An analysis of pooled data from multiple weight-loss trials conducted at Rutgers shows that increasing the amount of protein even slightly, from 18 percent of a person’s food intake to 20 percent, has a substantial impact on the quality of the food choices made by the person.
“It’s somewhat remarkable that a self-selected, slightly higher protein intake during dieting is accompanied by higher intake of green vegetables, and reduced intake of refined grains and added sugar,” said Sue Shapses, author of the study and a professor of nutritional sciences at the Rutgers School of Environmental and Biological Sciences (SEBS). “But that’s precisely what we found.”
Weight-loss regimens that employ calorie restrictions can often spur dieters to reduce the intake of healthy foods that contain micronutrients such as iron and zinc. Ingesting higher levels of proteins is often associated with healthier outcomes, but the link between protein intake and diet quality is poorly understood, according to researchers.
“The impact of self-selected dietary protein on diet quality has not been examined before, to our knowledge, like this,” said Anna Ogilvie, co-author of the study and a doctoral student in the Department of Nutritional Sciences at Rutgers SEBS.
The data was collected from more than 200 men and women participating in clinical trials at Rutgers funded by the National Institutes of Health over the past two decades. The analysis of food records and diet quality for this study was funded by the Institute for the Advancement of Food and Nutrition Sciences in Washington, D.C.
Participants were between the ages of 24 and 75 and registered a body mass index that categorized them as either overweight or obese. All participants were encouraged to lose weight by following a 500-calorie-deficit diet and met regularly for nutrition counseling and support over a six-month period.
The participants were given nutrition advice based on the guidelines of the Academy of Nutrition and Dietetics and the American Diabetes Association. They were encouraged to allot 18 percent of their caloric intake to lean protein, such as poultry, unprocessed red meat, fish, legumes and dairy, and to expend the balance of their calories on fruits, vegetables and whole grains. They were discouraged from ingesting saturated fats, refined grains, sugar and salt.
Participants kept detailed food records, which researchers analyzed for diet quality, specific categories of foods consumed and ratios and specific sources of protein.
The participants who self-selected their protein intake were then characterized by researchers into a lower-protein approach with 18 percent of overall calories coming from protein or a higher-protein approach with 20 percent of the overall food intake coming from protein.
The study concludes:
- Both low- and high-protein groups lost the same amount of weight – about five percent of their body weight over six months
- Higher-protein groups individuals chose a mix of healthier foods to eat overall
- Higher-protein group individuals specifically increased intake of green vegetables and cut back on sugar and refined grains
- Higher-protein group individuals were better able to retain their lean muscle mass
Other Rutgers researchers on the paper include Yvette Schlussel and Lingqiong Meng in the Department of Nutritional Sciences at SEBS.
Consuming adequate dietary protein is critical for maintaining optimal health, growth, development, and function throughout life. Dietary protein requirements in healthy adults (≥19 years old) are dictated largely by body mass and lean body mass, as well as net energy balance and physical activity . The Institute of Medicine (IOM) established the current Dietary Reference Intakes (DRIs) for protein in 2005, including the Estimated Average Requirement (EAR), Recommended Dietary Allowance (RDA), and the Acceptable Macronutrient Distribution Range (AMDR) .
The EAR for protein is 0.66 g per kg body mass per day (g/kg/d) and is defined as the minimum amount of protein expected to meet the individual indispensable amino acid requirements of 50% of the U.S. adult population. The RDA, however, is 0.8 g/kg/d, and reflects the minimum amount of dietary protein required to meet indispensable amino acid requirements, establish nitrogen balance, and prevent muscle mass loss for nearly the entire (i.e., 97.5%) U.S. adult population [2,3].
The RDA for American adults is similar to international adult protein recommendations established by the World Health Organization (0.83 g/kg/d) . The current protein RDA, however, is often incorrectly applied when used as the definition of recommended intake, rather than its true designation as the required minimum intake. This misapplication is problematic for healthy populations and aging adults, and disadvantageous for those with pathophysiological conditions that would necessitate higher-protein needs.
