Are we too obsessed about protein?

First, we were told to cut back on fat. Then the spotlight shifted to carbs. Now, the latest nutritional obsession seems to be protein — and lots of it.

The rationale? Protein is essential for maintaining muscle mass and muscle is more than just for movement. It plays important roles in bone density, glucose control, fat metabolism, and perhaps even longevity itself. Meanwhile, the U.S. Recommended Dietary Allowance (RDA) for protein is increasingly criticized as outdated and insufficient.

So how much protein should we actually be eating each day? The short answer: most Americans eat enough protein already. The longer answer: protein intake higher than the RDA may provide small benefits in muscle mass or strength. But resistance training is far more important than protein.

Let’s start with the guidelines. The US, Canada, UN/WHO, and the European Food Safety Authority all recommend 0.8-0.83 g/kg of bodyweight of protein daily for healthy men and women ages 19 and up. I created the table below to give examples of what the RDA for protein looks like for different people:

The RDA for protein was part of the Dietary Reference Intakes published by the Institute of Medicine in 2005. It has been updated since then but the protein RDA has not changed. The scientists looked at a variety of methods for determining protein requirements—factorial, nitrogen balance, plasma amino acid response, direct amino acid oxidation, 24 hour amino acid balance, and indicator amino acid oxidation—and selected the nitrogen balance method.

Nitrogen balance

Protein is the only macronutrient that contains nitrogen, which makes it unique — and measurable. The nitrogen balance method takes advantage of this fact to estimate a person’s protein requirements.

Here’s how it works: if we bring healthy volunteers into a lab we can measure exactly how much nitrogen they consume (from dietary protein) and how much they excrete (through urine, feces, sweat, and even skin). If the amount of nitrogen consumed equals the amount excreted, the person is in nitrogen balance. That means their intake is just enough to replace the protein their body is breaking down through normal processes like cell turnover and tissue repair.

Let’s say I gave you enough protein to provide 20 grams of nitrogen per day. If you excrete 20 grams of nitrogen, your body is in balance — you're breaking down and building up protein at equal rates.

But what if I gave you 20 grams of nitrogen, and you only excreted 15 grams? That would suggest you're retaining nitrogen — your body is holding onto some of it, likely using it to build new tissue. This implies your previous intake may have been inadequate. On the flip side, if you excreted 25 grams, that would indicate you're breaking down more protein than you're consuming and the protein intake was excessive.

Having settled on the nitrogen balance method, the experts relied on a meta-analysis by Rand, et al in 2003 that pooled together 19 small nitrogen balance studies conducted across the globe with a total of 235 men and women aged 19-99 yrs old (most of the people were in their 20’s). These were healthy individuals assumed to be moderately active and in energy balance (not gaining or losing weight). These were not athletes trying to gain muscle or overweight individuals trying to lose fat. These volunteers lived in a lab for weeks (even months), were fed precise amounts of food so researchers knew the exact amount of nitrogen intake, and then had their urine, feces, sweat, and skin flakes collected to measure nitrogen excretion. And then the whole process was repeated with different protein amounts. Credit to the scientists and the volunteers who participated in these studies.

What did they learn? They calculated that the average man and woman studied was in nitrogen balance with 0.66 grams of protein per kg of bodyweight per day. They added 2 standard deviations above that average to catch 97.5% of people, so they arrived at 0.8 g/kg daily. Interestingly, for the average person, the RDA would actually be above their protein requirement (according to the nitrogen balance method).

Drawbacks of the nitrogen balance method

The Institute of Medicine acknowledged that the nitrogen balance method — like all the methods it evaluated — has important limitations. First, it's difficult to capture all nitrogen losses. While urinary and fecal nitrogen can be measured accurately, losses through skin and sweat are much harder to quantify. As a result, nitrogen excretion is often underestimated, which can make protein requirements appear lower than they truly are.

Some nitrogen balance studies found that at increasing protein intakes the volunteers continued to be in positive nitrogen balance (nitrogen intake exceeded nitrogen excretion). These volunteers seemed to hang on to the nitrogen and either use it or store it somewhere. These data points were assumed to be artifacts as the lean body mass (LBM) of these volunteers remained stable. Therefore, these data points were not accounted for in the calculation of the RDA. However, Wolfe and colleagues show data that higher rates of protein consumption often lead to increased LBM suggesting that the protein was being used to build tissue. In their opinion, the recommended daily allowance (RDA) should be changed to “recommended minimum intake” to reflect that higher protein intake can be beneficial.

