By Lyle McDonald and Jamie Hale

From Chapter 4: Protein Requirements, part 1

“Over the past two to three decades, there has been an almost never-ending debate regarding human requirements for protein. Without going into the debate’s history, the basic argument has come down to whether or not athletes need more protein than average, sedentary individuals. Mainstream nutrition types, especially registered dietitians (RDs), have either maintained that athletes don't need any excess protein or that they already get more than enough. On the other hand, athletes and some sports nutritionists, who are often dismissed as quacks, maintain that athletes do in fact need more protein to support their heavy training. In the research world, this argument persists. In this chapter, we will look at both sides of the argument, provide background data for both cases, and offer our general recommendations.

Protein turnover and nitrogen balance

Every day, even as you sit doing nothing, your body is undergoing a constant breakdown and resynthesis of your body’s many tissues. This includes body protein, which, depending on the tissue you're talking about, breaks down and is resynthesized at some rate. The combination of protein breakdown and protein synthesis is referred to as protein turnover. (1)
Liver proteins turnover very quickly, muscle proteins somewhat less quickly, and tissues like tendons and ligaments turnover very slowly. An average sized human may breakdown and resynthesize 300 grams of protein per day. Of course, you don't need 300 grams of protein per day because most of the protein which is broken down is simply reused for synthesis.
However, no process in the body is 100 percent efficient, and protein breakdown is no different. Due to protein turnover, some percentage of protein will end up being oxidized (burned) and excreted as urea, creatinine, ammonia, or another substance. With normal dietary intakes, roughly four percent of the daily protein turned over may be lost. (2)
If you'll recall from chapter XXX (basic definitions), one of the defining aspects of protein is that it contains nitrogen, and researchers can measure protein loss by measuring nitrogen loss. Typically nitrogen is lost in the urine, but it is also lost in your feces, sweat, hair, skin, finger nails, and other secretions (3). Since it is extremely difficult to measure all sources of nitrogen loss, estimates are generally used for feces, skin, hair, and the rest. Early studies would sew people into bags so that all excreted nitrogen could be measured, and estimates have been developed from those studies. Generally speaking, only urinary nitrogen excretion is measured, and this represents roughly 80 percent of your total daily nitrogen loss.
Such measurements allow researchers to perform something called a nitrogen balance study. This study allows them to compare the amount of protein going into the body (via dietary protein) to that coming out of the body (urine, feces, etc). If an individual is consuming more nitrogen than they are losing, they are said to be in nitrogen balance. In other words, they are presumably storing body protein. If they are losing more protein than they are ingesting, they are in negative nitrogen balance and are losing body protein.
Since protein (amino acids, to be more accurate) is the primary source of nitrogen, a negative nitrogen loss implies that the body is breaking down body protein. However, a simple nitrogen balance study does not give any indication which amino acids are being lost or from where they are being lost. The loss could be coming from muscle tissue, or from the breakdown of liver proteins. Under extreme conditions of severe starvation, organ protein can be broken down and lost.
It's important to point out that nitrogen balance depends heavily on total calorie intake, especially intake of carbohydrates (1). Essentially, as carbohydrate intake goes up, so does nitrogen balance (the body retains more protein). As carbohydrate intake goes down, nitrogen balance falls as well. This is one of the reasons that protein requirements go up when you are dieting (and total calories/carbohydrates are reduced). Dietary fat has less of an impact on improving nitrogen balance than dietary carbohydrates (4). Individuals who eat more protein will lose more nitrogen as well, simply from the increased intake (2).”

References for part 1:

1.   Waterlow JC (1984) Protein turnover with special reference to man. Q J Exp Phys 69:409-38.
2.   Waterlow JC (1986) Metabolic adaptation to low intakes of energy and protein. Ann Rev  Nutr 6:495-526.
3.   Groff James L., Gropper Sareen S., Hunt Sara M (1995) Advanced nutrition and human     metabolism, Second edition. St. Paul, Minnesota: West Publishing Company.
4.   Richardson DP, et. al (1979) Quantitative effect of an isoenergetic exchange of fat forcarbohydrate on dietary protein utilization in healthy young men. Am J Clin Nutr 32:2217–26.

From Protein Controversies, part II:

Protein and bone health. Many nutritionists have bashed protein, claiming that a high protein diet will cause weak bones because protein can lead to increased urinary excretion of calcium. However, the scientific evidence speaks for itself. One large study, published in the Journal of Bone and Mineral Research, showed that both elderly men and women who consumed the most animal protein had the lowest rate of bone loss whereas those who consumed little protein had much higher rates of bone loss. Another study published in the American Journal of Clinical Nutrition showed that postmenopausal women (the group of people at highest risk for osteoporosis), who consumed high amounts of protein, particularly animal protein, had the strongest bones and were the least likely to suffer from hip fractures. Other studies have shown that low protein diets hamper recovery from fractures (as well as illness in general).

