Losing weight is seldom the goal—losing fat is, while preserving muscle. Unfortunately, your body is reluctant to give up its reserves of energy (its fat) and all too quick to metabolize your hard-earned muscle when the caloric deficit gets just a little too high.

Fortunately, what you eat while on a calorie-restricted diet can affect where the weight loss comes from. Diets high in protein, in particular, have benefits with regard to energy metabolism, appetite, overall caloric intake, and muscle retention.

Appetite and energy metabolism

Protein reduces appetite (and thus food intake) by increasing the levels of certain peptides, such as glucagon-like peptide 1 (GLP-1).^[1]

In addition, your body needs to expend 20–30% of the caloric value of protein to metabolize and store it, compared to 5–10% for carbs and just 0–3% for fat.^[2][3][4] In other words, the thermic effect of food (TEF) is highest for protein. Whether this makes a practical difference in human beings outside of a clinical setting is a much-debated topic,^[5] but diets high in protein have also been shown to mitigate the decline in resting energy expenditure (the calories the body burns at rest) caused by calorie-restricted diets,^[6] perhaps because protein promotes muscle retention.^[7][8]

A trial randomized 130 overweight people between a high-protein group and a low-protein group. Each day, the high-protein group consumed 1.6 grams of protein per kilogram of body weight (1.6 g/kg, so 0.73 g/lb) and the low-protein group 0.8 g/kg (0.36 g/lb). The trial’s 500 kcal daily deficit led to a weight loss of 9.9–11.2% over a year with no difference between groups, but the high-protein group lost more fat (14.3%  ± 11.8%) than the low-protein group (9.3% ± 11.1%), and men more fat than women.^[9]

The higher protein intake still benefited women, though, as it did in other trials. When a 10-week trial enrolled 11 obese women to compare a high-protein diet (30% of daily calories as protein) with a high-carb diet (55% of daily calories as carbs), the high-protein diet led to greater weight loss (4.4 kg, so 9.7 lb), most of which was fat (3.7 kg, so 8.2 lb).^[10] Relatedly, in a 12-week trial with 100 overweight or obese women, the high-protein and high-carb groups lost the same weight, but the high-protein group lost more fat,^[11] which is the same result as in the trial presented in the paragraph above.^[9]

Of course, adding exercise can help. In a 16-week trial, 90 overweight or obese women followed the same exercise regimen (aerobic exercise and resistance training) and were randomized to three groups: high protein, high dairy; adequate protein, medium dairy; and adequate protein, low dairy. The first group experienced greater fat loss and muscle gains.^[12]

And while the four trials above had enrolled young and/or middle-age women,^{[9][10][11][12]} there’s evidence that postmenopausal women on a calorie-restricted diet also lose less muscle when they eat more protein.^[13]

In a 20-week trial, 24 obese postmenopausal women took either 15% or 30% of their daily calories as protein. The 30% group lost less weight (8.4 vs. 11.4 kg, so 18.5 vs. 25.1 lb) but nearly as much fat (7.0 vs. 7.1 kg, so 15.4 vs. 15.7 lb), suggesting greater muscle retention.^[14]

Similarly, in a 6-month trial, 31 overweight or obese postmenopausal women (aged 65.2 ± 4.6) were prescribed a diet limited to 1,400 kcal (15% as protein, 65% as carbs, 30% as fat), with one group also taking 50 grams of whey protein twice a day and the other 50 grams of carbs twice a day. The protein group lost more weight (−8.0% ± 6.2%) than the carb group (−4.1% ± 3.6%). The protein group lost more muscle relative to their weight loss, but less muscle relative to their fat loss.^[15]

By eating more protein when on a weight-loss diet, you can lose more fat and less muscle.

What is the effect on blood lipids?