Lastly, the statistical models used to calculate the RDA assumed a linear relationship between protein intake and nitrogen balance. But we now know this relationship is curvilinear. As a person approaches nitrogen balance, the body becomes less efficient at using additional protein. This plateau effect means that using a linear model may underestimate how much protein someone really needs to reach balance.

Alternative methods for measuring protein requirement

A newer method developed in the 1990’s was the indicator amino acid oxidation technique (IAAO). Unlike fat and carbs, excess protein cannot be stored - it either has to be used for building things or oxidized and converted to carbs. This is a point worth repeating: we cannot store excess protein and it can end up as carbs and then potentially fat.

Here’s how it works:

If just one essential amino acid is missing, your body can’t build new proteins.

So, it burns the other amino acids instead (oxidation).

When you supply that missing essential amino acid, your body stops burning the others and uses them to make protein.

When amino acid oxidation levels off, you've met your protein requirement.

This technique is much easier to perform than the nitrogen balance method. IAAO can be done in 1 day (instead of weeks) and amino acid oxidation can be measured in the breath or the urine done, making it much easier to test many different protein intakes in the same person.

What do IAAO studies show? Humayan et al in 2007 used this technique in a group of 8 healthy young men and calculated the RDA protein for protein as 1.2 g/kg daily. The same researchers published similar results in small groups of older women in (2014, 2015) and also in older men (2015). In 2023, Matsumoto et al reviewed all the IAAO studies evaluating protein requirements in humans and found similar results: the RDA for young adults was 0.98-1.2 g/kg daily; for older adults it was 1.15 to 1.29 g/kg daily.

Is there a way to reconcile the difference between these different methods?

Humayan et al also looked at the original 19 nitrogen balance studies used to determine the RDA and added 9 more recent nitrogen balance studies. They used a different statistical method, a biphase linear model, to account for the curvilinear relationship between protein intake and nitrogen balance (less efficient protein use as one approaches nitrogen balance). They calculated that the RDA for protein should be higher at 0.99 g/kg daily closing the gap between the nitrogen balance and IAAO methods.

What if both methods are flawed?

Layman and colleagues argue that both of these methods are conceptually flawed. These methods assume that when protein is broken down (nitrogen excreted or protein oxidized), that the remainder of the protein molecule is not useful. These methods focus only on efficient use of protein for growth. Layman and colleagues argue that even when oxidized the protein molecule has other important metabolic benefits, including substrates for messenger RNA translation, initiators of neurotransmission, enhanced satiety, and stimulation of nitric oxide for vascular regulation. They suggest that consuming protein above the requirements shown by nitrogen balance or IAAO studies allows for these additional metabolic benefits. They argue for additional research to define specific total daily protein intakes that affect health indexes and to consider outcome variables, such as lean body mass or muscle strength.

Clinical trials

Clinical trials may be able to provide an answer to that question. A practical way to determine if the RDA for protein is adequate is to give people that amount and see how it impacts an outcome such as lean body mass or strength.

Campbell and colleagues conducted a study of 10 healthy men and women aged 55-77 years and gave them 0.8 g / kg of bodyweight of protein daily for 14 weeks. Resistance training was not performed. They tracked a number of outcomes, including thigh muscle size as measured on CT scans. They found that the thigh muscle size decreased after 14 weeks, suggesting that the RDA for protein may not be adequate to completely meet the physiological needs of these older people.

However, Bhasin and colleagues conducted a larger RCT of 92 functionally limited older men over 6 months (average age was 73 years old). One group was given 0.8 g / kg and another group given 1.3 g / kg of protein daily. Resistance training was not performed. The higher protein group did not increase lean body mass, muscle strength or power, or physical function compared to the lower protein group.

Nahas and colleagues in Brazil conducted a 2 RCT’s of post-menopausal women comparing the 0.8 g/kg (the RDA) to 1.2 g/kg of protein daily. In this study, both groups performed resistance training 3 x per week for 10 weeks. In the first RCT of 23 women, there was no difference in lean body mass as measured by DEXA scan. In the second RCT of 47 women, there was a small increase in functional capacity (i.e., how fast they could walk 400 meters) but no increase in strength. It is possible that post-menopausal women require a higher increase protein intake than 1.2 g/kg, a longer period of resistance training, or perhaps even hormone replacement therapy - these questions could not be answered in this study.

