DIETARY PROTEIN AND THE ROLE OF SOY

"Protein" as a name is derived from the Greek word “proteios” which means of the first rank or position, and of primary importance. The word was originally coined in 1838 and was chosen to represent the fundamental nature of protein’s role in human nutrition. However, the nutritional importance of protein is also because of its constituent amino acids. The 20 a-amino acids that are part of mammalian body protein are classified based on their nutritional importance into indispensable (essential) amino acids, conditionally indispensable (essential) amino acids and the dispensable (nonessential) amino acids. Thus, both protein quantity and quality are important to ensure the provision of all amino acids in the right balance to sustain normal bodily functions.

Protein Requirement

Dietary protein is an essential component of a healthy diet to support both growth and maintenance during stages of development, and maintenance alone during all other life stages.² Hence, the current definition of protein requirement is: “…the lowest level of dietary protein intake that will balance the losses of nitrogen from the body, and thus maintain the body protein mass, in persons at energy balance with modest levels of physical activity, plus, in children or in pregnant or lactating women, the needs associated with the deposition of tissues or secretion of milk at rates consistent with good health.”³

Protein intake recommendations for North Americans are provided via the Dietary Reference Intakes (DRIs) as the Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs).² The EAR is the average daily nutrient intake amount estimated to meet the requirement of half the healthy individuals in a particular life stage and gender group. The RDA is an estimate of the minimum daily average dietary intake amount that meets the nutrient requirements of nearly all (97-98%) healthy individuals in a particular life stage and gender group.² The DRI estimates are a minimum requirement and do not represent a “ceiling” or maximum recommendation. In addition, the protein RDAs are based on direct evidence only for young adults.² For all other life stages, the values are based on factorial (mathematical) calculations from the young adult estimates. 

Current Protein Intake Recommendations

Recommendations for young adults are set at 0.66 g/kg/d and 0.8 g/kg/d as the EAR and RDA, respectively (Table 1). These values are based on a comprehensive meta-analysis of available nitrogen balance studies conducted by Rand et al.⁴ The nitrogen balance method has been the traditional method to determine protein requirements. Nitrogen balance identifies the protein requirement in healthy adults as the “continuing intake of dietary protein that is sufficient to achieve body nitrogen equilibrium (zero balance) in an initially healthy person of acceptable body composition at energy balance and under conditions of moderate physical activity and as determined after a brief period of adjustment to a change in test protein intake.”⁴

Rand and colleagues acknowledged that there are several shortcomings of the nitrogen balance method. The drawbacks of this method have been covered in detail.^2,3 Briefly, the balance method measures a very small difference in whole body nitrogen intake and excretion, and tends to overestimate nitrogen intake and underestimate nitrogen excretion. The net result is an overtly positive balance which could lead to an underestimation of the requirement.⁵

Positive balances are observed in most adult studies, although it is biologically impossible for such balances to be sustained. Balance studies also require relatively long periods of test diet adaptation (7-10 days) because equilibration of the large (and slow changing) body urea pool requires at least 5–7 days after a change in test protein intake. The invasive nature of the nitrogen balance method makes it unethical to study most other vulnerable life stages. Clearly better non-invasive methodology that would allow direct data-driven determination of human protein requirements is needed.

Human Protein Requirements Determined by Novel Method

The indicator amino acid oxidation (IAAO) method is a minimally-invasive technique that uses stable isotopes to determine amino acid requirements;^5,6 making it an attractive alternative for better assessing protein and essential amino acid needs in different populations.⁷ The method is based on the physiological principle that excess amino acids cannot be stored; and therefore must be partitioned between incorporation into protein or oxidation. Thus, when one indispensable amino acid is deficient for protein synthesis, all other amino acids including an indicator amino acid (another indispensable amino acid, usually L-1-¹³C-phenylalanine) are in excess and therefore will be oxidized (Fig. 1).⁷

