December 2017 Issue
High-Protein Diets and Weight Loss
By Kaley Todd, MS, RDN
Vol. 19, No. 12, P. 32
Does the research prove their efficacy and long-term safety?
Protein continues to top the charts and be the nutrient du jour as a result of trendy weight loss diets and celebrity-backed eating plans. The popular Atkins and Paleo diets and others typically demonize carbohydrates and promote protein as the key to shedding pounds and building muscle.
Popular weight loss plans promote the concept that high-protein foods take more work to digest, metabolize, and use, therefore the body burns more calories as it processes them. Proponents of these weight loss diets also suggest that the increased digestion time leads to feeling fuller longer and thus results in lower calorie intake.
To be sure, protein is essential to building and repairing tissues, as well as generating enzymes, hormones, and other body chemicals, and is important in developing bones, muscles, cartilage, skin, and blood. However, is it as magical as it's professed to be, and can too much of a good thing be harmful?
Defining High Protein
According to the 2015–2020 Dietary Guidelines for Americans, protein should represent 10% to 35% of total daily calories. In the United States, the Recommended Dietary Allowance for protein is 46 g per day for women older than 13 and 56 g per day for men older than 18.1 The Institute of Medicine recommends adults get a minimum of 0.8 g protein for every kg of body weight per day.2
According to Christopher Gardner, PhD, director of nutrition studies at the Stanford Prevention Research Center and a research professor of medicine at Stanford University in California, few Americans consume inadequate amounts of protein and, in fact, most consume more than two times their requirements. "Even the ones who aren't trying to eat a lot of protein tend to eat beyond what they need, often in the form of bars or powders," Gardner says.
Although there's no clear definition of a high-protein diet, the upper limit for protein set by the Dietary Reference Intakes is 35% of daily calories; therefore, many practitioners use "35% or more calories from protein" as a defining benchmark.
Connie Diekman, MEd, RD, CSSD, LD, FADA, director of university nutrition at Washington University in St. Louis, and a former president of the Academy of Nutrition and Dietetics, considers a diet that includes more protein than carbohydrates to be a high-protein diet. Typically, such diets provide more than 35% of calories from protein. However, she says, "The need for protein can vary among individuals. Body builders need more than cross-country runners, and pregnant women need more than when they're not pregnant. It's a bit difficult to give a clear definition."
Do They Really Promote Weight Loss?
After decades of health professionals promoting a low-fat dietary approach to weight loss, research currently is providing evidence that a high-protein diet may be an effective approach. A meta-analysis of 24 studies published in The American Journal of Clinical Nutrition examined the effect of the macronutrient profile of diet on weight loss.3 Compared with an energy-restricted standard-protein diet, an isocalorically prescribed higher-protein diet aided in greater weight loss. In addition, high-protein diets were found to be more effective than lower-protein diets for preserving muscle mass and preventing metabolic slowdown during weight loss.
Some research has shown that high-protein diets may impact people differently. Whether this is due to taste preferences or genetics remains to be determined. A European study published in the International Journal of Obesity concluded that based on different gene types, high-protein diets are effective for weight loss and maintenance in 67% of the population.4 Results from the A TO Z study, which compared the Atkins, Zone, Ornish, and LEARN diets, showed variation in weight outcomes within each.5 More research is needed to understand this concept of high-protein diets and nutrigenomics.
