The Recommended Dietary Allowance of Protein: Title and subTitle BreakA Misunderstood Concept

At periodic intervals the Food and Nutrition Board of the Institute of Medicine produces a report entitled the Dietary Reference Intakes (DRI). The recent DRI report for macronutrients (energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids) was published in 2005.¹ For each macronutrient, a series of values is published, along with a detailed review of all data used to derive those values. Those values for macronutrients are the estimated average requirement, the recommended dietary allowance (RDA), the tolerable upper intake level, and the adequate intake. Of these terms, the RDA is the most widely recognized and has the greatest influence on daily nutrition practices.

The RDA is defined as follows: “The recommended dietary allowance (RDA) is an estimate of the minimum daily average dietary intake level that meets the nutrient requirements of nearly all (97 to 98 percent) healthy individuals.”¹ In addition, the intent of the most recent DRI was to extend, where possible, beyond the concept of minimal requirement to the expression of a goal for consumption. In this case, the RDA would ideally represent an optimal intake of the nutrient, based on health outcomes such as decreased risk of heart disease. This approach was used for a number of nutrients. However, considerable confusion has resulted from the uneven application of this approach. Most notably, the RDAs for carbohydrate, fat, and protein adhere to the strict definition of the RDA (ie, the minimal amount to meet nutrient requirement, rather than an optimal level of nutrient intake).

The RDA for carbohydrate was taken to be the minimal amount required to satisfy the energy requirement of the central nervous system (120 g for adults >18 years), because the central nervous system normally relies entirely on circulating glucose as an energy substitute. It was determined that there is no particular requirement for total fat intake, and therefore there is no RDA. Minimal values for required intake of linoleic acid (between 12-17 g, depending on age and sex) and n-3 polyunsaturated fats (between 1.1-1.6 g) were provided.¹ In addition, the requirement for protein for adults older than 18 years was determined to be 0.8 g of protein per kilogram body weight per day (g/kg/d).¹

Quiz Ref IDThe RDA for protein was based on the results of all available studies that estimated the minimum protein intake necessary to avoid a progressive loss of lean body mass as reflected by nitrogen balance. The Food and Nutrition Board acknowledged the conceptual limitation of relying entirely on results from nitrogen balance studies to determine the RDA, because this method does not measure any relevant physiological end point. Furthermore, the existing data were gathered almost entirely in college-aged men and a greater nitrogen intake is likely required to maintain nitrogen balance in elderly persons.² Regardless of whether 0.8 g/kg/d is an appropriate value for the RDA for elderly persons as well as for 18- to 50-year-old individuals, the point is that the RDA is functionally defined as the amount of protein needed to avoid a deficiency that would lead to a progressive loss of lean body mass (as reflected by negative nitrogen balance).

The minimal nature of the RDAs for carbohydrate, fat, and protein is underscored by considering the sum of the energy equivalents of the corresponding RDAs in relation to the estimated energy requirements published in the same DRI document.¹ Quiz Ref ID The estimated energy requirement for a 70-kg man and a 57-kg woman older than 18 years is 3067 and 2403 kcal/d, respectively.¹ However, when summed together, the RDAs for carbohydrate, fat, and protein for individuals of the same weight would provide approximately 970 kcal/d for the man and 886 kcal/d for the woman. The RDAs for macronutrients thus total approximately one-third of the calories to meet the requirement for energy balance.

The DRI report clearly distinguished the RDAs for macronutrients from recommended intake in the context of a complete diet. The Acceptable Macronutrient Distribution Range (AMDR) was published in the same document¹ to express recommended intakes of macronutrients as percentages of total energy intake. The AMDR accounts for the sum of macronutrient intake having to equal total energy intake, and that a diet composed only of the RDAs for protein, carbohydrate, and fat will fall well short of total energy requirements.