Over the past decade, the potential muscle-related benefits achieved by consuming higher-protein diets (i.e., > RDA but within the AMDR) have become increasingly clear. Increased protein intake contributes to greater strength and muscle mass gains when coupled with resistance exercise , allows for greater muscle mass preservation when consumed during periods of negative energy balance , limits age-related muscle loss , and, to a lesser extent, provides a greater muscle protein synthetic response when evenly distributed across meals [5,8].
A prospective, cross-sectional analysis of the National Health and Nutrition Examination Survey (NHANES) database demonstrates inverse associations between animal and plant protein intake and waist circumference, body weight, and body mass index (BMI) . Advances in this field of nutritional science have translated to a greater emphasis on higher-protein diets, protein quality, and supplemental protein in peer-reviewed literature, lay media, and the commercial food market. Despite greater awareness of how higher-protein diets might be advantageous for muscle mass, actual dietary patterns, particularly as they pertain to protein, have remained relatively unchanged in American adults as a whole .
The disparity between knowledge and action raises the question of whether this expanded understanding of dietary protein is truly meaningful if scientific data are not translated and ultimately reflected in nutrition guidance and, more importantly, in what people eat. As such, the purpose of this brief communication is to highlight common perceptions and benefits of dietary protein on muscle mass, to address misperceptions related to higher-protein diets, and to comment on the translation of academic advances to real-life application and health benefit.
Dietary Protein and Muscle Mass Perceptions
Skeletal muscle protein is dynamic and in constant flux, alternating between states of negative (i.e., muscle protein synthesis < muscle protein breakdown) and positive (i.e., muscle protein synthesis > muscle protein breakdown) protein balance, largely in response to fasting (i.e., postabsorptive) and feeding (i.e., postprandial), respectively. In the postabsorptive state, muscle protein serves as the primary repository of amino acids that is readily catabolized to release free amino acids that can be reincorporated into muscle protein or used to support other critical physiological needs, including serving as an energy substrate through carbon skeleton oxidation, as well as by providing gluconeogenic precursors to support euglycemia (Figure 1).
In addition, free amino acids derived from muscle protein breakdown are used in the synthesis of immune system components, plasma proteins, peptide hormones, and intra- and extracellular enzymes. Transient periods of negative protein balance in healthy adults are completely normal and reversed by feeding.
The magnitude of the postprandial stimulation of muscle protein synthesis, suppression of muscle (and whole-body) protein breakdown, and shift to a positive protein balance is mediated by dietary protein content, protein quality (i.e., based on an individual protein’s digestibility and absorption kinetics, and abundance of indispensable amino acids), and the format in which protein is consumed (e.g., mixed-macronutrient meal, isolated supplemental intact protein, or free-form amino acids) .
The collective optimization of these protein intake-related factors can potentiate the beneficial effects of other protein kinetic stimuli, such as the mechanical and metabolic effects of resistance and aerobic exercise, respectively, resulting in the enhanced remodeling and repair of existing muscle proteins and synthesis of new muscle protein, providing the conditions for muscle maintenance and growth .
Current Dietary Protein Recommendations
The current DRIs for protein have been in place since 2005 but are not without limitations. The EAR and RDA were derived from meta-analyses of nitrogen balance studies . The nitrogen balance method has many limitations and tends to overestimate nitrogen intake (via diet) and underestimate nitrogen excretion (via urine, feces, sweat, and integumental loss), thus falsely illustrating nitrogen balance . Nitrogen balance is also considered a crude measure that fails to provide any information as to what occurs within the system to modulate the body nitrogen pool and subsequent balance [14,15].
