A Food Insight Series on the Power of Protein: A Peek at Protein and Weight Management

Outside of the necessity of eating protein and the benefits attributed to foods that contain high-quality protein including low-fat dairy, lean meats, eggs, and soybeans, new studies suggest that there are some emerging and unique health benefits that deserve to be explored. The May 2009 issue of Food Insight introduced the Power of Protein series and provided some background about protein, including daily protein needs and some common dietary sources. This article will focus on the potential health benefits of protein related to weight management.

How Protein Can Help with Weight Management
Protein may have an effect on an individual’s ability to manage their weight in a variety of ways, by: 1) increasing levels of satiety; 2) maintaining lean body mass thus possibly increasing energy expenditure; and, 3) overall dietary compliance. While each of these individual relationships to protein and weight management is being studied independently, their inter-relationship to each other likely contributes to protein’s association with weight maintenance.

Protein and Satiety
Researchers have defined the term satiety as “the feeling of fullness or lack of interest to eat following a meal.” The term satiety as it relates to protein can be used to describe the potential effect of individual ingredients such as whey, casein, or soy, the effect of foods such as meats, poultry, fish, dairy, beans, etc., or of entire meals.  In 2002, the IOM Dietary Reference Intake report on protein suggested that protein has a more satiating effect than fat or carbohydrates—the other macronutrients. This suggests that small increases in protein consumption that are offset by decreases in fat or carbohydrate consumption may help increase feelings of fullness after a meal. It is theorized that if satiety is experienced, then weight loss may occur secondary to reduced calorie consumption over time.

Additionally, some studies suggest that different forms of protein, such as whey, or casein found in milk, may have differing affects on satiety. Whey protein is digested faster and causes a quick increase in circulating amino acids, while casein is digested more slowly and provides a more consistent release of amino acids. These circulating amino acids have an effect on certain hormones that influence appetite. Research shows that whey protein has a more rapid effect on hormones that affect appetite, and therefore may help increase satiety after a meal.  Other studies have shown that soy protein has satiating effects similar to whey and casein.  However, proteins are often consumed together from foods like milk and fortified cereals, drinks and bars, so more research that takes into account the matrix of nutrients that may be consumed in a given meal needs to be conducted before further conclusions can be drawn.

Protein, Body Composition, and Energy Expenditure
In addition to the short term satiating effects in the studies that have been conducted thus far, more recent research is also evaluating the effect of protein on lean body mass and energy expenditure.

The amount of lean muscle mass versus fat that an individual may carry also has an effect on the base level of daily calories that are needed for body maintenance (e.g., blinking, breathing, breaking down food) and repair (e.g., building new cells and tissues).  The ratio of lean muscle mass to fat mass that an individual may possess also has an effect on weight maintenance.  Because lean muscle mass is more metabolically active than fat, a higher lean muscle mass to fat mass ratio translates to increased total energy expenditure.  Therefore, fluctuations in lean muscle mass can affect overall energy balance.

Generally, when a pound of body weight is lost it includes about 75 percent fat mass and 25 percent protein.  However, several studies have suggested that high-protein diets may increase the percentage of body fat lost, while sparing lean muscle mass. For example, in a six-month study of overweight people, fat loss was almost twice as great in people who consumed a high protein diet (25 percent of energy) compared with a moderate protein diet (12 percent of energy). A 12-month study of overweight people showed similar results.

Protein and Weight Management Outcomes
Studies have evaluated different ratios of fat, protein and carbohydrate over various lengths of time and have found efficacy for a variety of different diet plans.  High protein diets, both mostly animal-protein based or plant-based, have been shown to be efficacious.  Helping others lose weight and/or maintain weight loss is a challenging prospect. However, a number of studies have reported that compliance with high protein diets has been easier for participants to maintain over longer time periods, which may have a greater impact on sustained weight loss.  There is also general agreement that sustained weight loss is difficult to achieve with drastic changes in diet and lifestyle.  Some studies have reported that it may be beneficial to modestly offset other energy sources with protein, but also that higher protein diets may offer the added advantage of compliance.

While the factors that affect weight loss are complex, it appears that an expanded role for protein and weight management may be emerging. Just as the body is complex, so too is the relationship between protein and weight loss. Future research will continue to evaluate the role that protein may play in increasing satiety and total energy expenditure and maintaining lean body mass, which likely act in synergy along with increasing physical activity and managing energy intake to improve weight management.

Currently, the best advice for healthy individuals is to consume a colorful diet rich in whole grains, fruits, vegetables, and fiber.  Additionally, eating a wide variety of proteins, including lean meats, fish, beans, soy, and low-fat and fat-free dairy products will help ensure optimal intake of both essential and non-essential amino acids.