鈥淚t builds muscle!鈥� That鈥檚 the usual answer I get when I ask in class about the importance of protein in the diet. Sort of correct since muscles are indeed composed mostly of protein, but the protein we eat does not directly build muscles. It is broken down during digestion into its constituent amino acids that are then pieced together by enzymes (also proteins) to become the proteins that make up muscle tissue.
However, to build muscle, you have to do more than supply the required amino acids, you have to sweat it at the gym. But no matter how hard you train, you cannot build more than a few grams of muscle a day. The amino acids needed to do this are easily supplied by proteins in the diet. Muscles are also in a state of flux, constantly breaking down and being repaired. There are plenty of amino acids in the diet to take care of the repairs needed as well.
It is not only our muscles that are built of protein. Our skin, hair, nails, tendons, ligaments and organs are also made of protein and even our bones have an internal protein scaffold. The antibodies we rely on to protect us from microbial invaders are proteins, as are the hormones insulin, glucagon, leptin ghrelin and somatotropin. Then there are the enzymes that catalyze the numerous biochemical reactions that go on in our body all the time.聽
Proteins are also part of hemoglobin molecules needed by red blood cells to transports oxygen. Special proteins, known as 鈥渓ipoproteins鈥� ferry cholesterol, an essential biochemical, to cells. Neurotransmitters such as dopamine, serotonin, norepinephrine and gamma-amino-butanoic acid are made from amino acids that are released when proteins are digested. Obviously protein intake is important.
Unlike carbohydrates and fats, our body does not have a protein reservoir, so we have to consume protein regularly. Studies have shown that 0.8 grams/kg body weight is enough to avoid an overt protein deficiency, but ideally, we should be consuming 1.0-1.6 grams/kg a day. The lower end of this scale is fine for adults under the age of 65, but above that age a higher intake is needed to compensate for the normal muscle loss that comes with aging. The source of protein is also relevant. An American study that followed over 400,000 people for 16 years found that swapping just 18g of meat protein, but not dairy or fish protein, for plant protein daily was associated with a 12鈥�14% lower risk of dying prematurely. This difference seems to disappear after the age of 75.
It was once believed that animal proteins are superior because they contain all nine essential amino acids while plant proteins have to be eaten in certain combinations, rice and beans for example, because neither contains all the essential amino acids. The thinking was that the amino acids missing in one can be supplied by the other. This has been shown to be incorrect. As long as the different plant proteins are eaten the same day, the body can use their amino acids to build new proteins. One of the essential amino acids, leucine, is particularly adept at stimulating muscle protein synthesis. There is also some evidence that distributing protein intake over three meals leads to more efficient muscle protein synthesis.
Now comes the important question, one I generally pose in class. Does your daily diet supply enough protein? For my 18-21 year-old students with an approximate average weight of 70 kg, the daily intake should be around 70 grams. I usually ask what they had for lunch, and I hear the likes of cheeseburger (25 grams protein), 2 slices of pizza (30 grams), peanut butter and jelly sandwich (10 grams), mac and cheese (15 grams), turkey sandwich (30 sandwich) or egg salad sandwich (20 grams). Typical of a student diet, I guess. It seems that with whatever else they eat for breakfast and supper, they are getting enough protein.
When it comes to protein consumption, more is not better. Some observational studies suggest that high protein intake in midlife, particularly from animal sources, correlates with higher circulating Insulin-like Growth Factor (IGF-1) levels and possibly increased cancer risk. Conversely, higher protein intake in older adults is associated with lower mortality. This suggests that in youth and middle age, moderate protein consumption may suffice but in later decades, adequate protein becomes protective against frailty.
I have given my lecture on proteins for many years and have always included a listing of foods with their protein content. Curiously, when I was asking the students and making comments about their intake, I never thought about my own protein consumption. However, the last time I gave the lecture, my mind flashed to my own diet, probably because I have been paying more attention to recent publications about increased protein needs above the age of 75. It seems that seniors exhibit 鈥渁nabolic resistance,鈥� meaning they require higher protein doses to stimulate the same muscle-building response seen in younger individuals. By my calculations, I should be consuming about 90-100 grams a day, which as it turns out I鈥檓 not doing. I think I better up my consumption of lentils, beans and tofu.
Lentil soup has 10-15 grams of protein, a serving of kidney beans or tofu, about 10 grams. A scoop of whey powder adds roughly 25 grams of protein. I鈥檓 happy to see a study that concluded no change in longevity after age 75 whether protein comes from a plant or animal source while at a younger age life expectancy is decreased if the protein in the diet comes from an animal source. So, bring on the chicken breast with its 20 grams of protein per 100 grams! I think I will forego protein-fortified water and skip the protein popcorn but I鈥檒l dip my apple slices in peanut butter and I do have my eyes on a chocolate-flavoured protein bar that has 21 grams of protein and only 3 grams of sugar and a negligible amount of sodium. Now off to the gym for some muscle building.
