Adapted from The Big Book of Food and Nutrition. Order your copy today!
In a Danish study, researchers put 65 subjects on a low-protein diet, a high-protein diet, or no diet. While the low-protein dieters lost an average of 5kgs, the high-protein dieters lost an average of 9kgs. But even more amazingly, the high-protein dieters lost twice as much abdominal fat. One reason may be that a high-protein diet helps your body control its levels of cortisol, a stress hormone that directs fat toward the belly.
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Made of amino acids, proteins are the nutritional building blocks for lean muscle mass. Stressing a muscle—by, say, raising and lowering a dumbbell—causes microscopic tears in the muscle fibres. When amino acids reach a muscle’s cells, they help repair damaged muscle fibres and make new, stronger ones in a process called protein synthesis. This process can’t happen, of course, unless you’ve got amino acids coursing through your body. There are 22 different kinds, 13 of which we produce ourselves. The other nine, called essential amino acids, must come from the food you eat.
The best kinds of protein—namely meat, dairy, and eggs—provide all nine and have a high “biological value,” your body can easily use them. Other foods may contain a fair amount of protein, but they’re considered “incomplete” sources, because they contain fewer than nine amino acids. Your body can form complete proteins by combining incomplete ones—this happens whenever you combine legumes, nuts, and grains—but you’ll need to consume as much as a quarter more plant-based foods to get all the benefits provided by animal protein.
Beyond just building muscle—which helps raise your metabolism and increase your daily calorie burn—protein helps keep you slim in another way, as well: it requires a lot of calories to digest. And of the three macro-nutrients, protein will make you feel full the fastest. Here’s a closer look at the most common types of protein.
Three essential amino acids—leucine, isoleucine, and valine—are called “branched-chain amino acids,” and they’re the very best amino acids you can get, because they act quickly and your body uses them so efficiently. Whey protein happens to be 10 percent leucine (other animal-based proteins have as little as 5 per cent), and it behaves almost like a hormone in your body. “It’s more than a building block of protein—it actually activates protein synthesis,” says Dr Jeffrey Volek, a nutrition and exercise researcher at the University of Connecticut in the USA. Found in dairy products and supplements, it’s also known as a “fast protein” because it’s quickly broken down into amino acids and appears in your bloodstream 15 minutes after you consume it, which makes it a very good protein to consume after your workout.
Casein, another dairy protein also sold in supplement form, digests slower and provides a slower-absorbing but more sustained source of amino acids. It’s ideal for providing your body with a steady supply of smaller amounts of protein for a longer period of time—such as between meals or while you sleep. Whey and casein are ideally consumed together.
Your body can use 94 per cent of the protein in a whole egg, making it, like whey and casein, an excellent source of high-quality protein. In fact, a recent review of more than 25 published studies on protein concluded that egg protein may help boost muscle strength and development more than other proteins do because of its high concentrations of leucine. And egg protein is also good for keeping you from getting hungry over a sustained period of time.
Besides not contributing to protein synthesis as much as other pro-teins, soy presents a number of other nutritional weaknesses as well. The researchers of a 2005 study in the Journal of Nutrition compared soy with casein and concluded that “the biological value of soy protein must be considered inferior to that of casein protein in humans.”
Though it’s promoted as a health food—especially for the heart—soy protein is anything but. In 2006, a study review in the American Heart Association journal Circulation cast doubt on the long-held notion that eating soy daily can lower LDL (bad) cholesterol. In fact, research now suggests that you’d have to eat the equivalent of 2 pounds of tofu every day to lower your LDL cholesterol a measly 3 per cent. Soy’s isoflavones have also been shown to raise estrogen levels in men and to negatively impact thyroid function when consumed in high amounts. And a 2008 Harvard study found a strong association between men’s consumption of soy foods and decreased sperm counts.
While Asian diets are often considered “healthier,” contrary to popular belief, Asians don’t consume huge amounts of soy protein: Studies say the average daily intake is around 10 grams in China and about 8 grams in Japan. Many Western cultures get north of 20 grams because soy is so abundant and inexpensive.
PROTECTING THE BUSYBODY MACRO-NUTRIENT
Muscle isn’t the only thing protein is good for. Protein provides the framework of all tissue—from filaments to tendons to organs. Enzymes, hormones, and antibodies are also proteins. In short, protein plays many underappreciated roles, and your body is constantly draining its protein reserves for other uses—making said hormones, for instance. The result is less protein for muscle building. To counteract this problem, you need to “build and store new proteins faster than your body breaks down old proteins,” says Dr Michael Houston, a professor of nutrition at Virginia Tech University.
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