11.2: Protein Requirements

How much protein do we need? We need to replace normal losses (maintain body tissues). For this, the need is based on normal body weight, with adjustments, if needed, for inadequate calories and for the dietary source of the protein. Children and pregnant or nursing women need additional protein. They’re either adding body tissue or “exporting” protein in breast milk. Some amino acids are required for purposes other than making protein, e.g., tryptophan is used to make the neurotransmitter serotonin (Chap. 15), but the amounts needed are small and are covered by a normal diet that meets protein needs.

Effect of Inadequate Calories

Meeting energy needs is a higher priority for the body than meeting protein needs. If a diet is deficient in calories, the body “burns” amino acids for its calories, rather than use them to make protein. In other words, if we don’t get enough calories, we need more protein because some of the protein is diverted for energy production (Chap. 9).

Low calories and low protein go hand-in-hand in many developing countries. Protein deficiency is made much worse by a calorie deficit. This creates a dilemma. Should the recommended protein intake be based on the average intake of calories, which is often inadequate, or should it be lower as if calories were adequate? (This isn’t of much concern in the U.S., where diets are usually sufficient or excessive in calories and we have the luxury of recommending nutrients in amounts well above a minimum.)

This is an important question because recommendations for protein intake are used in deciding food policies, and are used by other countries and world health organizations in providing aid. Protein-rich food (e.g., meat, egg, milk) costs much more than carbohydrate-rich food (e.g., rice, corn). Using protein-rich food to provide calories is an inefficient use of resources. When resources are limited, providing the least expensive form of calories must be considered along with providing adequate protein.

Effect of Dietary Source of Protein

Protein recommendations are higher when protein comes exclusively from plants. More is needed because of lower digestibility and quality. Proteins in whole grains, beans, and vegetables are about 80-85% digested, whereas animal proteins are about 95% digested. This difference is significant in diets with marginal amounts of protein.

When protein quality is low, extra protein is needed to provide adequate amounts of the essential amino acids. In Figure 11.2, the letter T represents the limiting amino acid in bread. Since the amino acids in bread come as a “package,” eating more bread to get more T means eating more “extra letters” as well. These extra letters represent the extra protein needed to provide enough essential amino acids.

Because of its high quality, animal protein is especially valuable to growing children. Essential amino acids are a larger part of animal protein than of plant protein. An infant’s need for essential amino acids is about 45% of the protein requirement, whereas for adults it’s only about 20%. It’s no coincidence that breast milk is an infant’s first and best food. Mother’s milk is an easily digested, rich combination of nutrients (including essential amino acids) tailor-made for a baby.

Growing children need not only high-quality protein but, as noted earlier, also lots of protein relative to their body weight. Animal foods (e.g., milk, eggs, meat) are not only more concentrated in protein than plant foods (e.g., beans, grain), but also have more fat—concentrated sources of calories are also scarce for many of the world’s children.

Recommended Dietary Allowance (RDA)

As with other nutrients, the RDA for protein is generous, with a large margin of safety to meet the needs of virtually all healthy people in the U.S. (Table 11-2). The RDA is based on the typical diet—a mix of plant and animal proteins and enough calories.

Table 11-2: Recommended Dietary Allowance (RDA) for Protein

As discussed in Chapter 4, there are several sets of RDA. Infants less than 6 months old grow fast, and their RDA is the highest at 1.0 gram of protein per pound body weight. The RDA of a man (or woman who isn’t pregnant or nursing) is 0.36 g/lb. For perspective, the RDA for a 150-lb adult is 54 g protein. A McDonald’s meal of a Big Mac, and medium fries and shake has 44 g protein (and 1490 calories).

The amount of protein in the typical American diet is much higher than the RDA. We eat lots of meat and dairy products. Also, animal proteins are part of many of our favorite foods. Eggs are in cookies, cake, and pecan and lemon pie. We add milk or milk products to many foods—sour cream on baked potatoes, cream in coffee, milk in pumpkin pie with whipped cream on top, ice cream above split bananas and below hot fudge, milk over our cereal, cheese on pizza and in burritos.

Protein Requirements of Athletes

Do athletes need more protein? Yes and no. Yes, when they’re building muscle or if their athletic endeavors make them sweat profusely (some nitrogen is lost in sweat, which protein in the diet replaces). No, they typically don’t need to consume more protein. Chances are, their diet already provides much more than they need. Not only is the typical American diet high in protein, but athletes eat a lot to meet their higher calorie needs. When people eat more, they take in more of all nutrients, protein included.

Whatever more an athlete needs, it isn’t much. Protein RDA even for fast-growing and active teenage boys (age 15-18) is only 0.41 g/lb (vs. 0.36 g/lb for a grown man). As stated by the RDA committee, “In view of the margin of safety in the RDA, no increment is added for work or training.” Keep in mind that protein itself isn’t the best source of energy (Chap. 9). Carbohydrate is a better and less expensive source.*

Believing that something helps, even if it doesn’t, has great psychological value. Many athletes take expensive amino acid supplements in hopes of increasing their athletic prowess. Free amino acids have more allure than ordinary protein, although ordinary protein is rapidly broken down to amino acids in the digestive tract.

Use of amino acid supplements by athletes is relatively new, so long-term effects, if any, aren’t known. Excessive protein (amino acids) does increase water loss through urine, which can raise the risk of dehydration. People taking amino acid supplements should keep in mind that fat-free sources of protein foods like cooked egg white† and fat-free milk are much cheaper and more likely to have a balanced amino acid content (although sometimes the supplements are simply “pre-digested” egg or milk protein). What about a peanut butter sandwich—a convenient, tasty, inexpensive, protein/energy supplement!

^{*Carbohydrate, unlike protein, doesn’t have the liability of aminos (this liability will be discussed shortly), and, unlike fat, it isn’t wholly dependent on oxygen for energy production.
†Routinely eating lots of raw egg white (e.g., 8 raw eggs a day “a la Rocky the boxer”) can cause a biotin (a B-vitamin) deficiency. Avidin, a protein in egg white, binds to biotin in food and keeps biotin from being absorbed. This doesn’t happen with cooked egg white; heat inactivates (denatures) avidin. Before biotin was identified, it was described as a dietary factor protective against raw-egg-white injury in rats.}