Protein

TL;DR. Protein is the only macronutrient you cannot store, so a supply has to keep arriving. Nine of the twenty amino acids are essential, meaning you must eat them, and the practical question is not whether a food is a "complete protein" but whether your whole day's eating covers all nine, which almost any varied diet does. The official minimum of 0.8 g per kilogram of body weight prevents deficiency; the amount that optimises muscle retention, especially in older people and people losing weight, is higher, around 1.2 to 1.6 g/kg. Protein is the most satiating macronutrient by a clear margin, which is the most practically useful thing about it.

Key takeaways

• There is no protein storage depot. Excess is burned or converted; shortfall is met by breaking down muscle.
• Nine essential amino acids must come from food. Leucine specifically triggers muscle protein synthesis.
• The RDA of 0.8 g/kg is a minimum to prevent deficiency, not an optimum. Older adults and people in a calorie deficit do measurably better at 1.2 to 1.6 g/kg.
• Protein is the most satiating macronutrient, and the thermic effect of processing it costs 20 to 30 percent of its own calories.
• "Complete protein" is a per-meal concern that turns out not to matter if the day's total is varied. Combining rice and beans at the same sitting is unnecessary.

What protein is

In short: Chains of amino acids folded into specific three-dimensional shapes; the shape is the function.

A protein is a chain of amino acids linked by peptide bonds and folded into a precise shape. There are twenty amino acids in the human genetic code, and the sequence determines the fold, and the fold determines what the protein does.

Proteins are not primarily a fuel. They are the working machinery:

You contain roughly 10 to 12 kg of protein as an adult, and you turn over about 250 to 300 grams of it every day, recycling most of the amino acids. Only about 40 to 60 g a day is actually lost and needs replacing, which is why the requirement is far lower than the turnover.

Essential and non-essential

Nine amino acids cannot be made by the human body and must be eaten:

Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine.

The mnemonic that works: PVT TIM HaLL (phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine, lysine).

Several more are conditionally essential: normally made by the body, but required from food during illness, injury, rapid growth, or prematurity. Arginine, cysteine, glutamine, glycine, proline, and tyrosine are in this group.

Leucine deserves separate mention because it is not just a building block. It acts as a signal: leucine activates the mTOR pathway, which switches on muscle protein synthesis. A meal needs a leucine threshold of roughly 2 to 3 g to trigger a robust synthesis response, which is one reason a small snack of protein does less than a proper serving, and why leucine content is the reason whey and animal proteins outperform some plant proteins gram for gram.

Protein quality, and why it matters less than the phrase suggests

In short: Quality scores measure the essential amino acid profile and digestibility; the practical implication is much weaker than the marketing.

Two scoring systems are in use.

PDCAAS (Protein Digestibility Corrected Amino Acid Score) is the older standard, capped at 1.0, which compresses the top of the scale so that several very different proteins all score a perfect 1.0.

DIAAS (Digestible Indispensable Amino Acid Score) is the newer FAO-recommended approach, measured at the end of the small intestine rather than in faeces, and not capped, so it discriminates better.

The pattern is real: animal proteins generally score higher because their amino acid profile more closely matches human requirements and they are more digestible.

The limiting amino acid concept explains most of the plant scores. Cereals (wheat, rice, maize) are low in lysine. Pulses (beans, lentils, peas) are low in methionine and cysteine. Each covers the other's gap, which is the biochemical basis for why nearly every traditional cuisine on Earth independently arrived at a grain-plus-pulse staple: rice and dal, beans and tortillas, hummus and pita, rice and beans, lentils and bread.

The complementary protein myth, corrected. Frances Moore Lappé's influential 1971 book Diet for a Small Planet argued you must combine complementary plant proteins at the same meal. She retracted that specific claim in later editions, and the retraction is correct. The body maintains a free amino acid pool and recycles a large quantity of protein daily, so amino acids eaten at breakfast are available to combine with those eaten at dinner. The American Dietetic Association and its successors have stated for decades that combining at each meal is unnecessary. Eating varied plant protein across the day is sufficient.

What remains true for plant-based eaters: total protein needs to be somewhat higher, perhaps 10 to 20 percent, to account for lower digestibility, and lysine deserves attention, which means including pulses regularly rather than living on grains and nuts.