Bone is a living, active tissue, just like any other in the body. It contains large amounts of cells that are made up partially of protein. Many of these cells have functions essential to maintaining bone health. Furthermore, eating low amounts of protein usually puts the body in a catabolic state, where muscle is cannibalized for its protein. It could be that in such a state, the body is producing lower amounts of anabolic, bone-building hormones such as testosterone, estrogen, and growth hormone. Our body isn’t stupid. Feed it a low amount of protein, and it will channel any that it gets towards essential functions such as keeping essential organs alive and neglect muscles, bones, and the immune system. Lastly, it is also known that protein induces an increase in an insulin-like growth factor, which promotes bone building.

Robert Heaney, whose team conducted the widely cited study showing a positive correlation between protein intake and urinary calcium loss, has taken it upon himself to revisit his original data. This is in light of all of the evidence pouring forth from prospective studies showing that protein intake is actually beneficial for bone. In his editorial in the American Journal of Clinical Nutrition, he critiqued his own study, stating that the women in the study were in a metabolic ward receiving abnormally low amounts of calcium. He speculated that under such conditions, protein may have some negative effects. In addition, he blasted special interest groups, such as vegetarians, for twisting the scientific evidence to suit their own political agendas.

A growing body of research suggests that eating a high protein diet does not harm bones if adequate dietary intakes of calcium and vitamin D are consumed. Although protein is essential for bone health, a high intake of protein, especially purified protein, may increase urinary calcium loss. This calcium loss could potentially cause a negative calcium balance, thereby increasing the risk of bone loss and osteoporosis. However, other nutrients in foods or the diet can offset the effects that protein has on calcium excretion.
Controversy regarding protein's effects on bone health may be explained by other nutrients in food sources of protein or the total diet. Researchers at Tufts University in Boston found that adequate dietary calcium helps to promote a favorable effect of dietary protein on the skeleton in older adults. Also, phosphorus (e.g., in milk, meat) and potassium (e.g., in milk, legumes, and grains) reduce urinary calcium loss, thereby offsetting protein-induced urinary calcium excretion.
Protein exists in close association with other nutrients in the diet. For this reason, it is important to consider protein's role in bone health in the context of other foods or the overall dietary pattern. Protein in dairy foods—milk, cheese, and yogurt—may be particularly beneficial for the skeleton because the calcium content of these foods is high in relation to their protein content. They also provide other nutrients such as phosphorus, magnesium, and zinc as well as vitamins A, D, and K, which are needed to build and maintain healthy bones. A number of studies suggest that milk's nutrient package has positive effects on bone health.

Dietary protein has several opposing effects on calcium balance, and its net effect on bone is not well established. It has long been recognized that increasing protein intake increases urinary calcium excretion. More recently, it has been observed that increasing dietary protein raises the circulating level of insulin-like growth factor-1, a growth factor that promotes osteoblast formation and bone growth. Other effects of protein on the calcium economy have been suggested in some studies, but they are less well established. Several studies have examined associations between protein intake, and bone loss and fracture rates. In the original Framingham cohort, subjects with lower total protein intakes and animal protein intakes had greater rates of bone loss from the femoral neck and spine than subjects consuming more protein. In another study, higher total (and animal) protein intakes were associated with a reduced incidence of hip fractures in post-menopausal women. In contrast, a high animal/plant protein intake has been associated with greater bone loss from the femoral neck and greater risk of hip fracture in older women. Higher total and higher animal protein intakes have also been associated with increased risk of forearm fracture in younger post-menopausal women. In a recent study, it was found that increasing dietary protein was associated with a favorable (positive) change in bone mineral density of the femoral neck and total body in subjects taking supplemental calcium citrate malate with vitamin D. Favorable changes were not in those taking the placebo. The possibility that calcium intake may influence the impact of dietary protein on the skeleton warrants further investigation.

Protein has both positive and negative effects on calcium balance, and the net effect of dietary protein on bone mass and fracture risk may be dependent on the dietary calcium intake. In addition to providing substrate for bone matrix, dietary protein stimulates the production of insulin-like growth factor-1 (IGF-1), a factor that promotes osteoblast-mediated bone formation.
Protein also increases urinary calcium losses by several proposed mechanisms. Increasing calcium intake may offset the negative impact of dietary protein on urinary calcium losses, allowing the favorable effect of protein on the IGF-1 axis to dominate. Several, although not all, studies are either compatible with or support this hypothesis. Protein supplements significantly reduced bone loss in elderly hip-fracture patients in a study in which both the protein and control groups received supplemental calcium. In an observational study, total protein intake was positively associated with favorable three year changes in femoral neck and total body bone mineral density in volunteers who received supplemental calcium citrate malate and vitamin D. Favorable changes were not found in volunteers taking placebos. In conclusion, an adequate calcium intake may help promote a favorable effect of dietary protein on the skeleton in older individuals.

References for part II:

1.   Dawson-Hughes B (2003) Calcium and protein in bone health. Prc Nutr Soc 62(2):505-9.

2.   Dawson-Hughes B (2003) Interaction of dietary calcium and protein in bone health in humans. J Nutr 133(3)852S-854S.
3.   Calcium and Bone Metabolism Laboratory at the Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging, Tu