In a 12-week weight-loss trial, 100 obese people were randomized to high daily protein (2.2 g/kg, so 1.0 g/lb) or low daily protein (1.1 g/kg, so 0.5 g/lb). Both groups lost the same weight, but here again, the high-protein group lost more fat, and it also experienced reductions in LDL-C and total cholesterol not observed in the low-protein group.^[8] In another 12-week weight-loss trial with 215 overweight or obese people, a high-protein diet led to greater reductions in fat, total cholesterol, and triglycerides.^[7]

Does the kind of protein matter?

Trials usually control for protein quantity rather than quality, but studies show that proteins with a higher percentage of essential amino acids (EAAs) tend to better correlate with fat loss^[16] and bone health.^[17] Most EAA-rich proteins are animal proteins: meat, fish, eggs, and dairy. The protein in milk is 80% casein and 20% whey protein.

In general, animal proteins are richer in essential amino acids than plant proteins, and so are better for fat loss, but the advantage is small. For fat loss, protein quantity trumps protein quality.

For more information, see our Optimal Protein Intake Guide and Protein Intake Calculator.

Bone health

The acid-ash hypothesis states the following: The metabolism of certain foods — namely protein and grains — increases acid production in the body, as evidenced by an increase in urinary acidity.^[23] To counteract this increase in acidity, bone is broken down to release calcium bicarbonate (a base) corresponding with an increase in urinary calcium excretion, which is thought to reflect negative body calcium balance or bone loss. Therefore, a high-protein or acid-producing diet accelerates bone loss and increases the risk of osteoporosis.

However, changes in urine pH don’t necessarily reflect changes in blood pH, which is maintained within a narrow range primarily by the renal and pulmonary systems in healthy people.^[24] Additionally, variations in diet have virtually no effect on blood pH, as any nutritional influence that slightly disrupts acid-base balance is immediately corrected by biochemical buffering systems that do not involve bone.^[24]

While an increase in urinary acidity has been correlated with an increase in urinary calcium excretion, dietary changes that increase urinary acidity do not lower body calcium balance.^[25] Relatedly, a higher-protein diet does not negatively affect dietary calcium retention because although it increases urinary calcium excretion, it increases intestinal calcium absorption by a similar magnitude.^[26][27]

Ultimately, the available evidence does not support the acid-ash hypothesis,^[28] and in accordance, higher-protein diets do not have a negative effect on bone health.

Meta-analyses of prospective cohort studies have reported that a higher-protein diet was either not associated with the risk of hip fracture,^[29] or, compared to the group with the lowest protein intake, there was an 11%–16% lower risk of hip fracture in the group with the highest protein intake.^[30][31][32]

With respect to bone mineral density (BMD), a meta-analysis of randomized controlled trials 12–24 months long reported that a higher-protein diet had a protective effect on lumbar spine BMD.^[33] A higher-protein diet also tended to have a protective effect on total hip BMD, although this finding was not statistically significant. The data from prospective cohort studies is mixed on whether higher-protein diets have a protective effect on BMD (some studies suggest a benefit with more protein, while others have reported no impact), but there is a lack of evidence indicating that higher-protein diets are associated with lower BMD.^[31][33]

Concerning protein intakes significantly greater than the Recommended Dietary Allowance (RDA), there are a couple of long-term prospective cohort studies in older adults that shed light on the topic. In one four-year study that included older men and women (average age of 75), the quartile with the highest protein intake (1.24–2.78 grams of protein per kg of body weight per day) showed the least BMD loss at the femur and lumbar spine.^[34] Compared to the quartile with the highest protein intake, the quartiles with the lowest (0.21–0.71 g/kg/day) and second-lowest (0.72–0.96 g/kg/day) protein intakes experienced a significant reduction in femoral neck BMD.

In a separate five-year cohort study that only included older women (average age of 75), a higher protein intake was associated with greater whole-body bone mineral content, and the tertile with the highest protein intake (about 1.6 g/kg/d) had significantly higher whole-body and appendicular bone mineral content than the tertiles with lower intakes.^[35]

Kidney health

The idea that a high-protein diet puts undue stress on the kidneys stems from early research in rodents and dogs that reported increased urea excretion, renal blood flow, glomerular filtration rate (GFR; a marker of kidney function), and kidney size in animals fed a high-protein diet.^[36] From this data, it was determined that a high-protein diet increases the workload of the kidneys, and thus may damage the kidneys over time and increase the risk of chronic kidney disease (CKD).

