Weight loss: can you lose weight effectively on a protein diet?

Protein and weight control

At a glance

Most people think of protein as a muscle-building nutrient. But as Kevin Tipton explains, new research suggests it could play an important role in weight loss and maintenance

Weight control is a significant issue for many people, including athletes, who may benefit from reduced fat mass to improve performance. In general, creating a calorie deficit (either through dietary energy restriction or increased energy output) for a sufficient period of time will result in successful weight loss.

For sedentary people, slow weight loss over a fairly protracted time period is likely to be the best strategy. Athletes, on the other hand, may desire more rapid weight loss for competitive reasons and, since they are already training at a high level, may not be able to increase exercise or activity levels to any great degree. Therefore, the dietary intake side of the equation becomes critical.

Most dieters, particularly exercisers, are more concerned with loss of fat and would like to retain as much muscle as possible to maintain performance. Unfortunately, hypocaloric dieting often results in loss of lean body mass (1), perhaps leading to compromised performance or increased injury risk (2). Clearly, a dietary strategy that minimises the loss of muscle while maximising fat loss would be desirable for most.

Recent evidence suggests that weight loss is influenced not only by the total energy consumption, but also by the macronutrient composition of the diet – ie carbohydrate, fat and protein contents. Recently, protein has received a great deal of interest particularly with regard to the loss of muscle relative to fat. Many recent studies have investigated the impact of manipulating protein intake during hypocaloric weight loss in overweight and obese individuals(1). Increased protein ingestion during weight loss, particularly in combination with exercise training, improves weight loss and decreases the loss of lean body mass. Moreover, a higher protein intake when calorie intake returns to normal (ie matches calorie output) decreases the rate of weight regain(3). Thus, at least for obese or overweight individuals, a higher protein intake seems to be the best choice during weight loss with a hypocaloric diet.
In addition to simply increasing the amount of protein to enhance weight loss and maintain muscle mass, there is now evidence that specific proteins may be important influential for this effect. In particular, dairy products have been touted as the key ingredients in a diet designed for optimal weight loss(4).

The effectiveness of dairy products is attributed to the calcium content and other bioactive ingredients, including a high level of whey proteins. Whey proteins have a high proportion of leucine, an amino acid associated with increased protein synthesis and fat oxidation (see below). It is important to note that not all studies demonstrate that increasing the dairy content of the diet results in superior loss of weight or improved body composition. Thus, it is fair to say that the jury is still out on the importance of dairy proteins for optimal weight loss(5).

Evidence in athletes

Whereas it seems clear that high-protein diets may be appropriate for overweight and obese populations(1,3), there is little information available on athletic populations. The metabolic turnover and activity level of athletes and regular exercisers is likely to be far removed from those of sedentary overweight individuals. Moreover, metabolic disorders are often present in inactive, obese people.

Athletes, on the other hand, are usually healthy (at least metabolically, even if not orthopaedically!) and this fact may impact the response to high-protein, low-calorie diets. As such, extrapolation of the results of studies on obese and overweight subjects to athletes may not be appropriate; the specific focus of studies on athletes is critical before making recommendations.

The impact of increased protein intake during weight loss in athletes is not entirely clear. One study used a technique called nitrogen (N) balance. N balance represents protein balance. If the number is negative, then the individual is considered to be losing protein and if positive, gaining protein (see figure 1).

Figure 1

A study from Virginia used the N balance technique in two groups of bodybuilders during weight loss(6). One group consumed a higher proportion of protein than the other during one week of hypocaloric dieting. The bodybuilders with the higher protein intake were in positive N balance, whereas those consuming moderate protein were in negative N balance. Extrapolated to muscle, the high-protein group gained around 700g of muscle for the week while the group consuming moderate protein lost around 600g of muscle. However, no difference between the groups could be detected in the changes in fat or muscle measured by underwater weighing. Whereas this change in muscle mass seems quite large for a single week, the method used to detect these changes is not particularly sensitive, which explains why no statistically significant difference was found between groups. Furthermore, these results demonstrate a common issue with N balance – at high-protein intakes N balance is often quite large and may not reflect measured changes. Nevertheless, these results can be used to support the idea that increased protein intake preserves muscle during low-calorie dieting in body builders.

