Best Source of Protein – Whey, Soy or Casein?

I’m sure that you’ve all come across these in the supplements that you are buying. I know that I have and I’ve often wondered about the effectiveness of each as a post-workout protein shake. In this article I want to take a closer look at each and use scientific studies to conclude which is the best source of protein. Let’s make a start…

Whey Protein

Whey is the most popular supplement to be used in protein powders. I’ve written more about whey protein before so check that out for the details. In summary, though, whey is derived from cows milk.

In a recent study by The Human Performance Laboratory at the University of Connecticut, the researchers concluded that “Despite consuming similar calories and protein during resistance training, daily supplementation with whey was more effective than soy protein.”

So, looks like the boffins believe that whey protein is better than soy protein. This was backed up by another study, albeit it several years earlier, which also concludes whey protein is a better post-workout option than soy because it is absorbed more rapidly.

Soy Protein

Soy is a protein derived from plants. It is considered a full protein because it contains all of the essential amino acids which your body cannot product and has to obtain from the diet. The interesting part about soy is that it contains higher levels of estrogen and some people believe that this can have a negative impact on men who take it.

In a research study completed by the University of Connecticut, researchers gave men one of three supplements before their workouts

  1. Whey Protein Isolate (WPI)
  2. Soy Protein Isolate (SPI)
  3. Maltodextrin Placebo

The subjects then performed 6 sets of 10 reps of squats at 80% of 1 rep max and their hormonal responses were tracked. The test found that “This investigation observed lower testosterone responses following supplementation with soy protein”. And they also found that “Importantly, there were no significant differences between groups in changes in estradiol concentrations.” Which means that estrogen levels were not increased as a result of soy supplementation when compared with the whey protein group or the controlled group.

It should be noted that this study was only undertaken over the course of 14 days so the longer term effects are not known.

Casein Protein

Casein protein is regarded as a much slower releasing protein supplement. It’s commonly found in weight loss shakes and protein powders which you take at night time. People often take a casein supplement at night before bed so that the protein is absorbed and made available to the muscles throughout the night. That’s a great idea, by the way, because your muscles require a constant supply of protein throughout the recovery process and not just post-workout. The recovery period can be up to 72 hours post-workout.

A study by McMaster University took three groups of healthy young men and got them to perform weight training using the leg extension, just with one leg. Following that they were given one of three drinks “containing an equivalent content of essential amino acids (10g)”. The drinks were:

  1. Whey Hydrolysate
  2. Soy Protein Isolate
  3. Micellar Casein

The conclusion of the study found that whey hydrosolate worked the best in terms of increasing protein synthesis both at rest and after exercise. Soy protein was the next best and casein was the least effective.

The researchers also concluded that “despite both being fast proteins, whey hydrolysate stimulated MPS to a greater degree than soy after resistance exercise” This means that the whey hydrolysate was more effective in this study than the soy.

Remember that this study was looking at the effectiveness of getting protein to the muscles as quickly as possible and this is just one part of what you might be trying to achieve.


Based on the research I have done it has confirmed that whey protein is the most effective form of protein powder for post-workout recovery, muscle growth and repair.

Soy protein is a good quality source of protein but is not quite as effective as whey. Also, although it does not appear to increase estrogen levels it does inhibit the production of testosterone which is not ideal when you are trying to build muscle so I would avoid it.

Micellar casein should not be used as a post-workout protein powder because it is not absorbed rapidly enough to cater for your muscle’s requirements. However, that’s not to say that it cannot be used for a different purpose and in fact it would be better suited if you require a slow release protein. For weight-loss or meal replacement purposes, for example.

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