How does protein affect weight loss?
To lose more fat and retain more muscle on a weight-loss diet, protein quantity matters more than protein quality.
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).
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. 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, 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, perhaps because protein promotes muscle retention.
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.
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). 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, which is the same result as in the trial presented in the paragraph above.
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.
And while the four trials above had enrolled young and/or middle-age women, there’s evidence that postmenopausal women on a calorie-restricted diet also lose less muscle when they eat more protein.
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.
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.
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. 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.
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 and bone health. 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.