The AMDR is defined as “a range of intakes for a particular energy source that is associated with reduced risk of chronic diseases while providing adequate intakes of essential nutrients.”¹ The definition of the AMDR is more in line with the concept of optimal nutrient intake, as opposed to the minimal required intake reflected by the RDA. In the case of protein, the AMDR was cited to be between 10% and 35% of the daily energy intake.¹ The AMDR for protein can be expressed in the same units as the RDA (g/kg/d) by accounting for the energy expenditure of the individual.Quiz Ref ID For example, the daily energy requirement of a sedentary 19-year-old reference man (76 kg and 1.76 m tall) is estimated to be 37.8 kcal/kg/d in the section on energy requirements in the DRI.¹ Ten percent of this caloric intake translates to a protein intake of 0.95 g/kg/d and 35% of energy intake translates to 3.3 g/kg/d. Thus, the RDA is below the lowest intake recommended by the AMDR when considered in the context of the overall dietary intake of macronutrients. Expressed differently, if the reference man were to consume the RDA of protein, it would constitute only 8.5% of his energy intake, which is below the lowest recommended percentage of the AMDR.

Whereas the AMDR expresses the recommended macronutrient intake in the context of a complete diet, the RDA (0.8 g/kg/d) has been widely applied by others as the appropriate amount, or even maximal amount, of protein intake.Quiz Ref ID For example, the Dietary Guidelines published by the US Department of Agriculture adopted a recommended daily protein intake of 0.8 g/kg/d.³ Governmental programs, such as the school lunch program for children and Meals on Wheels for elderly persons use 0.8 g/kg/d as the appropriate protein intake.⁴ More pervasively, dietary guidelines published in the dietetic literature, the popular press, and various nutritional computer programs promulgate a protein intake of 0.8 g/kg/d. For example, in a science advisory on dietary protein and weight loss, the American Heart Association statement indicated that “Most Americans consume more protein than their bodies need.”⁵ In fact, protein comprises an average of approximately 15% of calories for US adults,⁶ which is at the low end of the range recommended by the AMDR.

The problem of widespread misinterpretation and improper application of the RDA arises from the fact that the Food and Nutrition Board maintained the historically familiar term recommended dietary allowance but functionally defined it in terms misunderstood by most individuals. Thus, the term recommended implies that the RDA does not express a firm requirement, as set forth in the definition of RDA in the DRI. The distinction is that a requirement clearly defines the minimum amount needed, whereas no such implication is conveyed with the word recommended.

The definition of the term allowance in the context of the RDA is not obvious, but presumably relates to the permissible amount if the most conventional definition of allowance is used. Taken together, the logical semantical interpretation of RDA would be the permissible amount of a nutrient the committee recommends individuals should ingest each day, rather than the minimal amount of intake required to meet the nutrient requirement.Quiz Ref ID However, although the term allowance implies an upper limit of intake, the DRI separately addressed the safe upper level of protein intake. It was concluded that there was no evidence documenting an upper level of protein intake beyond which adverse effects would ensue. Consequently, the DRI indicated that there is no tolerable upper intake level for protein.¹

This discussion is relevant to nutrition policy only if there is evidence that the optimal level of protein intake differs from the minimal requirement. The wide range recommended in the AMDR (10%-35% of energy intake) implies uncertainty regarding the exact optimal level of protein intake. This uncertainty reflects in part a relative lack of research addressing this issue. In addition, the wide range reflects, by deduction, uncertainty regarding the optimal levels of carbohydrate and fat in the diet. For example, variations in the proportion of fat in the diet may inadvertently vary correspondingly in the proportion of protein because of an association between common sources of dietary protein and fat. These points notwithstanding, there is ample evidence that the optimal level of protein intake is greater than the RDA. A variety of studies have shown levels of protein intake above the RDA benefiting muscle mass, strength, and function,⁷ bone health,⁸ maintenance of energy balance,⁹ cardiovascular function,¹⁰ and wound healing.¹¹ Close examination of these and related research studies should enable a reasonable estimation of the optimal level of protein intake in a variety of circumstances.

The solution to the dilemma of widespread misinterpretation of the RDA for macronutrients, particularly for protein, would be to accept that a discrete value cannot be assigned to each macronutrient and to use the AMDR to express the recommended intakes. Additionally, the term minimal daily requirement could replace the term recommended dietary allowance to more accurately reflect its functional definition. The introduction of more clearly defined terms would potentially have a greater benefit for nutrition policy and for individual practices than would further deliberations on the exact value of the RDAs of the macronutrients.