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Study under review: Dietary Protein Requirement of Men >65 Years Old Determined by the Indicator Amino Acid Oxidation Technique Is Higher than the Current Estimated Average Requirement
Introduction
Large public health organizations like the World Health Organization (WHO) and the Food and Agriculture Organization of the United Nations (FAO) make recommendations for protein intake as part of their efforts to improve global health. Two numbers are often used in this context: Estimated Average Requirement (EAR) and Recommended Daily Allowance (RDA). The EAR refers to an intake value that is estimated to meet the requirement of half the healthy individuals in a group, and by definition implies a high risk of inadequacy (i.e. only 50% of a healthy population will have a higher intake). The RDA is set at an intake level that should meet the needs of about 97% of healthy individuals. The current EAR for protein intake in healthy adults according to the WHO and Institute of Medicine is 0.66 grams per kilogram of bodyweight per day, and the RDA is 0.8 grams per kilogram of bodyweight per day.
Recommendations for healthy adults are generally derived from studies conducted mainly in younger adults. This is potentially problematic for a number of reasons. For example, compared with younger adults, the muscles of older adults are less sensitive to smaller doses of protein and appear to require a greater amount of protein to fully stimulate muscle protein synthesis[1].
Inadequate protein intake results in loss of lean body mass, decreased muscle function, and even an impaired immune response[2] to stress and infection. Previous[3] research[4] has shown protein intakes at the current RDA can lead to negative nitrogen balance and loss of muscle mass in elderly individuals. Due to this, some recommendations[5] now suggest an intake of 1.2-1.5 grams of protein per kilogram of bodyweight per day for certain older populations. Observational research[6] has found associations in elderly people between a greater dietary protein intake and improved health.
Current recommendations are based on data from nitrogen balance studies[7], which are a type of study that attempts to determine protein requirements by measuring the amount of nitrogen (a key component of amino acids, the building blocks of protein) that is ingested and the amount that is excreted (via sweat, urine, feces, hair, and skin).
Unlike carbon, oxygen, and hydrogen (other building blocks of amino acids), nitrogen doesn't turn into gases and water during metabolism, so it can be measured as a specific marker of protein balance. Being in negative nitrogen balance means that the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested, and is associated with wasting diseases and muscle catabolism, while positive nitrogen balance means that the amount of nitrogen excreted from the body is less than the amount of nitrogen ingested, and is associated with growth and muscle buildup. The use of nitrogen balance to determine optimal protein intake dates back over 100 years.
However, the nitrogen balance method has several limitations[8]. For instance, variations in overall body nitrogen and amino acid metabolism can lead to a change in nitrogen balance[9], with or without a change in intake. If excretion is changing without a change in intake, it becomes difficult to determine whether or not the effect is due to the dietary protein intake, because this method is basing conclusions on specific dietary protein intakes. Also, testing requires five to ten days of adaptation[8] to each level of amino acid by a participant, and complete collection and quantification of all sources of nitrogen excretion (mostly in urine and feces, but also sweat) is difficult.
An alternative method for studying protein needs is the indicator amino acid oxidation (IAAO method), which is shown in Figure 1. This method is based on the concept that when one essential amino acid (EAA) is deficient for protein synthesis, then all other EAAs, including the “indicator” amino acid, will be oxidized. This is because the deficient amino acid becomes the limiting step in protein synthesis. The indicator amino acid (often phenylalanine, though lysine or leucine can also be used) is “labeled” with a stable isotope (like a tracking device). The appearance of the label in exhaled carbon dioxide is used as an indicator of a protein or amino acid requirement. This method was originally developed to determine requirements for individual amino acids. Researchers would vary the levels of one EAA and measure oxidation of the tracer. However, in the current study, the researchers varied the levels of all EAAs (except phenylalanine and tyrosine) in parallel, then observed how much of the other EAAs it takes for phenylalanine oxidation to drop, as measured by the tracer. Since there is no storage of amino acids, the indicator is either incorporated into protein or oxidized. This method is non-invasive, doesn’t require a week of adaptation at a set protein level, and can be measured through breath and urine samples.
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