Telling bodybuilders about the importance of eating a high-protein diet is preaching to the choir. Following a high-protein diet is considered essential to the pursuit of building muscle and strength. It’s also vital when the goal involves losing bodyfat.
One of the key elements of popular low-carb-diet plans is that they emphasize a high protein intake. Critics of low-carb diets frequently suggest that among their negative aspects are a loss of lean mass. They suggest that without sufficient carbohydrate intake, the body turns to muscle as a source of energy, resulting in a loss of muscle. That, in turn, results in a lowered resting metabolic rate, thus ensuring the return of the bodyweight lost through dieting—and then some. The scenario of losing and then rapidly regaining bodyfat is often referred to as “yo-yo dieting.”
Advocates of low-carb diets counter that the extra protein that is an integral part of such diets prevents the excessive loss of lean mass. Indeed, carbohydrates aren’t even considered essential in human nutrition because glucose, the sugar found circulating in the blood, can be made in the liver from other substrates, including the glycerol portion of fat, lactate and amino acids. That process is called gluconeogenesis. Studies show that about 57 percent of excess protein intake is converted into glucose in the liver.
A recent study examined the effects of two types of diets on physically fit human subjects.1 The diets consisted of a maintenance diet, containing the number of calories required to support normal daily activity and exercise while maintaining bodyweight, and a moderate energy-deficit diet that contained 80 percent of estimated daily calorie requirements. Both diets included 1.5 grams of protein per kilogram of bodyweight, which represented 30 percent of daily calories. That made both plans high-protein diets.
During the energy-deficit diet, subjects lost an average of a kilogram, or 2.2 pounds, of bodyweight. More important, the muscle-protein-synthesis rate dropped 19 percent. Measures of substances related to muscle protein synthesis also dropped significantly. Despite that, the nitrogen balance was maintained, which was attributed to the higher protein intake. The authors noted that other studies have also found preservation of nitrogen status with higher protein intakes, prompting them to say that “nitrogen retention is better maintained in response to energy deficits with protein intakes approximately twice the [recommended daily allowance].”
What caused the lower protein synthesis rate when calories were decreased? The authors suggest that the decreased release of insulin caused by the lower calories resulted in less activation of muscle protein synthesis. That’s a bit confusing because other studies show that insulin promotes muscle protein synthesis only in the presence of a high concentration of amino acids in the blood. The amount of protein in the diet should have provided that.
The rapid weight loss that occurs when people follow low-protein diets is mostly muscle, since muscle has lower energy density than fat. Muscle loss results in a lower resting metabolic rate, which ensures the regain of lost bodyfat. The key to sustained weight loss involves the preservation of muscle during dieting. The key to making that happen is to maintain protein quality and quantity when you diet. Ongoing research suggests that the important element involved in this process is the branched-chain amino acid leucine.
Of all amino acids, leucine is the most active in triggering protein synthesis. It does so by helping activate an upstream aspect of muscle protein synthesis called mTOR, which initiates a cascade of muscle-protein-synthesis activators. The result is increased muscle protein synthesis and added muscle and strength, or, if you’re dieting, preservation of lean mass. An important take-home message here is this: If you want to maintain muscle when dieting, make sure that you take plenty of branched-chain amino acids. The estimated daily leucine intake for purposes of maximizing muscle protein synthesis is nine grams a day, which is about twice the average protein intake.
Rat-based studies show that when leucine is added to the animals’ diets, they not only maintain more lean mass but also lose more bodyfat. The studies point to increased energy expenditure in the rats, even when they’re fed diets that are high in fat, the most concentrated source of calories. The rat studies show that muscle protein synthesis is maximal when subjects get 25 percent of the daily requirement of leucine. Amounts over 50 percent had no additional effect. Studies with more relevance to humans have shown that you need to double or triple blood leucine to maximize muscle protein synthesis.
In practical terms, that means humans need to eat meals containing 25 to 30 grams of protein and a minimum of 2.5 grams of leucine. That’s more easily accomplished by eating foods that are rich in leucine. One example of that is milk, which contains 10 percent leucine by weight. In contrast, wheat proteins contain 6.8 percent, so you’d need to eat considerably more wheat protein to get the right amount of leucine. Studies show that most Americans get 65 percent of their daily protein intake in a single large dinner meal, leaving less than 35 percent distributed over the remaining meals. Trouble is, muscle tissue is most catabolic during sleep, and breakfast meals often contain 15 grams of protein or less, not enough to offset the catabolic effects of an overnight fast. The solution is to eat a high-protein breakfast that contains at least 25 to 30 grams of protein, particularly the 2.5 grams of leucine needed to maximize muscle protein synthesis.
Getting a higher protein intake while following a diet offers other fat-loss benefits. Past studies showed that eating more protein induces greater thermogenesis, or the conversion of fat calories into heat. It’s been estimated to account for 5 to 10 percent of daily energy expenditure, which is far more than you get by eating carbs or fat. Leucine-induced muscle protein synthesis also uses calories, since the production of protein is a calorie-intensive event. Studies have shown increased use of ATP and increased mitochondria in muscle when subjects get more protein, both of which are evidence of increased calorie use.
One of the “secrets” of why higher-protein diets produce the greatest rate of bodyfat losses is that they increase satiety, or the feeling of being full. Protein is superior to both fat and carbs in that respect. Here again, leucine comes into play. Studies show that the leucine-induced stimulation of mTOR interacts with appetite centers in the brain to reduce the appetite sensation. In fat cells leucine aids in the release of leptin, which travels to the brain and turns off the hunger sensation.
A recent study showed that another reason higher-protein diets are more effective for bodyfat loss is that you just feel better when you’re following a diet that is higher in protein and restricted in carb content.2 It featured 21 healthy young men, average age 33, who drank two types of cocktails. One was high in protein and low in carbs, while the other was high in carbs and low in protein. After the high-protein, low-carb drink, the men reported greater feelings of pleasantness, satisfaction and relaxation, less feeling of sleepiness, along with more physical energy and mental alertness than after the low-protein, high-carb drink. The higher-protein drink also provided a greater feeling of fullness and fostered a 26 percent higher CCK response, along with a lower ghrelin response. CCK is a gut hormone associated with satiety, while ghrelin triggers feelings of hunger. The positive feelings after the high-protein drink peaked three to four hours after intake.
The gluconeogenesis induced by a high-protein, low-carb diet helps maintain more stable blood glucose and adds to the feeling of decreased hunger. In contrast, diets high in carbs and low in protein encourage increased release of insulin, which, in turn, lowers blood glucose, resulting in greater hunger and fatigue.
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1 Pasiakos, S., et al. (2010). Acute energy deprivation affects skeletal muscle protein synthesis and associated intracellular signaling proteins in physically active adults. J Nutr. 140(4): 745-751.
2 Boelsma, E., et al. (2010). Measures of postprandial wellness after single intake of two protein-carbohydrate meals. Appetite. 54:456-64.