224 ‒ Dietary protein: amount needed, ideal timing, quality, and more | Don Layman, Ph.D.

KEYPOINTS

• 01

  Muscle is an organ of metabolism as well as movement

  Don Layman, a professor emeritus of nutrition, grew up on a farm in the 1950s and has studied protein and muscle for decades. He sees muscle as doing two jobs. The first is mobility: most people who die after 65 die from some consequence of immobility, such as falls, fractures or hospitalization. The second is metabolism: skeletal muscle is the main site of insulin action and holds 75–80% of the body's glucose storage capacity. From that view, he says, you should plan a diet around muscle and around protein first.

• 02

  The RDA is a minimum to prevent deficiency, not an optimal intake

  The 0.8 g/kg RDA comes from short nitrogen-balance studies, often seven days long, in college-age students. The method measures nitrogen and multiplies it by 6.25, assuming protein is 16% nitrogen, so it counts anything containing nitrogen. The estimated average need is then raised by two standard deviations to cover about 97.5% of people. Layman says every essential amino acid also has metabolic roles at intakes far above that minimum. Leucine activates mTOR, arginine supplies nitric oxide, lysine feeds carnitine, and cysteine is used to make glutathione.

• 03

  Protein quality varies widely

  Animal proteins and most isolates, including soy isolate, are about 95% or more digestible, while raw plant sources may be only 60–70% available. On DIAAS, the digestibility-corrected quality score, Layman ranks whey isolate at about 100%, a ribeye at about 96%, soybeans at about 80% and shredded wheat at about 40%. A cereal label claiming 4 g of protein is still just nitrogen × 6.25. Of roughly 7,000 foods in the USDA database, about 4,000 have amino acid profiles and fewer than 300 have DIAAS scores. Layman is working on a simpler score based on three amino acids: leucine, lysine and methionine.

• 04

  At high enough intake, the protein source matters less

  Layman's rule of thumb: at about 120 g of protein a day, the split between animal and plant probably doesn't matter, because you will cover your essential amino acids. At 50 g it matters enormously. So a plant-based eater aiming for about 125 g is probably fine, while a vegetarian eating 56 g is 'going to get in trouble'.

• 05

  Older muscle is less responsive, but more protein can make up for it

  Muscle proteins turn over with half-lives of roughly 15–16 days. The body makes about 300 g of new protein a day while most Americans eat about 80 g (women about 70, men about 90), so most of it is recycled. With age, a given dose of protein triggers less synthesis than it does in a 16-year-old. Layman says raising intake from 0.8 to about 1.6 g/kg can bring a 65-year-old's muscle protein synthesis response up to that of a 20-year-old.

• 06

  Protein is an absolute need, so never set it as a percentage of calories

  A 75-year-old woman eating 1,200 calories still needs about 100 g of protein, which is 35–40% of her calories. Setting fat at 30% and carbohydrates at 50% and leaving protein with the remainder gets it backwards. Layman says you should set protein first and build the rest of the diet around it.

• 07

  The first meal of the day matters most

  After an overnight fast, protein synthesis is suppressed and muscle is catabolic until a meal provides about 3 g of leucine, which is roughly 30 g of high-quality protein. Layman advises 40–45 g at the first meal and again at the last meal, with smaller amounts between. In his study of people averaging 37 years old, 90 g split 30/30/30 beat 10/20/60. He now thinks the ideal might have been closer to 40/10/40.

• 08

  Big meals aren't wasted, but muscle has a ceiling per meal

  You will digest and absorb 100 g of protein at a meal, despite the myth. However, muscle can use only about 25–60 g of it per meal depending on quality, and the liver uses the rest. Total daily protein still matters. A very large lifter needing 250 g would do better with four or five meals than two, but Layman says that's about total supply rather than the leucine–mTOR signal.

• 09

  Higher-protein diets help weight loss mostly by preserving lean mass

  Layman's trials ran from 12 weeks to 16 months, the longest multicenter one with about 120 subjects. Calories were equal, and the higher-protein group lost about 8 lb more, roughly 6.5 lb of it fat. He estimates an energy advantage of about 170 calories a day, enough to explain the difference. For anyone over 40, losing lean mass during weight loss matters, and he says that's when protein becomes critical.