High-Protein Diets

The following is an exclusive excerpt from the book NSCA’s Guide to Sport and Exercise Nutrition, published by Human Kinetics. All text and images provided by Human Kinetics.  

High-Protein Diets

High-protein diets, often with concomitant carbohydrate restriction, have gained a lot of attention and become quite popular as a means to lose weight, improve body composition, curb hunger, and improve certain blood lipid profiles and insulin sensitivity (Brehm and D’Alessio 2008; Halton and Hu 2004; Kushner and Doerfler 2008; Noble and Kushner 2006). Published research findings demonstrate that diets higher in protein most likely aid in weight loss due to the satiating and thermic effect of protein (Brehm and D’Alessio 2008).

In fact, Johnston and colleagues (2002) demonstrated that the thermic effect of a meal containing relatively high protein (30% of energy as complex carbohydrate, 10% as simple sugar, 30% as protein, and 30% as fat), on average, was nearly two times greater than that of a high-carbohydrate meal containing equal calories (50% of energy as complex carbohydrate, 10% as simple sugar, 15% as protein, and 25% as fat).

An additional suggestion is that individuals who consume higher-protein diets are more likely to eat less at subsequent meals due to satiating effects (Halton and Hu 2004). Specifically, eating a high-protein meal has resulted in consuming 12% (Barkeling, Rossner, and Bjorvell 1990) and 31% (Latner and Schwartz 1999) fewer calories at the next meal. One reason higher-protein diets may be more satiating than high-carbohydrate diets is that protein, as opposed to fat and carbohydrate, is a relatively strong stimulator of the satiating gastrointestinal hormone cholecystokinin (CCK) (Johnston, Day, and Swan 2002). Elevated CCK levels have been shown to inhibit food intake in both rats and humans (Bray 2000).

Researchers in Denmark showed that when participants on a high-protein diet (46% carbohydrate, 25% protein, 29% fat) or a high-carbohydrate diet (59% carbohydrate, 12% protein, 29% fat) ate ad libitum (i.e., as much as they wanted), those on the high-protein diet consumed significantly fewer calories over the course of the study (Skov et al. 1999).

Specifically, participants on the high-protein and high-carbohydrate diets lost 8.9 kg and 5.1 kg of body weight and 7.6 kg and 4.3 kg of fat, respectively (Skov et al. 1999).

Layman and colleagues (2003) examined the effects of two different hypocaloric (~1,700 kcal/day) isoenergetic diets with varying carbohydrate-to-protein ratios on body composition. One of the diets had a carbohydrate-to-protein ratio of 3.5 (providing 68 g protein per day), and the other had a carbohydrate-toprotein ratio of 1.4 (providing 125 g protein per day). The two diets resulted in similar weight loss, but the diet that contained a greater percentage of protein led to greater fat loss, better lean muscle preservation, and ultimately improved body composition (Layman et al. 2003).

According to Brehm and D’Alessio (2008), among studies lasting up to 12 months, randomized, controlled trials repeatedly demonstrate that high-protein diets are comparable, and possibly superior, to low-protein diets when it comes to weight loss, preservation of lean body mass, and improvement in several cardiovascular risk factors. Therefore, diets that moderately increase protein and modestly restrict carbohydrate and fat may have beneficial effects on body weight and body composition (Brehm and D’Alessio 2008; Halton and Hu 2004).

Interestingly, as Kushner and Doerfler (2008) point out in a review, long-term data continue to indicate that total weight loss does not differ significantly between low-carbohydrate dieters and low-fat dieters. Therefore, although not all research agrees, high-protein, low-carbohydrate diets may be better for weight loss and body composition in the short term; but long-duration studies suggest that the traditional lower-fat, higher-carbohydrate diets may be just as effective.

Even though diets higher in protein and lower in carbohydrate seem promising, scientists point out that the long-term effects of a high-protein diet on overall cardiovascular and metabolic health need to be studied (Kushner and Doerfler 2008). However, to date, the majority of “high-protein” studies that evaluated the potential effects on cardiovascular risk profile actually show an improvement or reduced risk in comparison to traditional American diets.

Furthermore, increasing protein intake during weight loss at varying calorie intakes will prevent a negative nitrogen balance, which may also help lessen the loss of lean muscle tissue and ultimately resting energy expenditure (Stiegler and Cunliffe 2006).

However, adequate carbohydrate intake is also critical for several aspects of athletic performance and high-intensity exercise. Therefore, with physically active individuals, it is often unwise to advocate drastically decreasing carbohydrate intake because this may adversely affect muscle glycogen stores and performance (Cook and Haub 2007).

Hypocaloric diets that restrict carbohydrate intake are probably not prudent during the competitive season if the sport relies on heavy carbohydrate usage, as do long- and middle-distance running, swimming, basketball, wrestling, and others. However, variations of these diets may be beneficial in promoting weight loss for athletes in the off-season. An important point is that weight loss should take place in a competitive athlete’s off-season when possible. Since competitive performance is not a part of the off-season, attaining an ideal body weight and body composition through changes in dietary intakes at this time will not directly affect competitive performance.

NSCA’s Guide to Sport and Exercise Nutrition will lead you through the key concepts of sport and exercise nutrition so that you can assess an individual’s nutrition status and—if it falls within your scope of practice—develop customized nutrition plans. The book is available in bookstores everywhere, as well as online at the NSCA Store.