Likewise, the AMDR for protein (10–35% of total daily energy intake) was established by setting the lower end of the AMDR at the relative amount of protein believed to meet the set RDA of 0.8 g/kg/d, while the upper end is the mathematical difference achieved if carbohydrate (45–65% of energy) and fat (20–35% of energy) are consumed at the lower ends of their respective AMDR (i.e., 100% − 45% − 20% = 35% as the upper end of protein AMDR) . Carbohydrate and fat are important energy substrates and energy balance is critical to optimal health, but this derivation raises uncertainty about the physiological relevance underlying a recommended upper limit for protein consumption at 35% of total energy intake.
Similarly, the RDA may be sufficient to meet the dietary protein needs of healthy, relatively sedentary young adults, though investigators have argued that this recommendation should be reconsidered based on data from studies demonstrating the inadequacy of the RDA within certain populations when compared to greater requirements derived from the indicator amino acid oxidation method .
Accordingly, internationally recognized professional organizations recommend protein intakes on the order of double the current RDA for physically active individuals, including the joint recommendation to consume protein between 1.2–2.0 g/kg/d established by the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine .
The International Society for Sports Nutrition also recommends protein intake at levels higher than the RDA for physically active individuals (1.4–2.0 g/kg/d) . The definition of the protein RDA itself draws criticism given that it reflects the minimal amount of protein required to prevent deficiency, rather than an amount which may allow for optimal health.
The AMDR does provide for more flexibility in dietary protein intake recommendations in the context of the complete diet, yet most American adults habitually consume protein on the lower end of this range (i.e., 14–16% of total energy intake) .
Dietary Protein and Physical Activity
The benefits of consuming protein following resistance exercise training have been well-documented, especially as they relate to muscle hypertrophy and function . A recent meta-analysis showed significant positive associations between coupling resistance exercise with post-exercise protein ingestion and total fat-free mass, strength, as measured by one-repetition maximum, and muscle size, as measured by myofiber cross-sectional area .
The type and volume of exercise plays a determining role in muscle protein synthetic responses to post-exercise protein ingestion [19,20], as does age  and the training experience  of the individual. The type of protein consumed also factors into the net anabolic response, given that postprandial muscle protein and whole-body protein kinetic responses to free-form amino acids, isolated intact proteins, and mixed-macronutrient meals all differ [22,23,24]. As reflected in sports nutrition recommendations [1,17], holistic evaluation of varied experimental designs suggests that coupling post-resistance exercise protein ingestion (~20–30 g or 0.25–0.30 g/kg) with habitual protein intakes at ~1.6 g/kg/d promotes favorable muscle adaptations to exercise training .
Dietary Protein during Energy Deficit
Consuming higher amounts of protein during typical moderate energy-deficient weight loss diets (i.e., 500–750 kcal/d deficit ) preserves muscle mass in an otherwise catabolic physiological environment . However, the protective effect of higher-protein diets on muscle and whole-body protein homeostasis is compromised as the severity of energy deficit increases beyond 40% of daily energy needs, as a greater proportion of dietary amino acids are oxidized for energy production, thereby minimizing amino acid availability to support protein balance  (Figure 1).
However, most adults rarely experience acute or sustained periods of severe energy deficit, except for perhaps acute fasting for religious reasons, poorly-constructed drastic weight loss plans, preparation and/or recovery from bariatric surgery, or scenarios where food availability is severely limited (e.g., victims of natural disasters, emergency responders, etc.).
Regardless of the cause, these periods of severe energy deficit usually manifest only for short durations (e.g., 1–3 days) and, therefore, are likely physiologically tolerable. However, if energy expenditures are high and dietary energy and protein intake are limited for extended periods of time, for example during sustained, multi-stressor military operations [27,28], the consequences of severe energy deficit are much more problematic, especially if body mass and fat-free mass loss are so severe that immune system and muscle function and performance are compromised [29,30,31].
During those conditions, prioritizing energy intake, more so than focusing solely on protein per se, is vitally important to help prevent excessive muscle catabolism and conserve muscle function and performance. With moderate energy deficit, however, protein intakes on the order of double the current RDA (i.e., 1.6 g/kg/d) have proved efficacious in preserving muscle mass during weight loss .
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6566799/
Original Research: The findings will appear in Obesity