A lot of people mistakenly called low-carb diets “high-protein diets.” This is because the most well-known low-carb diet, the Atkins diet, allowed people to eat only a very small amount–less than 10 grams–of carbs each day. Consequently, people on the Atkins diet consumed a disproportionate amount of protein and fat as a percent of total daily calories.

Someone who’s not on a diet is probably consuming about 60 to 70 grams of protein per day, or more, depending on what they eat. When they go on a low-carb diet, they cut their total daily calorie intake significantly, while at the same time they increase their protein intake to between 70 and 100 grams per day. So it’s a higher protein load, but it’s still not an enormous amount of protein, and probably wouldn’t qualify as a “high-protein diet.” People on low-carb diets lose weight, but it’s because they’re consuming fewer calories overall. On these low-carb diets, even though the books tell you that you can eat as much fat or protein as you want, almost every published study showed that most people decreased their caloric intake. One theory is that the body goes into ketosis (which means burning fat rather than carbs), which may have an appetite-suppressing effect.

Protein is considered the most satisfying of the three macronutrients (protein, carbs and fat). There is some evidence that people are better able to adhere to low-calorie diets when they consume about 20% of total calories from protein, rather than 15%. But Christopher Gardner, who conducted the A to Z Weight Loss Study (which stands for Atkins to Zone), for example, reported that people lost a little more weight on the Atkins diet, in particular, because they were better able to adhere to their diet. In other words, it matters little which diet you’re on so long as you stick to it and reduce your overall daily caloric intake.

One of the biggest problems with so-called high-protein diets is that people think that all protein food is the same. Many animal sources of protein (such as meat and cheese) have high levels of saturated fat, and everyone should be sure that their diet is low in this type of fat. Everyone should focus on lean protein sources, such as low-fat dairy, poultry without skin, and fish, for example, instead of higher-fat protein sources, like steak and cheese.

Others who need to watch their protein intake include those with kidney problems. Anyone with poor kidney function, which often includes diabetics who have not managed their blood sugar levels well over time, and have consequent kidney problems, are often told to decrease their protein intake. In other words, a diabetic who wants to lose weight probably shouldn’t follow a high-protein diet.

One last group –pregnant women—is worth mentioning. Pregnant women should never be on a weight loss diet, and they should be absolutely sure to consume a high carbohydrate diet—at least 50-60% of total calories—to get the nutritional balance they need.

Healthy individuals looking to lose weight should aim for around 20 percent of their calories from protein, or 55 to 66 grams of protein per day (in a person who weighs about 150 pounds) from lean sources like fish, lean meats, tofu, beans, and grains.

When it comes to protein, the average American obtains the recommended daily allowance, which is 0.8 grams of protein per kilo of body weight or 3-4 grams per 10 pounds. For athletes, though, these recommendations are higher.

For “endurance athletes”—those whose sport is running or cycling, for example—the typical recommendation is 1.2 to 1.4 grams of protein per kilo of body weight. This number is even higher for “power athletes” like weight lifters, or linemen on a football team. They can be advised to get from 1.6 to 2.0 grams of protein per kilo.

A 2008 review paper found that 1.6 was the top end of the body’s ability to utilize protein, and, granted, 2.0 is really atypical. There’s not a specific organization that sets these recommendations.

The best protein sources for athletes are lean meats, as well as cottage cheese, egg whites or egg substitutes (to minimize cholesterol intake), and soy products, such as tofu. Greek yogurt, which is strained and thicker than regular yogurt, has a higher protein content per serving than regular yogurt, so that’s good, and protein powders can be pretty convenient as well. Other foods with a good amount of protein include whole grains and whole grain breads and pastas. Nut and nut butters offer protein, too, but the calories can really add up, so those interested in weight loss need to be careful.

Protein’s relationship to muscle is straightforward: muscle is made of protein. There are 20 amino acids, and 9 of them are essential to meet our protein needs. When muscles are used during exercise they break down, and they need to be rebuilt post-exercise. The body needs protein to do this. This means the athlete needs to consume enough protein to match—or even exceed—the muscle break—down related to exercise.

Athletes also need to make sure they stay in positive nitrogen balance. Protein is the only source of nitrogen in the diet, and while you need nitrogen, too much isn’t good for you. Anyone increasing their protein intake needs to drink plenty of fluids so any excess nitrogen can be flushed out.

Protein also plays a significant role in recovery for athletes. Along with carbs, protein supports the resynthesis of glycogen along with the rebuilding and repair of damaged muscle tissue.

As with people trying to lose weight, athletes also benefits from the fact that protein is satiating and makes you feel full longer. That helps anyone stick to a specific diet.

Even athletes can overdo it on the protein front, though. Someone whose intake is in the range of 35 to 40 percent, for example, might also be consuming too much fat and cholesterol, as well as squeezing out other important nutrients in favor of protein. Plus, since muscle is built by tearing muscle fibers during exercise and then rebuilding them during recovery, eating more protein than is used in that process is superfluous.

In general, so long as the person has good kidney function and their fluid intake is high (so they can flush out excess nitrogen), there would be nothing negative about a protein intake at 20-25%; beyond 30% is not well studied.