The Truth About Protein, Part 3: How Much Do Adults Really Need, and Where Should It Come From?

In this series, we’ve talked about essential amino acids, why adults over 40 need protein, and how protein and mTOR impact longevity (in case you missed them, here are the first and second articles). In this final article, we’ll dive into the not-so-accurate RDAs for protein, how much protein you really need to eat, and whether you should get it from animals, plants, or supplements. 

Protein RDAs: Where Did They Come From?

The recommended dietary allowance (RDA) for protein is set at 0.8 grams per kilogram of body weight (g/kg/day), equating to 68 daily grams for a 150-pound adult, for example. 

However, contrary to popular belief, the RDA—for protein or any nutrient—doesn’t reflect an adult’s ideal intake. Rather, the RDA represents the minimum intake needed to prevent malnutrition—not to grow new muscle with exercise, support bone health with age, or facilitate the most efficient protein turnover. 

The evaluation of protein needs actually started in the animal sciences field, as farmers in the early 1900s wanted to know how to get their animals to grow best. They looked at nitrogen balance, which translated into how we calculate human protein RDAs today. 

Essentially, nitrogen balance measures all of the nitrogen you’re eating (which comes from protein) and then captures how much nitrogen you lose—the difference between these would, in theory, show how much protein your body uses. However, nitrogen loss is difficult to fully grasp, as it can come out through the urine, stool, sweat, and hair—of which the latter two are hard to measure. Therefore, it’s thought that nitrogen balance underestimates protein requirements due to also underestimating the nitrogen “outs.” 

Not only that, but nitrogen balance studies also require people to eat controlled, experimental diets for several weeks. This allows the body to adapt to lower protein intakes, leading to a downregulation of certain protein-dependent processes and an underestimation of how much protein we actually need for optimal health—not just survival.

How Much Protein Do Adults Really Need?

It’s often touted that protein intake in developed countries is too high—you’ll especially see these claims thrown around from vegan or other plant-based proponents. But is it true? 

Although the RDA is set at 0.8 g/kg, we’ve seen that these recommendations may not be entirely accurate due to shortcomings in nitrogen balance studies. More recent research suggests that aiming for a protein intake of at least 1.2 to 1.6 g/kg/day would be ideal for achieving optimal health outcomes in adults—essentially doubling the current RDA. 

Other research from 2010 and 2015 aimed to nix the nitrogen balance method and find a new way of measuring protein requirements. Named the indicator amino acid oxidation technique, these research teams concluded that up to 1.2-1.3 g/kg/day is appropriate for healthy young men, older men, and older women. 

Whether you’re 16 or 60, the body needs to make nearly 300 grams of new protein per day, allowing for the replacement of every protein four times per year. This process—known as protein turnover—occurs no matter your age, but the efficiency of it decreases in older adults. According to protein expert Don Layman, Professor of Food Science and Human Nutrition at the University of Illinois Urbana-Champaign, doubling the protein RDA to 1.6 g/kg would allow older adults to respond the same way a teenager does in regards to muscle protein synthesis and protein turnover—essentially fighting back on age-related sarcopenia and muscle loss.

Highly active adults who resistance train are also in need of more protein. A comprehensive meta-analysis concluded that the average amount of protein needed to maximize lean body mass was 1.6 g/kg, with some people (like male bodybuilders or endurance athletes) requiring up to 2.2 g/kg. 

Should We Raise the Protein RDA?

As you can see, the new protein recommendations are still not fully in consensus. However, the most recent research lands around 1.2 g/kg, suggesting that increasing protein intake by up to two-thirds is warranted. But as one-third of adults over 65 are not even meeting the RDA of 0.8 g/kg/day, would it be pointless to raise it even further? 

Well, some people suggest that raising the recommended protein requirements may make people more likely to eat more. Why? Let’s say the RDA for you is 70 grams of protein. If you’re eating 50 grams daily, you might think, “Well, it’s close enough.” But if you’re supposed to be eating 120-140g of protein and falling so short, you might do something about it. 

