What to Combine Soy Protein With

Soy protein is rarely consumed 'alone': it is mixed with other proteins, added to shakes with fruit and oatmeal, combined with creatine or carbohydrates after training. The editorial team has broken down which combinations have a scientific basis, which are simply convenient, and what is better spaced out in time.

Why combine at all

Soy isolate is a quality complete protein, but it has several features that it makes sense to compensate for with combinations. The first is a lower leucine content than whey, the key amino acid signal for muscle protein synthesis. The second is a relatively low methionine content. The third is a medium absorption speed: soy digests more slowly than whey but faster than casein.

The review by Pinckaers and colleagues (2021) describes several strategies that allow increasing the anabolic response to plant protein: increase the dose, enrich the product with leucine or combine different proteins so that their amino acid profiles complement one another. All three strategies are practically implemented with the help of the right combinations.

Besides amino acids, combinations also solve everyday tasks: they improve taste and texture, add calories during mass gain or, on the contrary, increase satiety on a calorie deficit. Soy powder with water does not taste good to everyone, but with a banana and rolled oats it becomes a full-fledged breakfast.

At the same time, not every combination makes sense. Marketing 'matrices' with a dozen ingredients often contain negligible doses of each component, and some combinations can reduce the absorption of medications. Therefore we will further divide the options into useful, neutral and those that require caution.

Soy and other proteins: blends

The best-studied combination is soy with whey and casein. The study by Reidy and colleagues (2013) showed that a blend of approximately 25% soy isolate, 25% whey and 50% caseinate after strength training stimulated muscle protein synthesis similarly to pure whey, while the elevated concentration of amino acids in the blood lasted longer. The idea of such blends is to combine fast, medium and slow proteins.

For vegans, the logical partner for soy is pea and rice. Pea protein is rich in lysine, rice — in sulphur-containing amino acids, while soy has a fairly balanced profile. The analysis by Gorissen and colleagues (2018) showed that plant isolates differ in the content of essential amino acids and leucine, so combining them allows bringing the profile closer to animal proteins.

Milk as the liquid for a shake is a simple way to combine soy with dairy proteins if a person does not follow veganism and tolerates lactose well. A glass of milk adds a few grams of protein with a predominance of casein, which slows absorption and makes such a shake suitable, for example, for intake in the evening.

When choosing a ready-made blend, look carefully at the proportions. If soy is last in the composition, its share may be minimal, and the product will effectively be a whey one. For vegans it is important to make sure the blend has no dairy components.

Soy and carbohydrates, fats and fibre

After strength training, adding carbohydrates to a sufficient serving of protein changes little in muscle protein synthesis. However, after prolonged or repeated exertion during the day, carbohydrates are needed to restore glycogen stores. The ISSN position on nutrient timing (Kerksick et al., 2017) notes that in such a situation combining protein with carbohydrates aids recovery, especially when there is little time between sessions.

In practice this means: if you train twice a day or engage in endurance sports, a soy shake with a banana, oatmeal, honey or juice is appropriate. But if the goal is weight loss, the carbohydrates in the shake should be counted as part of the daily caloric content.

Fats and fibre slow gastric emptying and, accordingly, the delivery of amino acids into the blood. This does not make the protein less useful but shifts the peak in time. For a shake before bed or as a meal replacement, adding nut butter or seeds is a good idea, while immediately before training it is better to choose a lighter option.

Iron is worth mentioning separately. Soy contains non-heme iron and phytates, which reduce its absorption. Vitamin C, on the contrary, improves the absorption of non-heme iron. Therefore berries, citrus or kiwi in a soy shake are not only a taste but also a useful combination, especially for vegans and women who train.

Soy and other sports supplements

Creatine monohydrate is one of the most studied sports supplements, and it is convenient to add to a protein shake. There is no direct interaction between creatine and soy protein: each works by its own mechanism. For vegans, whose diet has no meat and fish, creatine is especially relevant, since they get it from food in a minimal amount.

Leucine enrichment is another strategy described in reviews on plant proteins. Adding a few grams of leucine to a serving of soy protein brings its amino acid 'signal' closer to whey. An alternative and simpler path is to increase the soy protein serving itself, which will simultaneously raise the content of other essential amino acids as well.

• Creatine:compatible, convenient to add to a shake.
• Leucine:can enhance the one-time anabolic response; an alternative is a larger serving of protein.
• Caffeine:compatible; if the shake is drunk before training, take into account the total daily dose of caffeine.
• Beta-alanine, citrulline:can be taken together, no interactions with protein are described.
• BCAA:with a sufficient serving of complete protein there is little additional point.

Multivitamins and minerals can also be taken in the same meal, but it is worth remembering the effect of phytates on the absorption of zinc and iron. If a doctor has prescribed an iron preparation to correct a deficiency, it is better to take it separately from soy products, with a source of vitamin C.

What is better spaced out in time

The most important interaction to be aware of is with levothyroxine. Soy products can reduce the absorption of this hormone, so people with hypothyroidism who take it should drink the soy shake a few hours after the tablet. The guidelines of the American Thyroid Association (Jonklaas et al., 2014) recommend taking levothyroxine separately from food and substances that can disrupt its absorption.

Calcium and iron in the form of supplements, as well as some antibiotics and other medications, can interact differently with products rich in protein and minerals. If you constantly take any medications, ask your doctor or pharmacist whether their intake needs to be separated from protein shakes.

You should not combine soy protein with concentrated isoflavone supplements 'to enhance the effect'. There is no benefit for muscles from this, and regarding long-term intake of high doses of isolated isoflavones the safety data for many groups of people are scarce, as EFSA has also pointed out.

Finally, combining several 'protein' products in one intake — a shake, a bar and a gainer — often leads to an excess of calories without additional benefit for muscles. It is better to distribute protein evenly over several meals throughout the day than to eat it all at once.

Editorial conclusions

Soy protein combines well with other proteins: blends with whey and casein have direct scientific confirmation, while combinations with pea or rice help vegans bring the amino acid profile closer to animal proteins.

Carbohydrates should be added when it is necessary to restore glycogen or gain mass, and vitamin C — for better absorption of iron from a plant diet. Creatine and most popular sports supplements are compatible with soy.

The only truly important interaction is with levothyroxine; also, there is no point in enhancing soy protein with isoflavone concentrates.

To understand in more detail, we recommend reading our articles 'How to Take Soy Protein: Dosage, Timing, Duration', 'What to Combine Pea Protein With' and 'Soy Protein: Forms Available and Which to Choose'.

References

1. Reidy PT, Walker DK, Dickinson JM, et al. Protein blend ingestion following resistance exercise promotes human muscle protein synthesis. J Nutr. 2013;143(4):410–416.
2. Pinckaers PJM, Trommelen J, Snijders T, van Loon LJC. The anabolic response to plant-based protein ingestion. Sports Med. 2021;51(Suppl 1):59–74.
3. Gorissen SHM, Crombag JJR, Senden JMG, et al. Protein content and amino acid composition of commercially available plant-based protein isolates. Amino Acids. 2018;50(12):1685–1695.
4. 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.
5. Kreider RB, Kalman DS, Antonio J, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr. 2017;14:18.
6. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670–1751.
7. EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS). Risk assessment for peri- and post-menopausal women taking food supplements containing isolated isoflavones. EFSA Journal. 2015;13(10):4246.