Deciphering the Theories
Weight loss experts have proposed various theories on why high-protein diets may help certain individuals shed pounds. One theory is that they decrease hunger and promote satiety. It's believed that higher protein intake may increase production of peptide YY and glucagonlike peptide 1 (GLP-1), hormones that help promote the feeling of fullness. In a controlled study of 12 healthy women, the group that consumed a high-protein diet experienced higher GLP-1 levels, greater feelings of fullness, and less hunger than the group that ate a lower-protein diet.6 Studies also have found that higher protein intake helps reduce levels of ghrelin, aka "the hunger hormone." A study of 15 healthy men showed consumption of a high-protein meal to have greater suppression of ghrelin compared with consumption of a high-carbohydrate meal.7
Another theory is that a high-protein diet may significantly increase the rate of metabolism or have a higher diet-induced thermogenesis, a metabolic response to food. Food intake results in a brief increase in energy expenditure attributable to the various steps of nutrient processing (eg, digestion, absorption, transport, metabolism, and storage of nutrients). Research using a room calorimeter to assess 24-hour energy expenditure in subjects found higher energy expenditure (71 kcal per day) in those consuming a high-protein diet.8 Other findings have reported that subjects following a diet containing 29% protein had a higher resting metabolic rate, burning 213 more kcal per day than those consuming the same calories with 11% energy from protein. According to a report by Frank Hu, MD, PhD, MPH, the Frederick J. Stare Professor of Nutrition and Epidemiology at Harvard T.H. Chan School of Public Health in Boston, "The synthesis of protein, the high ATP [adenosine triphosphate] cost of peptide bond synthesis, as well as the high cost of urea production and gluconeogenesis are possible reasons for the higher thermic effect of protein."9 However, Hu notes that most of the studies on high-protein diets and weight loss have been too short in duration to be conclusive.
Nonetheless, studies have shown that high-protein diets may help preserve lean body mass during weight loss. Researchers from the School of Sport and Exercise Sciences at the University of Birmingham in the United Kingdom investigated the influence of dietary protein on lean body mass loss and performance during short-term weight loss in 20 young, healthy athletes.10 They found that a diet composed of 35% protein was significantly superior to a 15% protein diet for maintenance of lean body mass.
While some studies have suggested the validity of these theories, additional research is needed to understand the actual mechanism. Gardner believes the findings based on these theories are trivial and isn't convinced. "I don't think there's sufficient research supporting the idea that you burn extra calories when metabolizing protein, or that eating protein leads to building more muscle, and lean tissue burns more energy than fat," Gardner says, pointing to a study published in the Journal of the American Dietetic Association that found that varying the protein content of several entrées didn't differentially influence energy intake or satiety rating.11
In addition to possibly promoting weight loss, dietary protein low in saturated fat that replaces carbohydrates (especially refined sources) may provide heart health benefits. Researchers in a crossover study at the Lipid Clinic at University Hospital in London, Ontario, randomly assigned subjects with moderate hypercholesterolemia to either a high-protein, low-carbohydrate diet or low-protein, high-carbohydrate diet. Researchers kept intakes of dietary fat, cholesterol, and fiber constant. The main sources of protein included turkey, cottage cheese, beef, fish, and ham. Exchanging protein for carbohydrate significantly reduced LDL cholesterol and triacylglyceride levels while increasing HDL.12
A 20-year prospective study of more than 80,000 women found that eating a low-carbohydrate diet high in vegetable sources of fat and protein resulted in a 30% lower risk of heart disease compared with women who ate high-carbohydrate, low-fat diets. However, eating a low-carbohydrate diet high in animal fat and protein didn't offer such protection, and smoking was an important variable.13
Findings from the Optimal Macronutrient Intake Trial for Heart Health (OmniHeart) showed that, in some healthful diets, partial substitution of carbohydrate with protein can lower blood pressure and improve LDL in amounts equivalent to, or greater than, the original diet.14 James Hill, PhD, director of the Colorado Clinical Nutrition Research Unit, says that although a high-protein diet may show heart health benefits, it's most likely secondary to weight loss.
Considering the health benefits of consuming a high-protein diet, it's critical to evaluate the protein sources. "As a whole, high-protein diets don't offer any heart health benefits," Diekman says. "But if the protein is from plant sources—beans, tofu, nuts—these protein sources would shift the fat content of the diet to more healthful fats and provide phytonutrients that do seem to help with heart disease prevention."
