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Does Protein Make You Fat?

Self-styled experts often attribute various side effects to long-term high-protein diets—dehydration, calcium and bone loss, kidney disease and even increased bodyfat. The latter is based on the fact that protein contains calories, and taking in too many calories inevitably leads to gaining bodyfat.

A gram of protein contains four calories, the same as a gram of carbohydrate. Fat is the most concentrated source of calories at 9 1/2 per gram, but according to many nutrition pundits, that doesn’t matter. It makes no difference if you focus on a particular macronutrient—protein, fat or carb; if you consume more calories than you burn as energy, the excess will be stored as bodyfat.

Low-carb-diet proponents vigorously object to what they consider to be an oversimplification. It’s more than just a case of excess calories causing excess bodyfat, they say. There is also a hormonal interaction, namely insulin.

Insulin is indeed the most fattening hormone in the body. Whenever it is secreted, fat is either being maintained or synthesized. Among other functions, insulin blocks the activity of various enzymes that are involved in fat mobilization from fat cells as well as the actual oxidation, or burning, of fat.

Because of that, having a simple carbohydrate—a high-glycemic-index carb—prior to training will block the use of fat as fuel due to the higher insulin release that results. That effect lasts for an average of four hours after the high-carb meal is eaten. The general recommendation is that if you consume any carb prior to training, it should be from a low-glycemic-index source, which will cause less of an insulin release and promote more fat burning.

Even so, the low-carb devotees go further, claiming that calories are less important than carb intake for losing bodyfat. As evidence, they point to published studies that found more fat loss in people who ate low-carb diets than those who ate more carbs, even when the diets contained the identical number of calories. Low-carb diets not only control the harmful effects of insulin but also produce a higher thermic effect after meals. “Thermic,” or “thermogenic,” effect refers to the dissipation of consumed calories into heat. It’s also known as futile energy cycles, since no work is done to dissipate the calories.

Critics of low-carb regimens call this “metabolic magic”—meaning nonsense. A calorie is a calorie is a calorie, they say, and when it comes to ultimate fat loss, how many calories you take in compared to how many you burn is the ultimate arbiter. As evidence, they produce studies showing that while low-carb diets do tend to bring more rapid and greater rates of fat loss initially, as time goes on, it evens out. At the end of a year the fat-loss rate for low-carb diets and other diets is about the same, assuming that they contained the same number of total calories. The naysayers hold that insulin alone cannot make you fat unless you take in an overabundance of calories.

Then there is the protein issue. One of the established tenets of low-carbohydrate regimens is that you must increase your protein. It’s based on a number of established roles of protein in the body. For one thing, a higher protein intake is known to help maintain lean mass, mainly muscle.

Critics of low-carb diets like to point out that the body requires a certain amount of carbohydrate to function properly, an assertion that is not based in science. In fact, there is no established carbohydrate requirement. One reason is that other substances can be converted in the body to the main carbohydrate it uses—glucose. As such, lactate, glycerol from fat and amino acids from protein can all be converted into glucose in the liver. Consequently, carbs are not essential.

All that said, you don’t want to avoid carbs all the time. In some cases they offer definite advantages, such as for those engaged in endurance sports or training. A minimal amount of carb also plays a role in anabolic recovery processes following training, a key reason that you should never consider a zero-carb diet.

The low-carb diet features a higher protein intake because the excess protein helps to spare muscle that might otherwise be degraded for energy. The branched-chain amino acids are particularly effective in that regard. Other reasons for taking in more protein as your calories or carbs drop is to help control appetite, since protein helps you feel full when you’re dieting. Eating more protein also appears to maintain the resting metabolic rate, which ensures optimal fat loss.

What about the notion that taking in too much protein can make you fat? The pragmatic experience of generations of bodybuilders disputes it. While the suggested optimal intake of protein for bodybuilders is 1.7 grams per kilogram—2.2 pounds—of bodyweight, in actual practice most bodybuilders get far more than that. Considering the ubiquitous presence of protein in meat, chicken, turkey, eggs, milk and so on, along with the generous intake of protein supplements and meal-replacement powders, it’s not that difficult for many bodybuilders to take in two to three times more than the recommended dose of protein.

If, in fact, protein was as fattening as some people assert, bodybuilders who eat that much would look like walking versions of the Goodyear blimp. Clearly, they do not. Plus, bodybuilders are nutritionally savvy enough to boost their protein during dieting conditions—which also should be hindering their fat loss but clearly does not. Even people who don’t engage in weight training often eat more protein than they need, yet they rarely, if ever, get fat—unless they eat too many carbs and calories along with the protein.

How can that be? For one thing, the usual fate of ingested protein differs between inactive and active people. In active people excess protein undergoes metabolic changes in which the nitrogen portion is removed and excreted as urea. What about the calories? In active people the excess calories in protein are oxidized in the liver and not stored as fat.

While there is an outside chance that excess protein can wind up as fat in sedentary folks, in reality, they also have to be overeating carbs and calories in relationship to their activity levels. That was shown in a recent highly publicized study published in the prestigious Journal of the American Medical Association.1

Twenty-five healthy men and women, aged 18 to 35, all of whom were overweight to varying degrees, stayed in a metabolic unit in a research lab for 10 to 12 weeks. First, they ate a “weight-stabilizing” diet—15 percent protein, 25 percent fat and 60 percent carbs— for 13 to 25 days, and during the last eight weeks they were randomly divided into three diet groups:


• 5 percent protein (low protein)

• 15 percent protein (normal protein)

• 25 percent protein (high protein)

It wasn’t just the protein intake that the researchers were looking at, however. All three groups were purposely overfed during the last two months of the study—specifically, they got 40 percent more calories than what they ate on the baseline, or maintenance, diet. The results showed that those in the low-protein group gained less weight than the others, but they also stored 90 percent of their excess calories as fat. The 6.6 percent increase in resting metabolism in the low-protein group was attributed to the metabolic cost of converting the excess calories into bodyfat. In contrast, in the normal and higher protein groups 50 percent of the excess calories were stored as fat. The rest were burned up in a thermogenic reaction.

Another difference was that neither resting energy expenditure or lean body mass increased in the low-protein group, but they did in the normal- and high-protein groups. The excess calories eaten by all three groups were in the form of fat, which contains the greatest concentration of calories. Despite that, the high-protein group had the least amount of excess calories stored as fat, which underscores the effects of that strategy, as discussed above—that is, more calories are dissipated during a higher protein intake.

Based on the results of this study, the authors say that overall calorie intake, not how much protein you eat, is what makes you fat. In addition, it should be noted that the subjects did not exercise but rather remained sedentary in a metabolic lab. Without question, vigorous exercise changes the way nutrients are used in the body. Not only does exercise burn off excess calories, but the muscle gains accrued during weight training will prevent any possibility of excess protein being converted into bodyfat.

—Jerry Brainum


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1 Bray, G., et al. (2012). Effect of dietary protein content on weight gain, energy expenditure, and body composition during overeating. JAMA. 307;47-55.

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