I point out these RCT’s because these were the only ones I could find that compared the RDA to higher amounts. All 3 were in older individuals. What about a broader review of all the RCT’s in older adults?

In 2013, the PROT-AGE group - a international panel of experts - reviewed the evidence and published their recommendations for optimal dietary protein intake in older people. They concluded that healthy older adults consume 1.0-1.2 g/kg of protein daily. The evidence they reviewed to make this recommendation is summarized this table below.

As you can see, 2 of the 6 studies did not give a specific recommendation - simply that the RDA should be greater than 0.8 g/kg. The Volpe review states “RDA of protein is probably sufficient for most sedentary or low active adults” which sadly describes many older adults. The Paddon-Jones 2012 reference states that the RDA should “perhaps” be 1.0-1.3 g/kg and the Morley 2010 reference states that because a certain percentage of older people consume less than the RDA, it is “suggested that protein intake be increased.” These studies hardly give a compelling endorsement to increase the RDA to 1.0-1.2 g/kg daily.

In 2014, the European Society for Clinical Nutrition and Metabolism (ESPEN Expert Group) - another international panel of experts - published their recommendation for protein intake and exercise for optimal muscle function with aging. They also concluded that for healthy older people the diet should provide 1.0-1.2 g/kg of protein daily and also added a recommendation for daily physical activity or exercise for as long as possible.

When I looked at the references they cited for this recommendation, they included the PROT-AGE report above and the lukewarm Volpe review also cited above. They also cite a Nordic Nutrition Recommendation review that concluded the “evidence is assessed as suggestive to inconclusive regarding an optimal protein intake higher than the estimated RDA…but an exact level cannot be determined.” They also cite the Healthy Aging and Body Composition (ABC) study which was a prospective observational study looking at 2,000 adults aged 70-79 years old followed over 3 years. This study reported that adults who consumed the highest amount of protein had less loss of lean body mass compared to those consuming the lowest amount of protein. Unfortunately, protein intake was assessed once during the 3 year study using a food frequency questionnaire which are notoriously unreliable.

In 2022, another panel of experts, the Health Council of the Netherlands, published a systematic review of 18 RCT’s to evaluate the effect of increased protein intake on health outcomes in older adults. They reported that their review “provides insufficiently convincing data that increasing protein in older adults >0.8 g/kg elicits health benefits.” In other words, these experts could not find compelling evidence to recommend increasing the RDA in older adults to improve lean body mass or muscle strength.

What about protein middle-aged adults?

Nunes and colleagues al did a systematic review and meta-analysis of 74 randomized clinical trials (RCT’s) in healthy adults (age 19 and up) that evaluated the impact of protein intake on lean body mass and strength. Unfortunately, this analysis could not compare the RDA vs higher intakes because in 80% of the RCT’s, the baseline protein intake was at least 1.2 g/kg daily. They essentially compared 1.2 g/kg daily to higher amounts.

The table below summarizes the results of differing protein intakes on lean body mass (LBM) in those already doing resistance exercise:

Figure 2

The vertical dotted line represents zero change in LBM and any confidence interval that crosses zero is considered not statistically significant. As you can see, they did see that in the group consuming less than 1.2 g/kg daily (at the bottom), there was a trend towards decreased LBM but these confidence intervals also included zero.

In the highest group consuming 1.6g/kg protein daily(twice the RDA!), nearly all of the RCT’s had results in which the confidence interval also included zero. When they pooled all the studies together, the result was a slight increase in LBM that was statistically significant. How slight? Compared to the control group, the 1.6 g/kg protein group gained 1.5 pounds of additional LBM.

Does the increase in LBM translate to increase in strength? Not by much. When the same group looked at RCT’s assessing strength, there was a small increase in bench press and lower body strength in the higher protein group, but no difference in hand strength (considered an indicator for overall muscle function and longevity). Given the heterogeneity of the RCT’s and risk of bias, the authors report that despite being statistically significant, the evidence was of “low certainty.” In addition, the magnitude of the protein effect was small: “effects on bench press strength, handgrip strength, and improved performance in physical tests in healthy adults seem to be trivial.”

The results of this paper are consistent with a 2020 systematic review and meta-analysis of 105 RCT’s of protein supplementation in healthy adults by Tagawa and colleagues. The “low” protein group averaged 1.0 g/kg vs the high protein group averaged 1.5 g/kg protein daily. Protein supplementation resulted in a slight increase in LBM of 1.1 pounds.