On the other hand, a more recent study found no effect of increased protein or branched chain amino acid (BCAA) intake on the amount of lean body mass lost during weight loss in athletes(7). Researchers in France had four groups of wrestlers restrict their calorie intake for 19 days. One group served as a control and consumed a typical dietary composition, while the others consumed a high-protein, a high-BCAA or a low-protein diet. All groups lost about 5% of lean body mass, but the high BCAA group lost significantly more fat than the other groups. The high-protein group lost no more fat than the low protein group. These results do not seem to match those of the earlier study, nor do the results fit with those from studies on obese individuals. Taken together, these limited – and apparently conflicting – data make it difficult to form solid conclusions on the effectiveness of high-protein intake during weight loss in athletes.

Guidelines for protein use

New research

We recently performed a carefully controlled study to further examine the impact of increased protein intake during low-calorie dieting in weight lifters. Two groups of athletes who normally consumed about 3,000-3,500 calories per day, including about 1.6g of protein per kilo of body weight per day (about 15% of total calories) consumed 60% of their normal calorie intake for two weeks (ie reduced it by 40%).

During the weight-loss regime, one group consumed a diet that resembled their normal dietary pattern. The other group consumed a high-protein diet – 2.3g protein/kg/day (about 36% of total calories). Both groups lost the same amount of fat, but the group consuming more protein lost little if any muscle while the other group lost an average of 1.5kg of muscle mass. Thus, the group with the ‘normal’ diet lost more total weight. Clearly, the difference in the loss of body mass was entirely attributable to a loss of lean body mass (see figure 2). We hope to follow up these admittedly preliminary results with future studies designed to further investigate details of body composition changes in athletes during weight loss with high-protein intake.

Figure 2

Our results suggest that the goals of the athlete should be carefully considered when making a recommendation for diet during weight loss. For many athletes, preservation of muscle mass may be very important. If so, then a high-protein diet may be advisable. If, however, the total amount of mass to lose – regardless of the type of tissue lost – is the critical factor, then a high-protein diet may not be advisable. Either way, for the vast majority of athletes, the carbohydrate level should not be compromised. In our study, the two groups ate the same amount of carbohydrate in an attempt to avoid declines in performance due to lower carbohydrate intake. This factor may also help explain the lack of performance decrements in either group. Clearly, much more research needs to be undertaken to determine the effectiveness of dietary composition for attainment of ideal body composition for athletes and others during weight loss.

Protein

What is a high-protein diet?

There are several ways to consider dietary protein content. In Table 1, the dietary composition of a weight maintenance and low energy diet for a typical 80kg athlete are presented. The diets are portrayed in two fashions – regular (normal) dietary composition and high-protein. Both are designed for weight loss. Note that the composition of the diet can be described as the absolute amount of the protein, the % of total energy (calories) as protein and the amount of protein ingested per kg of body weight per day (g/kg BW/d).

Table 1

Since we are concerned with maintenance of exercise capacity, the carbohydrate intake has been kept constant, from a % energy standpoint. Nevertheless, in order to reduce total energy intake, carbohydrate has to go down on a g/kg body weight basis. In order to increase protein, fat intake is reduced. Note that as the energy intake drops, the fat intake must be dramatically reduced to accommodate the increased protein. In practice, this issue may be problematic because very low-fat diets are often very unappetising. Menu design must be considered carefully in order to provide the appropriate dietary composition. In the scientific literature, the definition of a high-protein diet varies from ~27% up to ~70% of total energy intake, from an absolute amount of ~ 90g up to almost 300g of protein per day or from ~1.5 g/kg BW/d up to 3-4 g/kg BW/d – all depending on who makes the call!

References

1. J Nutr 2006; 136:319S-323S
2. Am J Clin Nutr 2006; 84:475-482
3. Am J Clin Nutr 2008; 87:1558S-1561S
4. J Am Coll Nutr 2005; 24:537S-546S
5. Nutr Metab Cardiovasc Dis 2006; 16:445-451
6. Int J Sports Med 1988; 9:261-266
7. Int J Sports Med 1997; 18:47-55
8. J Nutr 2001; 131:856S-860S
9. Forum Nutr 2003; 56:168-170

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