How to Take Soy Protein: Dosage, Timing, Duration
Soy protein is an ordinary dietary protein, so an 'intake regimen' here means not a course but the competent incorporation of the powder into the diet. The editorial team has gathered what is known from the positions of sports associations and studies of muscle protein synthesis: how much protein is needed per day, how much to put in one serving, when to drink it and whether it makes sense to take breaks.
Where to start: the daily protein requirement
Protein powder does not exist separately from the plate. Before thinking about scoops, it is worth assessing how much protein a person already gets from food. For people who train regularly, the International Society of Sports Nutrition (ISSN) in its 2017 position names a range of approximately 1.4–2.0 g of protein per kilogram of body weight per day. Similar figures are cited by the joint position of the ACSM, the Academy of Nutrition and Dietetics and the Dietitians of Canada.
The meta-analysis by Morton and colleagues (2018), which combined data from dozens of training studies, showed that the gain in lean mass from additional protein plateaus at approximately 1.6 g/kg per day. The upper limit of the confidence interval reached about 2.2 g/kg, so for those who want to 'play it safe', a reference point of 1.6–2.2 g/kg is reasonable.
The practical algorithm is simple: count the protein in your usual menu over several typical days, subtract it from the target daily amount, and the difference will show whether powder is needed at all. For many people who eat meat, fish, eggs and dairy products, a supplement merely conveniently covers 20–40 g rather than being the main source of protein.
For vegans and vegetarians the situation is different: a plant diet often has a lower protein density, so soy isolate can provide a significant part of the daily norm. The study by Hevia-Larraín and colleagues (2021) showed that a high-protein plant diet using soy protein provided the same adaptations to strength training as a mixed diet with the same amount of protein.
How much soy protein in one serving
Studies of muscle protein synthesis after a single intake show that the muscle response increases with the dose up to approximately 20–40 g of quality protein, after which the additional effect becomes small. Converted to body weight, the ISSN recommends aiming for 0.25–0.40 g of protein per kilogram in each meal.
For soy protein there is an important nuance: it has less leucine than whey — the amino acid that is the main 'signal' for triggering muscle protein synthesis. Reviews by van Vliet and colleagues (2015) and Pinckaers and colleagues (2021) propose to compensate for this in a simple way — slightly increase the serving of plant protein to obtain the same amount of leucine.
In practice this means that instead of the 'standard' 20–25 g of whey it is sensible to take 25–40 g of soy protein, especially for people with a large body weight and for older people, whose muscles respond worse to small doses of protein. Note: this is about grams of protein, not powder. A scoop of 30 g of product with 85–90% protein gives about 26–27 g of protein.
Below is an approximate calculation of a serving based on the range of 0.3–0.4 g/kg. This is not a medical norm but a convenient arithmetical reference point for healthy adults.
| Body weight | Protein per intake (0.3–0.4 g/kg) | Approximate isolate powder (≈90% protein) |
|---|---|---|
| 55 kg | 17–22 g | 19–25 g |
| 70 kg | 21–28 g | 23–31 g |
| 85 kg | 26–34 g | 29–38 g |
| 100 kg | 30–40 g | 33–44 g |
If you use a concentrate or a blend with a lower protein share, the powder serving will have to be increased. Always calculate by the label: it states the protein content per serving and per 100 g of product.
Timing of intake during the day
The ISSN position on nutrient timing (Kerksick et al., 2017) summarizes: the total daily amount of protein matters more than the exact minute of intake. The idea of a narrow 'anabolic window' of 30 minutes after training has found no convincing confirmation, although taking protein in the nearest hours before and after exertion remains entirely reasonable.
A convenient strategy is to distribute protein over 3–5 meals with intervals of approximately 3–4 hours. Such a distribution allows raising the concentration of amino acids in the blood several times a day and triggering muscle protein synthesis. The soy shake is easiest to assign to those meals where ordinary food provides little protein: breakfast, a snack or the post-workout period.
Regarding intake before bed: studies by the van Loon group were conducted mainly with casein, which digests slowly. Soy protein is absorbed faster, so before bed it can be mixed with milk or other slower protein sources or you can simply eat a full dinner. There are few direct data specifically for soy in this context.
Intake on an empty stomach before training is not mandatory. If you train in the morning and tolerate food poorly, a small serving of liquid soy protein 30–60 minutes before the session is usually tolerated more easily than a heavy breakfast. The main thing is that the total daily amount matches the goal.