However, as with almost everything in the nutrition world, this is not a black-and-white answer—as we can see with the varying RDA estimations from different researchers. Protein requirements can also change drastically with factors like age, illnesses, activity level, and body composition. Plus, low-protein enthusiasts have one thing right—people with underlying kidney or liver disease need to limit their protein intake. However, higher-protein diets in healthy people have not been shown to damage or place extra stress on these organs. 

Where Should Adults Get Their Protein From?

Moving on from the RDA, the amount of protein you get per day doesn’t even matter if your body can’t use it—or if it doesn’t contain the right amino acids. As we learned in the first article in this series, we have nine essential amino acids and six to eight conditionally essential amino acids (that’s a debate for another day). 

Although we need all of these essential amino acids, protein experts like Don Layman state that three of them are the most critical for muscle health and longevity—leucine, methionine, and lysine. Leucine is vital for muscle repair and influences lipid and energy metabolism, as it is a rare amino acid that activates the fatty acid oxidation pathway—the way we break down and use fat for energy. Dr. Layman states that unless you’re eating a meal with at least 30 grams of protein—which contains at least 2.5 grams of leucine—you’re in a catabolic state of muscle breakdown. 

Plant Protein Vs. Animal Protein Bioavailability 

The quality of a protein is determined by its bioavailability and amino acid profile. Speaking strictly in the nutritional sense (debates about ethics and environmental effects could be an entire book themselves), animal protein is best. Vegans may even have higher protein requirements than meat-eaters because plant proteins are typically inferior to animal proteins in both bioavailability and essential amino acid profiles. 

Isolated proteins (like whey protein isolate) typically have a bioavailability of 99% or more, while plant proteins (like beans, for example) might have a 50-70% bioavailability. Few plant-based protein isolates (like soy protein isolate powder) have a similar bioavailability to animal proteins, but soy-based foods like tofu will decrease in bioavailability. 

Protein bioavailability is often measured by the Protein Digestibility Corrected Amino Acid Score (PDCAAS). Whey protein isolate receives a perfect PDCAAS score of 1.0, while vegan protein powders like pea protein are pretty close, at 0.93. However, whole food-based plants are much lower—for example, black beans score at 0.75 and peanuts at 0.52, while whole animal proteins like beef, milk, and eggs score at 1.0.

Plants have lower PDCAAS scores because most do not contain all of the essential amino acids and may also contain anti-nutrients like phytates or tannins that prevent some of their protein from being fully digested and absorbed. Plant-based protein powders are often more bioavailable than the plant themselves because processing methods can remove some of these anti-nutrients. 

So, where should adults get their protein from? Well, from a bioavailability standpoint, animal proteins are best. According to Dr. Layman, a leucine-focused diet is recommended to prevent muscle breakdown. If you are unable to consume leucine-rich foods consistently—beef, chicken, turkey, sardines, tuna, pork, and Parmesan cheese—an amino acid beverage or supplement could be beneficial. If preventing muscle breakdown is your goal—whether due to exercise or increasing age—supplemental amino acids offer a rapid delivery system to the muscles. 

As discussed in the previous article in this series, you want high-quality protein in the right amounts—but not all day, every day. You also want to give yourself a break from eating protein (like short-term fasting is) to allow for mTOR inhibition and autophagy to take place and support longevity. 

The Bottom Line

Although the ideal RDA varies depending on which doctor or researcher you ask, many agree that 0.8 g/kg is too low—especially for aging adults. Most of the recommendations fall in the 1.2-1.6 g/kg range, suggesting that increasing protein intake by up to two-thirds or double is warranted. 

When it comes to protein sources, there’s no getting around the fact that we absorb and utilize animal proteins better—especially isolated versions, like whey protein isolate. However, if you are vegan, plant-forward, or simply don’t eat much meat, isolated plant protein powders (like pea protein) or leucine-rich amino acid supplements or drinks can help to bridge the gap and support muscle maintenance with age. 

Reviewed by: Heather L. Makar

References:

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