The Power of Protein
Once relatively ignored, protein has been thrust into the nutrition spotlight by the explosion of interest in low-carb diets. In the body, protein has five important biological functions. Protein:
- Provides building blocks for the body to build its own proteins.
- Provides building blocks for hormones and enzymes.
- Is involved with immune function.
- Can be converted to blood sugar (a process known as gluconeogenesis).
- Can be burned for energy.
According to dietary guidelines set by the Institute of Medicine, adults need 0.8 grams of protein for every 2.2 pounds of body weight every day. That translates into about 45 grams of protein for someone who weighs 120 pounds, 60 grams for someone who weighs 160 pounds, and 80 grams for someone who weighs 220 pounds. Given the ready availability of protein, meeting this recommendation isn't difficult for most people.
Getting much too little protein can lead to malnutrition; getting somewhat too little can increase the chances of having a hemorrhagic stroke. What is unclear is whether there are health benefits or hazards from getting too much protein? On the benefits side, eating more protein (and less of something else) has been linked with improved weight loss, better cholesterol levels, and decreased incidence of heart disease and stroke. On the hazards side, eating extra protein could damage the kidneys and bones.
Good sources of protein
The average American gets about 15 percent of his or her calories from protein. Good sources of this nutrient include:
| Food | Serving size | Grams of protein |
|---|---|---|
| Ground beef | 4 ounces | 33 |
| Chicken | 4 ounces | 31 |
| Tuna | 4 ounces | 33 |
| Cottage cheese | 1 cup | 15 |
| Tofu | ½ cup | 10 |
| Peanut butter | 2 tbsp | 10 |
| Lentils | ½ cup | 9 |
| Skim milk | 1 cup | 8 |
| Peas | ½ cup | 8 |
| Cheddar cheese | 1 ounce | 7 |
| Eggs | 1 | 6 |
| Almonds | 1 ounce | 6 |
| Whole wheat bread | 2 slices | 6 |
| Rice | 1 cup | 4 |
| Corn | 1 ear | 3 |
Pay Attention to the Protein "Package"
Foods don't deliver pure protein. Instead, protein comes packaged with fat, carbohydrates, vitamins, minerals, and other substances. Some excellent sources of protein are rich in saturated fats, which aren't so good for the heart. Others deliver decent doses of unsaturated fats, which are good for the heart and other body systems. Other elements, such as plant-based estrogens and the heterocyclic amines formed when meat is cooked to a well-done degree may also contribute to health and disease.
| Protein, fat, and saturated fat in different protein packages | |||||
|---|---|---|---|---|---|
| Amount | Calories | Protein (grams) | Total fat (grams) | Saturated fat (grams) | |
| Prime rib, lean | 4 ounces | 289 | 31 | 17 | 6.8 |
| Cheddar cheese | 2 ounces | 114 | 7 | 9 | 5.9 |
| Hot dog, beef | 1 | 148 | 5 | 13 | 5.3 |
| Beef tenderloin, lean | 4 ounces | 252 | 32 | 13 | 5.2 |
| Duck | 4 ounces | 228 | 27 | 13 | 4.7 |
| Turkey, dark meat | 4 ounces | 212 | 32 | 8 | 3.5 |
| Peanut butter | 2 tbsp | 188 | 8 | 16 | 3.3 |
| Bacon | 3 slices | 126 | 9 | 9 | 3.2 |
| Chicken, dark meat | 4 ounces | 232 | 31 | 11 | 3.1 |
| Ham, boneless | 4 ounces | 201 | 25 | 10 | 2.8 |
| Egg | 1 | 101 | 7 | 7 | 2.2 |
| Coho salmon | 4 ounces | 201 | 28 | 9 | 2.1 |
| Tofu | ½ cup | 183 | 20 | 11 | 1.6 |
| Almonds | 1½ ounces | 246 | 9 | 22 | 1.6 |
| Turkey, light meat | 4 ounces | 177 | 33 | 4 | 1.5 |
| Chicken, light meat | 4 ounces | 196 | 35 | 5 | 1.2 |
| Skim milk | 8 ounces | 83 | 8 | 0.3 | 0.2 |
| Lentils | ½ cup | 115 | 9 | 0.4 | 0.05 |
| Peas | ½ cup | 62 | 4 | 0.2 | 0.04 |
Protein and Prevention of Chronic Disease
Dietary protein may do more than just provide building blocks for enzymes and structural proteins. Eating protein in place of easily digested carbohydrates may ease weight loss, at least in the short term. It may also help prevent type 2 diabetes, heart disease, and some types of cancer. Keep in mind, though, that the protein package matters. Data from two long-running Harvard studies of female nurses and male health professionals suggests that eating nuts and fish can reduce the chances of developing type 2 diabetes, while eating processed meats may increase it.
