Casein or Whey for Weight Loss: Which Works Best?

If you want to lose weight, should your protein shake contain whey or casein? Is one better than the other? Or will they have much the same effect on weight loss?

Protein and Weight Loss

When it comes to getting lean, protein has a couple of important roles. First, it does a better job at filling you up than carbohydrate or fat. Eat a protein-rich diet, for example, and chances are you’ll end up eating fewer calories throughout the day.

Protein also has a muscle sparing effect. If you don’t get enough protein while you’re on a diet, you’ll end up losing muscle as well as fat.

On the basis of how quickly they’re digested, proteins are divided into so-called fast and slow proteins. And the two that come to mind whenever someone mentions fast and slow proteins are whey and casein (pronounced kay-seen), both of which come from milk.

Whey is considered a fast protein because it’s rapidly absorbed by the body. Casein, on the other hand, is a slow protein that takes longer to break down and absorb. 

And it’s the slower digestion rate of casein that should, in theory at least, give it the upper hand in the weight loss department. Why? 

First, because casein hangs around in your stomach longer than whey, it should keep you feeling fuller for longer. As a result, you won’t feel as hungry, which should make it easier to create the calorie deficit required for weight loss.

Second, because the protein is being released into your system more slowly, casein should do a better job at minimizing the loss of muscle during weight loss.

Like I said, that’s the theory. But does it actually work like that in practice? If you want to lose weight, what’s the best protein supplement to use?

Casein vs Whey for Weight Loss: The Research

The research on hunger, appetite and food intake is a bit of a mixed bag. Some studies show that whey leads to higher ratings of satiety and fullness than casein [1], while others point to casein being more effective than whey at reducing daily food intake [2].

However, short-term changes in satiety and food intake don’t always lead to long-term changes in body weight [1]. That’s why we need to turn to longer trials that look directly at the effect of casein and whey on body composition over a period of weeks and months.

Back in 2012, a team of French scientists looked at how fast and slow proteins affected body composition in a group of obese men and women [3].

Subjects were divided into four groups, and given two different protein supplements – whey or casein.

Protein intake was also distributed differently throughout the day. Two of the groups ate roughly the same amount of protein in each of their four meals. Groups three and four ate most of their protein for the day in a single meal.

Here’s what happened:

For one, the source of protein had no effect on the perception of hunger. As you might expect, subjects felt hungrier before compared to after a meal. Hunger also varied significantly according to meals, with subjects reporting less hunger at breakfast than at lunch and dinner. But subjects given casein didn’t feel any less hungry than those given whey.

What’s more, after six weeks of dieting, there was no significant difference in the rate at which fat was lost. Whether they took whey or casein, everyone in the study lost roughly the same amount of fat.

In the final week of the study, casein was more effective than whey at inhibiting whole body protein breakdown, leading to a greater positive protein balance. This hints at the possibility that casein may do a better job at preserving muscle mass during a period of weight loss.

However, the loss of lean body mass (a reasonable proxy for muscle mass), was not significantly different in the whey and casein groups. 

This may be because protein breakdown measured at the whole body level doesn’t tell you what’s happening to muscle protein. It tells you that protein is being broken down, but it doesn’t tell you where.

Given that changes in whole body protein balance only showed up in the sixth week of the study (and assuming that changes in whole body protein breakdown were indicative of what was going on at a muscular level) you’d need a study lasting much longer than six weeks to detect any effect of whey or casein on changes in lean mass.

A similar study, which looks at how whey and casein affect weight maintenance following a period of weight loss, also shows little differences between the two proteins [4]. 

If casein was making it easier to control hunger and appetite than whey (or vice versa), you’d expect to see one group regain the lost weight more quickly than the other.

But that isn’t what happened. Whether subjects were given whey and casein, the end result was much the same.

How Mixing Nutrients Affects Digestion

One of the problems with the concept of fast and slow proteins is that mixing nutrients, such as eating protein with carbohydrate and fat, changes the rate at which amino acids reach the bloodstream. 

Simply adding table sugar to milk protein, for example, has been shown to slow digestion time and slightly improve the utilization of that protein [5].

Consuming a fast or slow protein as part of a mixed meal, or even a few hours after a mixed meal (where nutrients are still being digested and absorbed), is going to affect the way that protein is digested.

The way that protein supplements are processed and stored can also affect their digestion rate. 

There was an interesting study published by a team of Dutch researchers, who looked at how three different types of casein – micellar casein, calcium caseinate and sodium caseinate – are digested and absorbed [6].

Surprisingly, micellar casein appeared to be more digested and absorbed a lot more quickly than expected of a slow protein. In fact, it was even faster than the calcium caseinate, a finding the researchers describe as “unexpected.”

A certain type of protein may well behave differently from company to company, from batch to batch, or even based on its shelf life. ⁣

Final Thoughts

Much has been made about the difference between fast and slow proteins. However, while casein and whey are digested and absorbed at different rates, there’s very little evidence to show that this has any meaningful impact on your weight loss results over a period of weeks and months.

If you want to take whey, take whey. If you want to take casein, take casein. As long as you’re lifting weights 3-4 times a week, and your diet ticks the right boxes in terms of calories and protein, it’s not going to make much difference one way or the other.