With increasing intake of the limiting amino acid, oxidation of the indicator amino acid will decrease due to its increasing incorporation into protein (Fig. 1). Once the requirement is met for the limiting amino acid/protein, there will be no further change in the oxidation of the indicator amino acid with increasing intake of the test amino acid. The inflection point where the oxidation of the indicator amino acid stops decreasing and reaches a plateau is referred to as the “breakpoint” (Fig. 1). The breakpoint, identified with the use of two-phase linear regression analysis, indicates the EAR of the limiting (test) amino acid/protein.^5,7 This method is well-suited for studying protein requirements across the life cycle and in at-risk populations because IAAO requires only oral isotope provision, collection of breath samples and a single study day adaptation.⁵

Using the IAAO method described above, we have determined the protein requirements across the life cycle (Table 1), beginning with young adult men,⁸ 6-10 year old children,⁹ pregnant women during early (~16 week) gestation and late (~36 week) gestation,¹⁰ elderly women (>65 years old)¹¹ and men (>65 years old),¹² and in octogenarian (80+ year old) women.¹³ Our estimates are 30–70% higher than the current recommendations. While on a body weight (g/kg) basis the IAAO determined requirements are higher, when expressed as a percent of energy, the IAAO estimates range from 10–20% across different life stages, thus, are well aligned with the acceptable macronutrient distribution range (AMDR) for energy from protein. Conversely, the current DRIs for all life stages call for a protein intake of only 7–10% of energy and therefore are not practical.

Soy: A High Quality Protein

Beyond the amount of protein, the type of protein is an important consideration, and is currently extensively being discussed by the Food and Agriculture Organization (FAO).^14,15,16  Protein quality is defined as the capacity of food sources to provide individual amino acids in sufficient quantity to the body. In general, animal sources are considered of higher quality, since they contain all essential amino acids at higher concentrations on a mg/g protein basis.

Soy and other plant sources, such as legumes and pulses, are considered good quality protein sources. But in general, plant proteins are low in one or more essential amino acids; for example, cereals are low in lysine and pulses/legumes are low in methionine. Furthermore, plant protein sources are also affected by cooking and processing methods and by the presence of anti-nutritional factors.¹⁷

However, plant-based diets,¹⁸ primarily based on pulses/legumes,¹⁹ are being encouraged as a sustainable diet choice by the FAO²⁰ and the Institute of Medicine (IOM).²¹ Sustainable diets have a low environmental impact and contribute to food and nutrition security and a healthy life for present and future generations. To provide dietary guidelines that include plant-based recommendations for environmental sustainability,²² we need a better understanding of the role of individual foods to support protein synthesis in vivo —particularly of plant dietary protein sources. Toward this objective we have embarked on a recent set of studies using our novel IAAO method to test the protein quality of food sources.²²

In vivo protein quality is primarily determined by two factors – digestibility, followed by bioavailability of amino acids for protein synthesis. We have determined that methionine is ~87% and 72% bioavailable from casein and soy protein isolate, respectively.²³ We also reported that lysine from cooked white rice is highly bioavailable (97%);²⁴ however, dry heat (browning of rice) reduced lysine bioavailability to 70%. Recently we showed in adult men that tryptophan bioavailability from cooked cornmeal is 80%, though lysine bioavailability from corn is 71%.²⁵

A key point is that the IAAO method measures both digestibility and bioavailability of amino acids because it is based on an end-point carbon oxidation measure and therefore accounts for all losses of dietary amino acids during digestion, absorption, and cellular metabolism. Our results have the potential to be applied in designing plant-based diets for vulnerable populations, such as young children, to meet individual limiting essential amino acids.

Conclusion

Protein nutrition plays a critically important role in overall health. Therefore, it is important to ensure that recommendations for dietary protein intakes match human demands. However, current protein recommendations may not be adequate to meet in vivo demands--

especially in different life stages. On the other hand, protein-rich animal sources are environmentally demanding to produce, and plant-based proteins such as soy are considered sustainable. To ensure all amino acids are provided to meet body demands, consume a variety of plant-based protein sources each day.

Table 1. Protein Requirements Determined in Humans vs Recommendations across the Life Cycle

¹Dietary Reference Intakes; ²Indicator Amino Acid Oxidation derived requirement estimates; ³Estimated Average Requirement; ⁴Recommended Dietary Allowance

Figure 1. Indicator Amino Acid Oxidation (IAAO) Concept

EAR, estimated average requirement

RDA, recommended dietary allowance

SD, standard deviation

REFERENCES

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