How much protein do you actually need?

In short: 0.8 g/kg prevents deficiency; 1.2 to 1.6 g/kg is better for muscle preservation, ageing, and weight loss; above about 2 g/kg there is no further benefit for most people.

The RDA of 0.8 g per kg of body weight per day is derived from nitrogen balance studies and is defined as the amount sufficient for 97.5 percent of healthy adults to avoid losing body protein. It is a floor, not a target, and this distinction is routinely lost.

For an 80 kg adult, that spans 64 g/day at the RDA to 128 g/day at 1.6 g/kg. In food terms, 128 g is roughly a chicken breast, a tin of chickpeas, two eggs, a pot of Greek yoghurt, and the protein incidentally present in bread and vegetables. It is achievable without powders.

Anabolic resistance is the reason the recommendation rises with age, and it is one of the more important practical findings in nutrition of the last twenty years. Older muscle needs a larger dose of protein in a single sitting to trigger the same synthesis response: where a young adult may maximise the response at around 20 g of high-quality protein, an older adult may need 30 to 40 g. Combined with the fact that older people often eat least protein at breakfast, this matters for sarcopenia, the age-related loss of muscle that predicts falls, frailty, and loss of independence. Spreading protein across three meals rather than loading it all at dinner is the practical advice, and it has trial support.

Protein and satiety: the most useful practical fact

In short: Gram for gram, protein reduces subsequent eating more than carbohydrate or fat, and it costs more calories to process.

The thermic effect of food is the energy spent digesting and processing it:

So 100 kcal of protein nets about 75 usable kcal, against about 98 for fat. That is real and too small to build a diet on.

The satiety effect is larger. Controlled feeding studies consistently rank protein above carbohydrate above fat for suppressing subsequent intake, mediated by gut hormones (PYY, GLP-1, CCK) and by slower gastric emptying. In practice, high-protein diets tend to produce spontaneous calorie reduction: people eat less without being asked to. That is the main reason protein is useful for weight management, more than any thermic accounting.

The 2011 Australian "protein leverage" work by Simpson and Raubenheimer proposed a stronger version: that humans regulate protein intake in absolute terms and will keep eating until protein needs are met, so a diet diluted in protein (which describes much ultra-processed food) drives overconsumption of everything else. The hypothesis is contested and the experimental support is genuinely interesting.

Protein sources, compared

In short: Animal foods deliver more protein per calorie with better amino acid profiles; plant foods deliver protein with fibre and without saturated fat.

Two things stand out. Nuts and nut butters look protein-rich per 100 g and are not protein-dense per calorie, because they are mostly fat. And bread, pasta, rice, and potatoes contribute a substantial share of most people's protein simply because of how much of them gets eaten, which is why frank protein deficiency is very rare in any country with adequate calories.

Does high protein damage your kidneys?

In short: In healthy kidneys, no. In existing kidney disease, yes, and protein restriction is a standard part of treatment.

This is one of the most persistent myths and it has a real fact at its centre.

In people with normal kidney function, controlled studies of high-protein diets, including intakes above 2 g/kg maintained for months in resistance-trained people, show no adverse effect on glomerular filtration rate, albumin excretion, or other markers. The increase in GFR seen with higher protein is a normal adaptive response, the same way muscles enlarge with use, not a sign of damage. Major reviews and position stands from sports nutrition and nephrology bodies concur.

In people with chronic kidney disease, protein restriction genuinely slows progression and is a mainstream part of management, typically 0.6 to 0.8 g/kg under dietitian supervision. This is where the myth comes from: an appropriate treatment for a diseased organ has been generalised into a warning for healthy people.

Two genuine cautions do apply. Hydration: metabolising more protein produces more urea, which needs water to excrete, so higher intakes need more fluid. And existing kidney stones: high animal protein raises urinary calcium, uric acid, and oxalate, and lowers citrate, all of which favour stone formation in people who already form them.

Timing, distribution, and the supplement industry

In short: Total daily protein matters most; even distribution across meals is a real second-order effect; the "anabolic window" was oversold.

The anabolic window. For years, the standard advice was to consume protein within 30 to 60 minutes of training. Subsequent work, notably Brad Schoenfeld's meta-analyses, found the effect largely disappears once total daily protein is controlled for. If you have eaten protein within a few hours either side of training, the timing is not the limiting factor.

