How to choose protein powder: how much you need, and whey or soy
4 min read
Start strength training, and the question "should I take protein powder?" comes up right away.
QUIZAdding protein supplements to strength training: how much extra muscle does it add on average? A. About 0.3 kg B. About 3 kg C. About 10 kgShow answer
A. About 0.3 kg.
That's the average difference in a study pooling 49 trials (1,863 people). Supplementing does something, but think of it as a small boost, not a substitute for training.
Protein powder isn't medicine or a special food. It's protein in powdered form. So the first thing to think about isn't "which protein powder is best," but whether you're already getting enough from food.
Contents
With strength training, it works. But the effect is small
A meta-analysis of 49 randomized controlled trials (1,863 people) that added protein supplements to at least six weeks of strength training found that supplementing led to greater gains in strength and fat-free mass (the weight of muscle and everything other than fat) [1].
But the difference wasn't big. The difference in fat-free mass averaged about 0.3 kg, and in one-rep max on lifts like the bench press, about 2.5 kg. Better to think of protein powder as something that gives the effects of training a small boost, not a substitute for training.
How much do you need?
In the same meta-analysis, once total daily protein went beyond about 1.6 g per kg of body weight, more protein barely added any fat-free mass [1]. At 60 kg body weight, about 100 g a day is one guide to the upper limit.
Meanwhile, Japan's Dietary Reference Intakes (2025 edition) set the recommended protein intake for ages 18–64 at 65 g for men and 50 g for women [2]. That's an amount for staying healthy, not for building muscle.
If you regularly eat meat, fish, eggs, soy foods, and dairy, food often covers the recommended amount. The order to think in is: if you lift and food alone doesn't reach 1.6 g/kg, make up the shortfall with protein powder.
POINT
Protein powder isn't "drink it and you'll build muscle." It's "something to make up what your meals are missing."
Whey or soy?
Protein powders include milk-based whey and casein, and soy-based protein. A meta-analysis of nine studies comparing effects when combined with strength training found no difference in gains in strength or fat-free mass between soy and animal proteins such as whey [3].
- If milk tends to upset your stomach, or you prefer plant-based → soy
- If you choose by taste, how well it mixes, or price → go with what you like
What to check on the label
| What to look at | Why |
|---|---|
| Protein per serving | Amounts per scoop vary by product. Compare with how much you're short |
| Sugar and calories | Flavored ones can be high in sugar. Check if you don't want to gain weight |
| Ingredients | Whey, soy, etc. If you have a milk or soy allergy, always check |
| Size and price | Try a small size first to test the taste and how your body reacts |
Who should be careful
If you have kidney disease, or a doctor has told you to limit protein, don't add protein powder on your own judgment; talk to your doctor. The same goes if you're pregnant, or take medication for a chronic condition.
Summary
- Combined with strength training, protein supplements make strength and muscle a little easier to gain (the effect is small)
- Beyond about 1.6 g per kg of body weight, more protein barely added muscle
- The recommended amount for health is 65 g for adult men and 50 g for women (Japan's Dietary Reference Intakes, 2025). First check whether your meals cover it
- No difference in strength-training results was seen between whey and soy
As I wrote in "I cut back on strength training, and somehow got more stamina," building bigger muscles and having energy for the day aren't the same thing. Decide what you're taking it for, then choose.
If you choose by these guidelines
References
1. Morton RW, Murphy KT, McKellar SR, et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine, 52(6), 376–384.
2. Ministry of Health, Labour and Welfare (Japan). Dietary Reference Intakes for Japanese, 2025 edition (in Japanese).
3. Messina M, Lynch H, Dickinson JM, Reed KE. (2018). No difference between the effects of supplementing with soy protein versus animal protein on gains in muscle mass and strength in response to resistance exercise. International Journal of Sport Nutrition and Exercise Metabolism, 28(6), 674–685.