We probably don’t have to spend too much time selling you on the idea that protein is important, so we won’t. The potential benefits of high-protein diets are multifactorial; protein facilitates recovery, supports hypertrophy, and enhances satiety to an extent. You’ll also commonly hear that protein has a higher thermic effect of feeding than carbohydrate and fat. This is true, but more of a “fringe benefit” than an important factor influencing diet design. The effects of dietary protein on energy expenditure are relatively negligible, but optimizing your protein intake can help with appetite management, glycemic control, recovery from exercise, and muscle hypertrophy.
All proteins are made up of a variety of amino acids. There are 20 naturally occurring amino acids, which can further be classified as “essential” or “nonessential.” We can make enough of the nonessential amino acids to get by without eating them, but we can’t synthesize the essential amino acids in sufficient amounts. Thus, we need to get adequate amounts of essential amino acids from our diet, which is why we call them “essential.” If you want to categorize things even further, branched-chain amino acids are a special subcategory of essential amino acids, and leucine is a particularly special branched-chain amino acid that is responsible for initiating muscle protein synthesis after a meal.
Building muscle ultimately boils down to net muscle protein balance; we see a transient increase in muscle protein synthesis following meals (blue areas in the image below), which is later followed by a reduction in synthesis and an increase in muscle protein breakdown (green areas in the image below) until the next meal with adequate leucine and essential amino acids. The more that synthesis exceeds breakdown, the more muscle we stand to gain. If we fail to take advantage of opportunities to stimulate muscle protein synthesis throughout the day, we may miss out on some gains. So, if we want to initiate muscle protein synthesis and ensure that we have all the building blocks available to create new muscle proteins and support hypertrophy, then we want to make sure our diet has at least a few meals throughout the day that provide plenty of leucine and plenty of essential amino acids.
Muscle protein synthesis (MPS; blue area) transiently increases after protein feeding; after a while, protein synthesis tapers off and muscle protein breakdown (MPB; green area) increases. (Image adapted from Jorn Trommelen's figure that appeared in this article on Stronger By Science.)
It’s important to recognize that not all protein sources are equivalent. There are numerous scales for rating “protein quality,” and while they all differ to an extent, the important characteristics dictating the quality of a protein source are pretty clear. On one end of the spectrum, a really high-quality protein will tend to have plenty of leucine per gram, plenty of essential amino acids per gram, a sufficient amount of each individual essential amino acid, and good digestibility. This is generally true of most animal-derived protein sources, such as dairy (including whey and casein), egg, meat, poultry, and seafood.
In comparison to animal-derived proteins, plant-based protein sources tend to have lower amounts of leucine and essential amino acids per gram of protein, poorer digestibility, and they generally lack a sufficient amount of at least one essential amino acid. This is unfavorable in theory, not only because of leucine’s role in stimulating protein synthesis, but also because we need all of the essential amino acids in sufficient amounts in order to synthesize new muscle protein to the maximal degree. In addition, our muscles can only use the amino acids that reach them, so poor digestibility can be a bit of a problem. Unfortunately, the potential issues don’t stop there. A 2019 study compared three different plant-based protein blends to whey protein isolate, while taking great care to ensure that all four protein supplements had the exact same leucine content (2.6g), protein digestibility corrected amino acid score (1.0), and total essential amino acid content (12g). Despite all the effort dedicated to making these treatments as equivalent as possible, the whey protein isolate still caused substantially larger increases in blood leucine and blood amino acid levels during the four hours following ingestion, which would theoretically be more favorable for muscle-building purposes.
There are quite a few studies, as reviewed by Van Vliet et al, showing that plant-based proteins generally induce smaller increases in muscle protein synthesis than animal-based proteins when we look at a single dose or meal. As such, one might reasonably expect that plant-based proteins are inferior for hypertrophy, but it’s not quite that simple. In studies that compare supplementation with a plant-based versus an animal-based protein supplement, a recent meta-analysis reported that effects on lean mass gains are pretty similar. You could argue that the results leaned slightly in favor of animal-based protein supplements, but it only amounted to a difference of less than half a kilogram of lean mass, which was not significantly different from zero. When taking a more comprehensive view of entire diets, a recent study suggests that vegan diets and omnivorous diets can support hypertrophy to a similar degree, as long as that vegan diet provides plenty of protein that is effectively distributed throughout the day.
Incorporating Protein Into a Diet
The “standard” protein recommendation within the evidence-based fitness community is to aim for 1.6-2.2g/kg of protein per day. This is based on sound evidence, but also assumes that you’re lifting weights, intending to maximize muscle growth (or retention), are not in a sizable caloric deficit, and have a fairly average body-fat percentage (let’s say roughly 10-20% for males, or roughly 18-28% for females). So, if you’re a lifter and the previous description applies to you, then a good recommendation would be to aim for between 3-6 decently sized servings of protein per day, with each yielding at least 0.3g/kg of total protein and at least 2-3g of leucine, with a total daily protein intake of 1.6-2.2g/kg. If your body-fat percentage is substantially lower or higher than the ranges mentioned previously, then scaling your protein intake relative to total body mass probably isn’t the best bet; you might be better off aiming for somewhere in the range of 2-2.75g/kg of fat-free mass instead.
For lifters, a very general guideline would be to aim for around 1.6-2.2g/kg of protein per day, but this recommendation should vary based on your body composition, goal, energy intake, and exercise habits. Image credit: Morton et al, 2018 https://pubmed.ncbi.nlm.nih.gov/28698222/
If you exercise regularly but don’t perform resistance training, then your protein needs are a little lower, and if you don’t perform any type of regular exercise, then your protein needs are even lower, although individuals in these situations may still opt for high protein intakes to facilitate fullness and satiety. If you’re in a calorie deficit then your protein needs might be increased, and this is especially true if you’re in a particularly large deficit or you’re already quite lean and aiming to get even leaner.
So, advisable protein intakes could range from as low as 1.25g/kg of fat-free mass (sedentary, weight-stable person with moderate-to-high body-fat percentage and a preference for lower protein intake) to up around 3.1g/kg of fat-free mass (highly active, very lean lifter with a preference for higher protein intake in a notable calorie deficit). There are some medical conditions in which some degree of protein restriction is warranted, but this range represents the extreme ends of the spectrum for healthy individuals, so the majority of people fall somewhere in the middle. In order to account for all of the important factors that influence optimal protein intake, MacroFactor makes individualized protein recommendations based on your body composition, goal, energy intake, and exercise habits.