A 2018 meta-analysis of randomized controlled trials that compared the effects of a high-protein diet (1.8 grams of protein per kilogram of body weight per day, on average) to a low-protein diet (0.93 g/kg/d) in healthy adults reported that higher protein intakes may slightly increase GFR.^[37] Other data indicates that a high-protein diet does not adversely affect blood markers of kidney function or blood pressure.^[38][39]

Given these findings, a high-protein diet does not appear to pose a serious threat to kidney health. In further support of this conclusion, the issue at hand can be viewed through a different lens altogether; that is, is an increase in GFR a risk factor for CKD in healthy people? Such a relationship has yet to be clearly established.^[37]

In fact, an increase in GFR in response to an increase in solute load (e.g., nitrogen from protein) is a normal adaptive mechanism.^[40] For example, GFR can increase by as much as 65% during pregnancy^[41] but does not increase the risk of CKD.^[42] Also, surgical removal of a kidney is not associated with a deterioration in kidney function in the long term (> 20 years), despite the increase in workload.^[43][44][45]

While a low-protein diet is recommended for people with CKD to help prevent disease progression,^[46] this does not mean that a high-protein diet is harmful in all cases. The available evidence suggests that, in healthy people, a high-protein diet does not adversely affect kidney function or increase the risk of CKD.

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Nutrition sure can seem complicated. You already had to worry about getting enough protein in your diet, should you now make sure you only get quality proteins?

And what makes a “quality protein” anyway?

Different scales have been developed to answer this last question. They rate and rank proteins based on two criteria: bioavailability and amino acid profile.

Bioavailability

Biological Value (BV), Net Protein Utilisation (NPU), and Nitrogen Balance (NB) rate proteins based on nitrogen measurements. They measure how much nitrogen people excreted, calculate how much protein that represents, and compare this number to how much protein was ingested. In such a way, they determine the protein’s bioavailability.

All three scales are based on two assumptions, both of which have been challenged: first, that dietary protein is the body’s sole source of nitrogen; second, that all nonexcreted protein has been used to make bodily proteins. In truth, some of the protein we ingest can be converted to glucose, especially if the protein’s digestion is fast and the body’s glycogen stores are low, and some can be fermented by our microbiota,^[47] especially if the protein’s digestion is slow.

The BV scale is still in use today, though mostly in promotional material and in the media, and so it needed mentioning despite being outdated. The current official scale, used notably by the FDA, is the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which takes into account not just the bioavailability of a protein but also its amino acid profile.

A protein is considered highly bioavailable if it’s easy to digest, absorb, and (after conversion into its constituent amino acids) make into other proteins. Some protein rating scales, such as the BV scale, rank proteins based solely on bioavailability.

Amino acid profile

Proteins are composed of amino acids, some of which your body can synthesize and others not. The nine you need yet cannot synthesize, and thus need to ingest, are called essential amino acids (EAAs). Among those, branched-chain amino acids (BCAAs) are crucial to your muscles,^[48] with leucine being especially anabolic.

mg/kg/day = daily requirement in milligrams (of a given amino acid) per kilogram (of body weight) per day
Complete/Milk/Pea/Rice/Soy/Whey = milligrams of amino acid per gram of complete/milk/pea/rice/soy/whey protein (mg/g)
References: World Health Organization. Protein and Amino Acid Requirements in Human Nutrition, page 245, table 49. 2007.^[49] Kalman. Foods. 2014.^[50] Gorissen et al. Amino Acids. 2018.^[51] USDA Food Composition Databases (accessed: 2018 Sep)

A protein is called complete when, proportionally to its overall amino-acid content, it has enough of each EAA. The main advantage of animal proteins is that most are complete.

