Even before the low-carb, high-protein hype, Americans were eating more protein than necessary. In fact, the average American consumes 161 percent of the Recommended Dietary Allowance for protein each day, according to the most recent U.S. Department of Agriculture surveys.

More protein, in particular animal protein, often means more fat. And we know eating more fat, especially saturated fat, puts us at an increased risk for heart disease, says Kava. "We know that people who eat lots of fruits and vegetables tend to have lower incidences of cancer," she adds.

Some dieters may not feel good when they restrict carbohydrates. When your body doesn't get the glucose it needs from carbohydrates, it produces ketones from fat. Our brains don't run as efficiently on ketones as they do on glucose, so some people may feel dizzy or fatigued.

In addition, Cornell University researchers found that animals fed lower-protein diets voluntarily exercised more. And, humans and animals on lower-protein, lower-fat diets actually consume more calories in the form of carbohydrates but are less likely to convert the energy to body fat.

Yet some people believe we were meant to follow a high-protein, low-carbohydrate plan because, they say, that's how our prehistoric predecessors ate. While the two diets do have some characteristics in common, such as a low-carbohydrate content, they also vary in that today's diets typically include more fat, more dairy and less fruit than was eaten by people in the Stone Age, says Loren Cordain, Ph.D., a professor in the Department of Health and Exercise Science at Colorado State University in Ft. Collins, Colo.

Even the low-carb connection is skewed, says Cordain, because Stone Age inhabitants would have eaten unlimited fruits and vegetables, which are limited in today's high-protein plans. In addition, the main sources of protein in the Stone Age came from wild animals, which didn't have the same types of fat as in today's feed-lot cows and pigs, which likely contribute to the high rates of heart disease we see today, says Cordain.

People would benefit, adds Cordain, by eating more fruits and vegetables and cutting down on sugar and salt. In addition, he says the best sources of protein in today's diet are those that are lean, particularly seafood and fish.