Distribution does appear to matter, modestly. Studies comparing an even split (roughly 30 g at each of three meals) against a skewed pattern (10 g, 15 g, 65 g) with identical totals find better muscle protein synthesis with the even split. The effect is small in young people and larger in older ones.

Before bed. Slow-digesting casein before sleep raises overnight synthesis and has reasonable trial support. It is a marginal gain, not a foundation.

Protein powders are food, conveniently packaged, and there is nothing wrong with them. Whey is a by-product of cheese making, fast-absorbing, and highest in leucine. Casein is slower. Soy, pea, and rice proteins work; blended plant proteins cover amino acid gaps better than single sources. The genuine cautions are that supplements are regulated far more loosely than food (Chapter 81), that independent testing has repeatedly found heavy metal contamination in some plant-based protein powders, and that most people meeting their needs from food gain nothing from adding powder.

BCAA supplements (branched-chain amino acids: leucine, isoleucine, valine) are one of the clearest cases of a supplement whose evidence does not support its sales. Taking three of the nine essential amino acids in isolation does not build muscle, because synthesis requires all of them; studies find BCAAs alone raise muscle protein synthesis far less than a complete protein containing the same leucine. If you are eating enough protein, they add nothing.

Deficiency, and the two forms of it

Frank protein deficiency is rare where calories are adequate, and severe where they are not.

Kwashiorkor presents with oedema (the swollen belly familiar from famine photographs), skin and hair changes, and an enlarged fatty liver, classically attributed to protein deficiency with adequate calories. Marasmus is total energy deficiency: extreme wasting without oedema. The clean protein-versus-energy distinction has been questioned for decades, and current understanding involves infection, micronutrient deficiency, aflatoxin exposure, and the gut microbiome as well.

The subtler and far more common problem in wealthy countries is sarcopenia: the progressive loss of muscle mass and strength with age, beginning in the fourth decade and accelerating after 60, driven by inactivity, anabolic resistance, and inadequate protein. It is a leading cause of falls, fractures, and loss of independence, and it is substantially preventable with resistance exercise plus adequate protein. That combination, not either alone, is what the trials support.

The bottom line

• Protein cannot be stored, so intake must be continuous. Nine amino acids are essential and leucine additionally acts as the switch that turns on muscle synthesis.
• The 0.8 g/kg RDA is a deficiency floor. 1.2 to 1.6 g/kg suits most people who care about body composition, ageing, or weight loss; athletes go higher.
• Protein quality differences between animal and plant sources are real and easily handled by variety. You do not need to combine complementary proteins within a single meal.
• Protein is the most satiating macronutrient and costs the most to process, which is why higher-protein diets tend to reduce total intake without conscious restriction.
• High protein does not damage healthy kidneys. It is restricted in existing kidney disease, which is where the myth comes from.

Sources and notes

Amino acid requirements, nitrogen balance, and the 0.8 g/kg RDA derivation follow the FAO/WHO/UNU expert consultation and the US Institute of Medicine Dietary Reference Intakes. PDCAAS and DIAAS scores follow FAO's 2013 report on dietary protein quality evaluation. The complementary protein correction follows Lappe's own revision in later editions of Diet for a Small Planet and successive American Dietetic Association position papers. Anabolic resistance and per-meal leucine thresholds follow Wolfe's and Phillips' work. Optimal intake for resistance training follows Morton et al., British Journal of Sports Medicine, 2018, which found benefit plateauing near 1.6 g/kg. Thermic effect and satiety ranking follow Halton and Hu's review. Protein leverage is Simpson and Raubenheimer, Obesity Reviews, 2005. The absence of kidney harm in healthy people follows Devries et al., Journal of Nutrition, 2018, and ISSN position stands. Sarcopenia prevention evidence follows the PROT-AGE and ESPEN expert group recommendations.

Open questions. Whether intakes above roughly 1.6 g/kg confer additional benefit in trained individuals is contested, and individual variation may be larger than the group means suggest. Long-term cardiovascular effects of sustained very high protein intake, and whether the source matters more than the amount, are not well characterised.

👉 Next: fats, where forty years of dietary advice went through a genuine and instructive reversal.