Most, but not all. Take beef protein powders: you might assume they’re made from meat, which is to say from the animal’s muscles, when most are actually made from collagen boiled from the animal’s skin, bones, and other connective tissues. Now, dietary collagen is far from useless; it’s been shown to promote skin and joint health, and it probably promotes bone health too; but it isn’t a complete protein.^[52] Rich in glycine and proline but poor in BCAAs, it isn’t a good primary source of protein, and is probably not the best muscle builder (though it has shown benefit in elderly women on a low-protein diet^[53] and in elderly men^[54]).

Conversely, most plant proteins are incomplete, but according to the table above, the proteins in soy, pea, and rice are nearly complete: rice is relatively poor in lysine; soy and pea, in methionine. Of course, incomplete proteins can complement one another — this is true of proteins from foods as well as from supplements. The amino acid profile of a 70:30 pea:rice protein blend is similar to that of whey.

Alas, soy and pea protein powders are usually very high in salt. Salt is used in the process that makes soy and pea protein powders, and it cannot all be washed away. Check the label of your soy or pea protein powder to ensure you don’t end up exceeding your tolerable upper intake of salt (sodium) for the day: 2.3 g for most adults.

Although pea and rice are gaining in popularity, alone and in combination, soy is still the most popular vegan source of protein powder. On the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) scale, soy protein isolates scores 0.97 (97%), so at first glance it appears to be the virtual equals of any animal protein. However, this is because the PDCAAS scale truncates any number superior to 1 (100%), with the rationale that any amount of amino acids beyond the requirement pattern confers no additional benefit to the individual consuming the protein. Otherwise, whey protein concentrate could score a more-than-perfect 1.07 (107%).^[55]

A dietary protein is considered complete if it contains enough of each of the amino acids your body needs yet cannot synthesize. Some protein rating scales, such as the PDCAAS scale, rank proteins based not just on bioavailability but also on amino acid profile.

The real world

The PDCAAS scale has displaced the BV scale, not because it is more accurate, but because it is more pertinent: it rates protein not only based on bioavailability but also on amino acid profile, so as to better reflect human needs. Better, but not perfectly, which is why the Food and Agriculture Organization has already proposed to replace it with yet another scale: the Digestible Indispensable Amino Acid Score (DIAAS).

The two scales differ notably by where the sample is taken. To determine the PDCAAS of a protein, you analyze the feces. To determine the DIAAS of a protein, you analyze the content of the ileum. In other words, PDCAAS looks at how much protein was absorbed after it has gone through your small and large intestines, whereas DIAAS looks at how much protein was absorbed after it has left the small intestine.

The food you ingest passes through your small intestine before passing through your large intestine, where most of your gut’s microbiome (the bacteria inhabiting your gut) can be found. Since this microbiome can use amino acids and peptides that you yourselves never absorbed, PDCAAS may overestimate a protein’s bioavailability.^[55] DIAAS overcomes this issue.

DIAAS = Digestible Indispensable Amino Acid Score | PDCAAS = Protein Digestibility-Corrected Amino Acid Score
Reference: Mathai et al. Br J Nutr. 2017.^[55]

The DIAAS scale, however, doesn’t address the main problem of all protein rating scales, which is that, even when they take into account bioavailability, amino acid profile, and ileal digestibility, and even if they took into account gluconeogenesis, digestion speed, and the effects of exercise on endogenous protein (i.e., muscles), they would still fail to reflect real life.

Trials need to eliminate confusing factors, so a subject is fed only one kind of protein, and that on an empty stomach. But when do you eat only one kind of protein over a whole day? And, except at breakfast maybe, when is your stomach really empty? Other food components, such as fiber and anti-nutrients (trypsin inhibitors, tannins, etc.), can all affect how much of your protein you absorb.

Rating each protein in isolation fails to reflect real life. You don’t need each protein you ingest to be complete; in a balanced diet, incomplete proteins, rich and poor in different amino acids, can complement one another (especially when eaten at about the same time). But a protein’s rating can be one of the criteria you consider when selecting a protein powder (alongside price, mixability, digestion speed, etc.).