Evidence from a 20-year study that looked at the relationship between low-carbohydrate diets and type 2 diabetes in women reported that low-carbohydrate diets high in vegetable sources of fat and protein modestly reduced the risk of type 2 diabetes.15 However, low-carbohydrate diets that were high in animal protein or fat sources didn't show this benefit. Such findings suggest protein type was critical in determining the healthful benefits of consuming a high-protein diet.
Overwhelmingly, the literature favors plant protein over animal protein for health benefits. A large Harvard study of health care professionals found higher animal-derived protein intake to be positively associated with cardiovascular mortality and higher plant protein intake to be inversely associated with cardiovascular mortality.16 Substitution of plant protein for animal protein, especially from processed red meat, was associated with lower mortality, suggesting the importance of protein sources.
Although short-term studies have found benefits to consuming a high-protein diet, many experts have concerns about possible long-term consequences. One primary issue is that weight loss generally isn't sustained beyond one year. "Research does show that high-protein diets can aid weight loss if overall calories are controlled," Diekman says. "But the challenge is that many of these diets aren't maintainable. If the shift in calories, no matter what the source, increases after weight loss, then the weight returns. A common outcome of a high-protein, low-carbohydrate diet is weight loss that does not last."
Hill encourages practitioners to work with their clients and patients to find reasonable and sustainable ways to stick with a healthful eating plan. "Following a diet over time is difficult. With any diet, compliance is essential," Hill says.
David L. Katz, MD, MPH, founding director of Yale University's Yale Griffin Prevention Research Center, and current president of the American College of Lifestyle Medicine, expresses similar concern about following a high-protein diet. "Although we don't have sufficient long-term studies, I would be cautious about following one [high-protein diet] for any long period of time," Katz says. "Any diet is good for weight loss in the short term, because it imposes discipline and restricts choices. In reality, good diets work; bad diets may work but only temporarily," Katz says. Although there's some evidence that protein helps with satiety, Katz says, "excess protein turns into body fat just like excess calories from any source."
Moreover, there's concern that recommending protein-enriched meal replacements as part of a weight management program could lead to changes in liver biomarkers or renal function and reductions in bone density. Although research is inconclusive, some studies suggest high-protein diets may place additional stress on the kidneys and may increase the risk of kidney disease in certain populations.17-19. "For people who already have impaired kidney function or have kidney disease, high-protein diets are bad for them, because that creates more work for the kidney. Those individuals should eat low-protein diets," Gardner says.
Given the current research and possible long-term consequences of following a high-protein diet for weight loss, dietitians can use the following recommendations as a guide when counseling patients:
• Evaluate protein sources. RDs must remember that different protein sources appear to have different effects on health. Clients who want to follow a high-protein diet must remember this doesn't mean they're to eat more meat. Although lean meats certainly can provide high-quality protein and be included in a healthful diet, dietitians should emphasize plant foods. The displacement of refined carbohydrates with protein may have health benefits; however, the use of plant sources of protein, such as nuts, legumes, and soy, may provide even greater benefits. It's essential to consider the protein "package"—the fats, carbohydrates, vitamins, minerals, and other nutrients that invariably come with protein.
• Rethink the plate. Gardner believes the challenge with studies on high-protein diets is that they focus on nutrients rather than whole foods. He believes practitioners need to encourage clients to look at foods in their entirety rather than in individual macronutrients or micronutrients. He recommends dietitians encourage clients to fill their plates primarily with plant-based foods. Gardner stresses the importance of introducing new fruits, vegetables, and whole grains and showing clients how plants can taste delicious.
As clients increase their protein intake, dietitians should remind them to displace other foods such as high-fat protein sources. These switches can impact the diet for better or worse. Katz reminds practitioners that too much protein can turn into body fat just like excess calories from any food source. So eating more nutritious protein sources instead of low-quality refined carbohydrates such as white bread and sweets should be encouraged.
— Kaley Todd, MS, RDN, is the dietitian for Sun Basket, a healthful meal kit delivery service. She's a freelance writer, clinician, and culinary and communications specialist. She's also the founder of Kaley Todd Nutrition at http://kaleytoddnutrition.com.
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