Protein (amino acids) is the only macro-nutrient that can build and help retain muscle when losing fat. Yes, carbohydrates have a protein sparing effect, but if your protein intake isn’t sufficient, carbs won’t be able to spare your muscle during fat loss. And as carbs have to be reduced, due to calorie restriction during a cut, protein is the most important to retain muscle during fat loss.
Protein does also for a fact provide the most satiety of all the macronutrients.
A meta-analysis by Clifton et al (1) found that a 5% or greater protein intake difference between diets at 12 months was associated with a threefold greater effect size for fat loss. Also, a study by Weigle et al (2) showed that in ad libitum conditions, increasing protein intake from 15 to 30% of total energy resulted in a spontaneous drop in energy/calorie intake by 441 kcal/day. This led to a body weight decrease of 4.9 kg in 12 weeks. (3) This just shows how great protein can be for fat loss, due to increasing satiety. So if you want to lose weight, be sure to eat more protein.
Of the three macronutrients, protein has the highest thermic effect and is the most metabolically expensive. (3) As reported by Jéquier (4), the thermic effect of protein (expressed as a percentage of energy content) is 25–30%, carbohydrate is 6–8%, and fat is 2–3%. Given this, it is not surprising that higher protein intakes have been seen to preserve resting energy expenditure while dieting. Meaning, during a caloric deficit, the body reduces energy expenditure (metabolic slow down / adaptive thermogenesis) to try to maintain current body weight. Eating more protein will increase energy expenditure and will prevent the decline in energy expenditure during fat loss.
The thermic effect even varies between different types of protein, for e.g. whey had a higher thermic effect than casein, which had a higher thermic effect than soy protein. This is probably because whey is a fast acting protein and gets absorbed very rapidly in the intestines.
Importantly, the thermic effect of each macronutrient can vary within and across individuals. However, protein still remains most thermic.
Protein is also the least fattening macro-nutrient. Antonio et al (5) did a 1-year crossover trial using resistance-trained subjects, comparing protein intakes of 3.3 vs. 2.5 g/kg. There were no differences in body composition (importantly, no significant increase in fat mass), despite a significantly higher caloric intake in the higher protein group (an increase of 450 vs. 81 kcal above baseline).
All in all, protein has the highest thermic effect of feeding. It increases non-exercise activity thermogenesis (NEAT), increases thermic effect of exercise (TEE), increases fecal energy excretion, reduces intake of the other macronutrients, due to increased satiety, suppresses hepatic lipogenesis and has a lean mass preserving effect, which may be amplified in trained subjects undergoing progressive resistance exercise. (3)
However, the required intake of protein varies from individual to individual and some people might be sensitive to high protein intake, although it has been reported that protein intake at 4.4g/kg long-term had no adverse effects on a comprehensive list of measured clinical markers, including a complete metabolic panel and blood lipid profile.
If a high protein diet makes you feel sluggish, fatigued and just not in a good state of health, first take into consideration that the meat that you are eating might contain too much fat. Hence, it will be good to first try eating more lean meats and cutting down a bit on your fat intake to see how your body responds to this for at least a week or two. But if you still don’t feel any better, just cut back on protein, and replace it with carbs. If at a later time you get an appetite for more protein then do so. The body’s need for macro-nutrients can actually change over time, so it’s best to listen to your body and what it needs for the present.
What is the optimal amount of protein to retain muscle?
Recent position stands from the Academy of Nutrition and Dietetics, Dietitians of Canada, American Colleges of Sports Medicine, and the International Society of Sports Nutrition recommend a protein intake of 1.2–2.0g/kg/BW/day (body weight per day) to support metabolic adaptations and repair and remodeling of skeletal muscle tissues in healthy physically active adults. Morton et al. have demonstrated that a dietary protein intake of ~1.6g/kg/BW/day is maximal to support resistance-exercise-induced strength and lean mass gain in healthy adults in energy balance. (6) This amount of protein is enough to support metabolism and growth when eating maintenance or a surplus of calories.
When dieting (losing fat), the need for protein increases, but this all depends on the amount of lean body mass that you have.
Studies show that 2.3g/kg is superior to prevent muscle loss, however, the risk of muscle loss only becomes significant the leaner you get, as well as with the greater amount of muscle you have. (7) A higher dietary protein intake (3.0g/kg/BW) may also strengthen the immunity of individuals engaging in intense training bouts. The harder you train (intensity, frequency, duration, etc…), the more protein your body will need in order to sustain itself.
However, this needs to be individually tailored for each person. You might need a bit more or maybe a bit less protein. As a recommendation: start off with 2.2g/kg/BW/day for a few weeks, and then increase it by 20-50g for a few weeks and see how you feel. If you feel better with more protein, stick to it, but if you feel worse then rather lower it back down. Then try lowering it even more than 2.2g/kg/BW/day. Replace the calories with carbs and see how you feel. Signs to look for is a better pump, more strength and endurance in the gym. Don’t be afraid to experiment with what works for you.
Don’t start with very high protein just because someone told you to do so, because when you eat too much protein (required for your individual needs), there will be fewer calories left for other important macros with nutrients such as carbs. You might also not have enough energy to perform a good enough workout, and your strength and exercise performance will suffer and you might lose muscle mass as a result, because the body needs that anabolic stimulus along with protein in order to build and maintain. The ability to maintain strength and keep exercise performance high, is what will give the anabolic stimulus and resultant in muscle retention. Remember, protein alone cannot retain your muscle if you don’t exercise and give your body a reason to keep the muscle. If you don’t use it, you’ll lose it.
Loss of strength and bad workouts = muscle loss. But don’t go and beat your muscles to death, hoping to make them grow that way or retain as much as possible, because that would be stressful on the body and will be more catabolic than anabolic.
Also just remember that 2g of protein/kg/BW/day doesn’t mean if you weigh 80/90kg that you should be eating 116-180g of meat, but that you should be consuming 116-180g of protein, which is two different things.