Metabolic Pathways
Research now shows that when you eat a high-protein meal, less of the protein calories are converted into fat than are carbohydrate calories when you consume a high-carb meal. For instance, in a study done at the University of Cincinnati College of Nursing, women who dieted by cutting carbs and focusing on protein lost twice as much weight after six months than women who merely cut calories (ADA annual meeting, November 2002). In this research, about 50 women between the ages of 30 and 60 were divided into two groups. One group ate a diet that limited fat to 30% of calories. The other women ate a very-low-carbohydrate diet that was high in protein.

Ketone Effects
The women who barely touched carbs for six months went into what's called a state of ketosis. Ketosis occurs when your blood-sugar level goes so low that your body fat is broken down by your liver and changed into ketone bodies. Ketone bodies are chemicals that the body makes when insulin, a hormone-like substance involved in carbohydrate metabolism, drops very low and fat is mostly burned for energy.

When ketones build up in the blood they can end up in urine or be released in the lungs making your breath smell fruity. (Blood tests of these dieting women confirmed that the bodies of those on a high-protein diet had entered a state of ketosis.) The result: high-protein dieters in this research lost an average of 18.5 pounds after six months. Just as important is the fact that 10 of those pounds were fat. The other group of women only lost about nine pounds each, five of which were fat. (Each of the women in both groups ate between 1200 and 1500 calories a day.)

Protein Basics
Obviously, protein plays a more basic role in the body than just helping to spur weight loss. Actually, it's the body's second most abundant substance (after water), used to build the "stuff" of which skin, muscles and other organs are made. Hair and nails are also primarily built of protein.

But protein is much more than a simple bodily scaffold. Enzymes, substances vital to chemical reactions within the body, depend on protein, as do disease-fighting antibodies and message-carrying hormones. Protein is also required to make brain chemicals called neurotransmitters, particularly dopamine and norepinephrine, both of which help improve alertness and concentration. "If you're feeling lackluster and not up to speed, you may need protein foods," says Molly Siple, MS, RD, author of Healing Foods for Dummies (IDG Books).

Protein needs do change throughout life. Pregnant and breast-feeding women need more protein to support fetal growth and milk production, and children need protein to power their growing bodies. Older people often need more protein to keep up their immune defenses. In addition, your protein needs will increase if you are sick or injured.

Whence Protein?
When many folks think of protein, they envision a thick steak, hardboiled egg or fish filet. And animal-based foods, including dairy foods, are excellent sources of protein. "Eggs are one of nature's finest products," Siple says. But plant-based foods can also provide plenty of protein. For example, while half a chicken breast contains a respectable 31 grams of protein, a cup of soybeans isn't far behind, with 29 grams. Similarly, a cup of whole milk contains 8 grams of protein; a cup of soymilk, 7 grams.

Getting the protein you need from plants is easy if you eat a variety of foods throughout the day; you don't need to worry about eating specific combinations of foods at each meal. "A well-balanced diet of vegetables, fruits, grains and legumes supplies an ample amount of protein to meet [the] body's needs," says Jay Solomon, author of Seven Pillars of Health: Nutritional Secrets for Good Health and Long Life (Prima Publishing).

Protein Protects Your Bones
As high-protein diets have gained in popularity, some experts have cautioned that these diets may compromise bone strength. But new research shows that eating extra protein strengthens bones instead of weakening them.

A study done by researchers from the Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University shows that eating more protein and cutting back on carbohydrates, like those in wheat, does not cause the body to take calcium out of the bones but actually slows what's called bone resorption (a process that would otherwise weaken bones).

Consequently, when you eat more protein and hold back on carbohydrates, according this research, bones get stronger (Journal of Clinical Endocrinology & Metabolism 3/04). The scientists found that people who boosted their daily protein intake by 58 grams a day had 25% more bone growth factor, along with lower levels of natural chemicals that show bone is being lost. (Fifty-eight grams is about an eighth of a pound, so the extra protein was the equivalent of about three small pieces of steak.)

Losing More Body Fat
One reason more and more researchers are looking into the benefits of following a high-protein diet to lose weight is the fact that this diet apparently preserves more muscle tissue and encourages the loss of more body fat. For instance, a study performed by researchers at the University of Illinois at Urbana-Champaign found that giving women dieters more protein made them less hungry between meals and also lowered their triglyceride levels (blood fats that have been linked to heart disease risk). In 10 weeks, these women lost about 10 pounds; in addition, they took off significantly more body fat than other women who ate more carbohydrates (Journal of Nutrition 2/03).

Staying Satisfied
The high-protein approach to eating has an additional advantage: you tend to feel more satisfied, which keeps you from raiding the fridge and consuming needless calories. When researchers from Arizona State University assigned people to eat either a high-protein or a high-carbohydrate diet-both with restricted calories-for six weeks, they found that both groups of dieters lost weight. However, the folks on the protein-rich eating plan "reported more satisfaction and less hunger in month one of the trial" (Journal of Nutrition 3/04).

The lesson of this continuing research is that while all calories count, if you make more of your daily calories protein calories, you can count on losing a few extra pounds.