Eating too much protein, though, may pose problems, with some research suggesting it could harm the kidneys and bones.
Protein and Weight Loss — the Questions
Most diets, like the weight loss they promise, fade away over time. There are a few exceptions. One that needs no introduction is the low-carb, high-protein diet promoted by Dr. Robert Atkins. It has lasted for more than 30 years and has been tried by millions of people. Is this just a case of excellent marketing, or can eating more protein and fewer carbohydrates help people shed pounds?
After years of dismissing Atkins' claim as snake oil, researchers around the country and around the world are now taking it seriously, and putting it to some serious tests. They want to answer (and in some cases already have answered) five key questions about the role of protein in weight loss:
- Does it take more energy to digest protein? Yes. Digestion isn't a passive event. The body must expend energy to digest and store food. Called the thermic effect of eating, it accounts for 10 to 15 percent of the total energy expended each day. It turns out that the thermic effect is higher for high-protein diets than it is for diets high in fat or carbohydrates, meaning the body burns more calories simply digesting the foods in a high-protein diet than it does for the once-standard high-carb, low-fat diet. (Halton and Hu, Journal of the American College of Nutrition, 2004)
- Do higher-protein diets help people feel full and satisfied with fewer calories? Possibly. In several short-term studies, higher-protein meals suppressed hunger and enhanced the feeling of being full and satisfied (called satiety).
- Do higher-protein, possibly high-fat diets have unhealthy effects on cholesterol levels? No. Surprisingly, such diets seem to have beneficial effects on LDL (bad) cholesterol, HDL (good) cholesterol, and triglycerides.
- Do higher-protein diets help people eat less? It seems so. In a study that combined the results of 10 separate studies, volunteers who ate high-protein meals took in an about nine percent fewer calories each day than those who ate lower-protein meals. (Eisenstein and colleagues, Nutrition Reviews, 2002).
- Do higher-protein diets lead to greater weight loss than low-fat or other traditional diets? It depends on your time frame.
Protein and Weight Loss—Some Answers
The majority of diets have never been tested in rigorous clinical trials. That's not the case for low-carbohydrate, high-protein diets. Fourteen early studies yielded inconsistent results, most likely because they included relatively few volunteers and lasted only a short time. Newer, larger, and longer studies lasting six months or a year indicate that a low-carbohydrate, high-protein diet allows for more rapid weight loss than a low-fat, high-carbohydrate diet (Skov and colleagues, International Journal of Obesity and Related Metabolic Disorders, 1999; Brehm and colleagues, Journal of Clinical Endocrinology and Metabolism, 2003; Foster and colleagues, New England Journal of Medicine, 2003, Samaha and colleagues, New England Journal of Medicine, 2003; Yancy and colleagues, Annals of Internal Medicine, 2004; Stern and colleagues, Annals of Internal Medicine, 2004). However, after a year weight loss appears to be the same regardless of diet type.
What's the bottom line? High-protein, low-carbohydrate diets may make it easier to lose weight initially. But they aren't any better than low-fat, high-carbohydrate diets for keeping weight off. That takes daily exercise and a diet that limits caloric intake while offering plenty of choices and minimizing restrictions so it's sustainable over the long term.
Possible Hazards of High-Protein Diets
In theory, eating a lot of protein could harm the bones and the kidneys. Digesting protein releases acid into the bloodstream. The body neutralizes this acid with calcium. It is possible, though not yet proven, that this calcium comes from bones. If that is, indeed, the case, following a high-protein diet for a long time could weaken bones and lead to osteoporosis. A high-protein diet also forces the kidneys to work harder to filter the byproducts of protein digestion. In people with healthy kidneys, this extra work doesn't pose a problem. But in people with compromised kidney function, eating a lot of protein, especially nondairy animal protein, may accelerate the decline in kidney function (Knight and colleagues, Annals of Internal Medicine, 2003).