Again in both these reviews, the high baseline protein intake may have resulted in smaller gains in LBM. But protein alone may not have a strong impact on muscle mass or strength. A better question might be, “Does adding protein while resistance training lead to more gains in muscle compared to resistance training alone?

Resistance training and protein supplementation

In 2017, Morton and colleagues did a systematic review and meta-analysis evaluating the effect of protein supplementation on muscle gains during resistance training. They analyzed 49 RCT’s in which both groups performed resistance training but one group received protein supplementation and one did not. Outcomes included muscle mass (i.e., fat free or lean body mass) and strength (i.e., one rep max). Unfortunately, this review was also not able compare the RDA to a higher amount: the control groups averaged 1.4 g/kg daily and the protein supplement groups averaged 1.8 g/kg daily.

This figure shows the effect of protein supplementation on strength (1 rep max):

As you can see, most of the confidence intervals hover around zero (only 3 studies did not). When pooled together, the results slightly favor protein supplementation to increase muscle strength with a greater benefit in trained individuals. You can see the magnitude of the effect was small: 5 pound increase in 1 rep max overall and a 9 pound increase in trained individuals.

What I found most interesting was that on average resistance training alone increased the 1 rep max by 59 pounds. Protein supplementation augmented that by an additional 5 pounds. My take home lesson: resistance training was a far more potent stimulus for muscle strength. Protein supplementation was icing on the cake.

This figure shows the effect of protein supplementation on fat free mass (similar to lean body mass):

Again, most of the studies had confidence intervals that crossed zero. When pooled together, the results slightly favored protein supplementation, especially in trained individuals. The magnitude of effect was small: 0.7 pound increase in fat free mass overall and 2.3 pound increase for trained individuals.

The authors reported protein supplementation beyond 1.6 g/kg daily during resistance training provided no further benefit on gains in muscle mass or strength.

The authors concluded that their “main finding was that dietary protein supplementation augmented resistance training induced increases in 1 rep max and fat free mass.” I like their wording: protein augments the benefits of resistance training. Resistance training being the foundation.

Resistance training is essential for older adults as well. Mertz et al conducted a RCT on the effect of daily protein supplementation with or without resistance training for 1 year on muscle size and strength in older men and women (average age = 70 years old). They randomly divided the 208 participants into various groups - the two groups to highlight are one with whey protein supplementation alone and one with whey protein supplementation plus supervised heavy resistance training 3 days a week. The control group consumed about 1.1 g/kg of protein daily (already above the RDA) while the whey supplement group consumed 1.5 g/kg. This is based on self-reported adherence to the whey protein supplements (self-reports can be unreliable).

At the end of 1 year, the researchers found no significant change in muscle mass or strength in the whey protein supplementation alone. In the whey protein plus supervised heavy resistance training for 1 year group, they observed an increase in muscle strength. Unexpectedly, they did not see a an increase in quadriceps size. They write that this group averaged 2 sessions per week over the year (instead of 3 as intended) and most participants took a 3-4 week vacation during the year that also decreased the number of workout sessions.

This study showed evidence that resistance training added to protein supplementation was the key. Mertz et al concluded that “these results provide strong evidence that an increase in protein intake alone does not add a benefit in preserving muscle mass or strength in healthy older adults living independently and eating in accordance with current guidelines.”

Summary

According to NHANES data (National Health and Nutrition Examination Survey), the average American consumes about 1.1-1.2 g/kg of protein daily - well above the RDA. Even if we apply new statistical methods to the old nitrogen balance data or use the newer IAAO methods to update the RDA, most Americans would still be consuming enough protein.

What if you are currently eating the RDA of 0.8 g/kg of protein daily? Would you benefit from eating more protein? Probably yes - especially if you are resistance training. While the data I reviewed are mixed, there appears to be a small benefit to increasing from 1.2-.1.4 g/kg to 1.8 g/kg. Therefore, it is plausible that increasing from 0.8 g/kg to 1.8 g/kg would have similar (or perhaps even more) benefit on muscle mass or strength. One review discussed above did suggest that increasing protein intake above 1.6 g/kg provided no meaningful gains.

I think rather than obsessing about protein, we should be obsessing about resistance training. As the research shows, that is where we can get our biggest return on investment when it comes to building and maintaining muscle mass and strength regardless of our age. Why resistance training? I’m reminded of the famous bank robber Willie Sutton who said, “Because that’s where the money is.”