, which is a good thing), while the intake
of no nutrients can result in a negative muscle protein balance.
Which protein is best? In general it’s your choice – both protein from plant
sources and animal sources seem to work equally well in increasing muscle
protein synthesis as a result of exercise.5 The amino acid leucine
seems to act as a major stimulus for protein synthesis; good sources of leucine
include spirulina, soy protein, egg white, milk, fish, poultry, and meat.
Can I eat too much protein?
If you overeat protein, this extra protein can be converted into sugar or fat
in the body. However, protein isn’t as easily or quickly converted as
carbohydrates or fat, because the thermic effect (the amount of energy
require to digest, absorb, transport and store protein) is a lot higher than
that of carbohydrates and fat. While 30% of the protein’s energy goes toward
digestion, absorption, and assimilation, only 8% of carbohydrate’s energy and 3%
of fat’s energy do the same.
You might have heard the statement that a high protein intake harms the
kidneys. This is a myth. In healthy people, normal protein intakes pose little
to no health risk. Indeed, even a fairly high protein intake – up to 2.8 g/kg
(1.2 g/lb) – does not seem to impair kidney status and renal function in people
with healthy kidneys.6 In particular, plant proteins appear to be
Here’s additional reading on this topic:
Dear Mom and
Summary and recommendations
- For basic protein synthesis, you don’t need to consume more than 1.4 to 2.0
g/kg (around 0.64-0.9 g/lb) of protein per day.
- Nevertheless, consuming higher levels of protein (upwards of 1g per pound of
body weight) may help you feel satisfied after eating as well as maintain a
healthy body composition and good immune function.
- You should consume some protein before and after training to ensure adequate
- Flatt JP 1978; Tappy L, 1996; Blom WA et al., 2006; Latner JD, Schwartz M,
- Lemon et al 1981; Tarnopolsky et al 1988; Tarnopolsky et al 1991.
- Flatt JP 1978; Tappy L, 1996; Blom WA et al., 2006; Latner JD, Layman et al
2003; Schwartz M, 1999; Tangney CC, et al. 2005; Kishino Y & Moriguchi S
1992; Marcos A, et al 2003.
- Dreyer et al 2006; Koopman et al 2006; Biolo et al 1995; Phillips et al
1997; Norton et al 2006; MacDougall et al 1995.
- Brown et al 2004; Anthony et al 2007; Kalman et al 2007.
- Poortmans JR & Dellalieux O 2000.
- Am Diet Assoc 2003; Millward DJ 1999.
Association. Position of the American Dietetic Association and Dietitians of
Canada: Vegetarian diets. J Am Diet Assoc 2003;103:748-765.
Anthony TG, McDaniel BJ,
Knoll P, Bunpo P, Paul GL, McNurlan MA. J Nutr 2007;137:357-362.
Biolo G, Maggi SP, Williams
BD, Tipton KD, Wolfe RR. Am J Physiol 1995;268:E514–E520.
Blom WA, Lluch A, Stafleu A,
Vinoy S, Holst JJ, Schaafsma G, Hendriks HF. Effect of a high-protein breakfast
on the postprandial ghrelin response. Am J Clin Nutr 2006;83:211-220.
Boelens PG, Nijveldt RJ,
Houdijk AP, Meijer S, van Leeuwen PS. Glutamine alimentation in catabolic state.
J Nutr 2001;131(9 Suppl):2569S-2577S.
Brown EC, DiSilvestro RA,
Babaknia A, Devor ST. Nutr J 2004;3:22-27.
Dreyer HC, Fujita S, Cadenas
JG, Chinkes DL, Volpi E, Rasmussen BB. J Physiol 2006;576:613-624.
Flatt JP. The biochemistry of
energy expenditure. In: Bray GA ed. Recent advances in obesity research. London:
Furst P & Stehle P. What
are the essential elements needed for the determination of amino acid
requirements in humans? J Nutr 2004;134(6 Suppl):1558S-1565S.
Kalman D, Feldman S, Martinez
M, Krieger DR, Tallon MJ. Effect of protein source and resistance training on
body composition and sex hormones. JISSN 2007;4:4.
Kishino Y & Moriguchi S.
Nutritional factors and cellular immune responses. Nutr Health
Koopman R, Zorenc AH,
Gransier RJ, Cameron-Smith D, van Loon LJ. Am J Physiol Endocrinol Metab
Latner JD & Schwartz M.
The effects of a high-carbohydrate, high-protein or balanced lunch on later food
intake and hunger ratings. Appetite 1999;33:119–128.
Lemon PW & Nagle FJ.
Effects of exercise on protein and amino acid metabolism. Med Sci Sports Exerc
Lemon PW, Tarnopolsky MA,
MacDougall JD, Atkinson SA. Protein requirements and muscle mass/strength
changes during intensive training in novice bodybuilders. J Appl Physiol
MacDougall JD, Gibala MJ,
Tarnopolsky MA, MacDonald JR, Interisano SA, Yarasheski KE. Can J Appl Physiol
Marcos A, Nova E, Montero A.
Changes in the immune system are conditioned by nutrition. Eur J Clin Nutr
2003:57 Suppl 1;S66-S69.
Millward DJ. Optimal intakes
of protein in the human diet. Proc Nutr Soc 1999;58:403-413.
Norton LE & Layman DK. J
Phillips SM, Tipton KD,
Aarsland A, Wolf SE, Wolfe RR. Am J Phisiol 1997;273:E99–E107.
Reeds PJ. Dispensable and
indispensable amino acids for humans. J Nutr 2000;130:1835S-1840S.
Rennie MJ & Tipton KD.
Protein and amino acid metabolism during and after exercise and the effects of
nutrition. Annu Rev Nutr 2000;20:457-483.
Schwartz MW & Kahn SE.
Insulin resistance and obesity. Nature 1999;402:860-861.
Soeters PB, van de Poll MC,
van Gemert WG, Dejong CH. Amino acids adequacy in pathophysiological states. J
Nutr 2004;134(6 Suppl):1575S-1582S.
Tangney CC, Gustashaw KA,
Stefan TM, Sullivan C, Ventrelle J, Filipowski CA, Heffernan AD, Hankins J. A
review: which dietary plan is best for your patients seeking weight loss and
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Tappy L. Thermic effect of
food and sympathetic nervous system activity in humans. Reprod Nutr Dev
Tarnopolsky MA, Atkinson SA,
MacDougall JD, Chesley A, Phillips S, Schwarcz HP. Evaluation of protein
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