Dietary protein requirements and recommendations – why is it so complex?
The Paper of the Month for April is from Nutrition Research Reviews and is entitled ‘Dietary protein requirements and recommendations for healthy older adults: a critical narrative review of the scientific evidence' by Yusuke Nishimura, Grith Højfeldt, Leigh Breen, Inge Tetens and Lars Holm.
How much dietary protein should a healthy recreationally active 70+ year old woman eat daily? The simple question ought to be answerable by looking up a concise official nutritional recommendation – but it can’t! Why is that? This review discusses the reasons behind this confusion.
There is consensus on how the answer to ‘how much protein’ should be found. Overall, three approaches exist to estimate the daily requirements for dietary protein: 1) the amount of protein we ingest should replace the amount of protein we lose, 2) the rate at which our body proteins are broken down is balanced out by the rate our body builds new protein, and 3) our body’s protein mass should remain unaltered over time at any average amount of protein intake in our diet. The three approaches are evaluated by different methodologies. While the first and second are investigated in experimental settings across a few hours or from days to weeks, the third approach is evaluated differently. First, you can determine the body size and muscle mass of older adults and link this with their dietary habits, including how much protein they eat. Secondly, you can follow people for months or even years, monitoring what they eat and how their body composition changes. Finally, you can ask research volunteers to consume different protein containing diets and follow how their body adapts.
Despite the consensus of the three overall approaches, scientific disagreement about the dietary protein recommendations remains. This unresolved debate may be due to many complex factors around protein as a dietary source. There are 20 different, amino acids, the building blocks of protein. Different amino acids are required in varying amounts depending on the availability of other amino acids. Different dietary protein sources contain different amounts of each amino acid. Thus, some protein sources better satisfy our body’s requirements for amino acids than others (a basic principle of estimating a protein’s nutritional quality). In practice, this means that ingesting high amounts of proteins with insufficient content of certain amino acids will not satisfy needs and our body protein mass will not be fully supported. Another reason is that the requirements for dietary protein depend on the general diet, specifically how much energy you eat. If you do not eat enough energy, more protein will be used for energy metabolism rather than as building blocks for tissues such as muscle. In contrast, when you eat sufficient energy, you will waste dietary protein beyond what the body can use (i.e. more is not always better). These factors explain why a simple answer doesn’t exist and challenge a “one-amount-fits-all” recommendation for dietary protein intake guidelines.
Future scientific efforts to determine the protein needs of the healthy, active 70+ year old woman, should control for all these factors, apply measures according to several of the approaches, and interpret information with caution. Then we may be able to give an accurate and precise answer.