The majority of protein is digested, and the amino acids not used for gut fuel are metabolized in the intestinal mucosal cells and transported by the portal vein to the liver for protein synthesis or gluconeogenesis.12 In the liver, nonessential amino acids are largely deaminated, and the amino group (nitrogen) removed is converted into urea for excretion in the urine.13 It has been shown that in subjects without and with mild type 2 diabetes, ~5070% of a 50-g protein meal is accounted for over an 8-hour period by deamination in the liver and intestine and synthesis to urea.14 It has been assumed that the remaining carbon skeletons from the nonessential amino acids are available for glucose synthesis, which would then enter into the general circulation.
While there's evidence of a protein threshold of 20 g to 30 g to trigger muscle protein synthesis, more research is needed. "It's premature to put it in the dietary guidelines. Randomized, controlled trials haven't been done," Anderson says. More research also is needed on the potential benefits of protein in weight loss, satiety, and blood glucose and blood pressure management. In the meantime, it makes good nutrition sense to recommend to clients and patients to snack on foods that provide a good supply of protein compared with fat and carbohydrate, both for possible appetite and blood sugar control.
For athletes, protein powders and high-protein snacks are plentiful. But how necessary are they, and can they really improve performance? The International Society of Sports Nutrition recommends consuming high-quality protein, and singles out milk-derived whey protein isolate and casein, egg white powder, and soy protein isolate as proteins that provide essential amino acids that readily aid in muscle tissue synthesis.16
Several studies have suggested that this pattern may not be the most beneficial. Instead, evenly distributing protein intake throughout the day has been found to be optimal. A recent study showed that muscle protein synthesis was 25% higher over a 24-hour period when the same quantity of protein was evenly distributed across breakfast, lunch, and dinner.18 While it hasn't been studied, the same may be true of including protein-rich snacks in the diet. "There's increasing evidence that distribution of protein so that more is consumed at breakfast may be beneficial. The idea of 30% of daily protein intake at each meal is being promoted, with some protein snacks between meals," Anderson says.
High-protein snacks also can help maintain normal blood glucose levels. In a study of 20 healthy males, who were given a variety of midmorning snacks, those given the snacks with the greatest protein-to-carbohydrate ratio, including plain yogurt and skim milk, had the lowest blood sugar levels.11 Researchers determined that the improvement in blood sugar was due to improved insulin action, rather than to increased concentrations of insulin.
The threshold for abnormal nitrogen synthesis and breakdown is reported to be at a fasting glycemia between 6 and 7 mmol/l (108125 mg/dl).80 Therefore, with moderate energy restriction there is need for a generous dietary protein intake (~1 g/kg body weight).
It seems as though snacking may have replaced baseball as America's favorite pastime. National dietary surveys have found that about 90% of adults,1 83% of adolescents,2 and 97% of children3 snack every day, sometimes several times per day. Whether or not that's a good thing largely depends on the quality of the snacks being consumed. Many of the most common snacks, such as chips and soda, are high in both salt and sugar. It's no coincidence then that increased snacking is associated with decreased protein intake.4 However, there's much research to suggest that choosing snacks high in protein, rather than high in salt and sugar, could provide a host of health benefits.
Gluconeogenesis also is increased, and hepatic extraction of alanine, a key amino acid gluconeogenic precursor, is accelerated.19 Excessive rates of hepatic glucose production, proteolysis, and amino acid oxidation in type 1 diabetes are all reduced by insulin administration,82 but proteolysis and amino acid oxidation are more resistant to the suppressive effects of insulin.83 Normalization of protein metabolic rates may, therefore, require long-term tight metabolic control.84 Today, with improved glycemic management in type 1 diabetes, a more normal protein synthesis, breakdown, and oxidation should occur.