Duration of intake and whether breaks are needed
Soy protein is a food product, not a drug, so the concept of a 'course' does not apply to it. The body does not get used to protein and does not 'stop responding' to it over time. The training studies included in the meta-analysis by Messina and colleagues (2018) lasted from several weeks to several months and revealed no need for breaks.
A question often asked in this context is isoflavones. Soy isolate contains a certain amount of these plant compounds, although their content strongly depends on the purification technology. Meta-analyses of clinical studies (Hamilton-Reeves et al., 2010; Reed et al., 2021) found no effect of soy or isoflavones on testosterone and estrogens in men at doses corresponding to ordinary nutrition and even higher.
A sensible approach is to consider soy powder part of the diet and apply to it the same principles as to any food: variety and moderation. If protein covers the larger part of the daily protein intake, it is worth alternating it with other sources — legumes, pea, dairy products, if they are in the diet.
It makes sense to take a break only when adverse reactions appear: bloating, rash, itching in the mouth. Such symptoms require not a 'rest from protein' but finding out the cause together with a doctor, since soy is one of the major food allergens.
Typical mistakes and how to prepare a shake
The most frequent mistake is to count powder instead of protein. The second is to drink protein 'in reserve' when the daily norm is already covered by food: in that case the excess protein simply becomes a source of calories. The third is to ignore total caloric content: 40 g of powder in 2.5% fat milk gives a noticeable amount of energy, which is important during weight loss.
- Liquid:water gives a minimum of calories; milk or plant drinks add protein, carbohydrates and improve the taste.
- Volume:250–350 ml of liquid per 25–35 g of powder is usually the optimal consistency.
- Mixing:soy isolate dissolves worse in cold water than whey; a shaker with a ball or a blender solves the problem.
- Temperature:soy protein can be added to baked goods, porridges and pancakes — heating changes the structure of the protein but does not reduce its nutritional value.
People with sensitive digestion should start with half a serving and gradually increase it over the course of a week. Soy isolate contains significantly fewer oligosaccharides than whole beans, but in concentrates there are more of them, and it is precisely they that can cause bloating.
Another practical point is storage. Keep the opened package tightly closed in a dry, cool place, use a dry spoon. The finished shake is best drunk within a few hours or stored in the refrigerator, because protein drinks are a favourable environment for microorganisms.
People taking levothyroxine should space out the intake of the medication and soy protein by a few hours, since soy can reduce the absorption of the hormone. People with kidney diseases should coordinate any protein supplements with a doctor.
Editorial conclusions
Soy protein is taken the same way as any quality protein: first the daily requirement is determined (for those who train, usually 1.4–2.0 g/kg), and the powder is used to cover the difference between this goal and the ordinary diet.
It is advisable to calculate the serving for one intake as 0.3–0.4 g of protein per kilogram of body weight with a small reserve — because of the lower leucine content in soy. The timing of intake is flexible: it is more important to distribute protein evenly throughout the day than to catch the 'anabolic window'.
Breaks in intake are not needed, and long-term use within ordinary food amounts is well studied. The only signal to stop is individual intolerance or medical restrictions.
If you are choosing a product, we recommend reading our materials 'Soy Protein: Forms Available and Which to Choose', 'What to Combine Soy Protein With' and 'Who Should Not Take Soy Protein'.
References
- Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition position stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20.
- Kerksick CM, Arent S, Schoenfeld BJ, et al. International Society of Sports Nutrition position stand: nutrient timing. J Int Soc Sports Nutr. 2017;14:33.
- Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement. Nutrition and athletic performance. Med Sci Sports Exerc. 2016;48(3):543–568.
- Morton RW, Murphy KT, McKellar SR, et al. 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. Br J Sports Med. 2018;52(6):376–384.
- Messina M, Lynch H, Dickinson JM, Reed KE. 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. Int J Sport Nutr Exerc Metab. 2018;28(6):674–685.
- van Vliet S, Burd NA, van Loon LJC. The skeletal muscle anabolic response to plant- versus animal-based protein consumption. J Nutr. 2015;145(9):1981–1991.
- Hevia-Larraín V, Gualano B, Longobardi I, et al. High-protein plant-based diet versus a protein-matched omnivorous diet to support resistance training adaptations: a comparison between habitual vegans and omnivores. Sports Med. 2021;51(6):1317–1330.
- Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. Neither soy nor isoflavone intake affects male reproductive hormones: an expanded and updated meta-analysis of clinical studies. Reprod Toxicol. 2021;100:60–67.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
