Blog Post

Does Slow Digesting Protein Before Bed Help Muscle Growth?

Whey vs Casein digestion speeds

Whey protein is well known for being a “fast-digesting protein source” while casein is a “slow-digesting protein source.”

Having whey protein post-workout when you want protein rapidly and having casein before bed when you want to be minimising muscle protein breakdown overnight makes sense based on this logic.

In nutrition though, it is not uncommon for things to make sense when you look at a single mechanism, but fall apart when you look at the big picture. In this case, the big picture thing we really care about is – does this approach lead to more muscle gain/retention in the long term?

That is the question this post is going to be looking at. The intention is to cover:

  • The theoretical side of the discussion.
  • The research on the topic- focused on outcomes.
  • What you should do in practice if your goal is muscle growth/retention. 

Rate of Protein Digestion – Differences Between Whey and Casein

Adding context to this discussion, it makes sense to first look at the differences in the rate of absorption of protein from whey and casein.

study on this topic found that if 30g of casein or whey was consumed after a 10-hour fast, the following was found:

–  Both protein sources reached their peak blood leucine levels after around an hour, but whey spiked it significantly higher.

– After about 4 hours, leucine returned to the baseline in the whey group. 

– After 7 hours, leucine was still well above the baseline in the casein group.

A graphical representation of this courtesy of can be found below.

Digestion time of whey vs casein

You can clearly see that while whey peaks higher, casein remains at a relatively high level for far longer. 

Another interesting finding in this study was that whey stimulated whole-body protein synthesis without affecting whole-body protein breakdown much, while casein did the opposite. Casein’s ability to reduce muscle protein breakdown for an extended period further builds the case for casein before bed.

How Do Other Proteins Compare?

To further add context though, ideally would look at how the rates of digestion and absorption compared to other forms of protein.

This is a pretty difficult thing to quantify well unfortunately though.

One of the most comprehensive summaries I have seen so far comes from a 2006 study and can be found below.

Digestion speeds of different protein sources

These are all just estimates based on research. They can also be variable for a variety of reasons, so I would not view these as perfect measurements.

This chart clearly complicates the whole “casein is a slow-digesting protein” aspect. Based on this casein is faster digesting than a lot of other sources.

Complicating matters though is that most other sources seem to categorise things in the following order:

  1. Fast = Whey
  2. Medium = Egg, pea, rice and soy
  3. Slow = Casein

The latter classification is likely more accurate and relevant. But I personally found it hard to find research supporting this type of classification, even though it is commonly repeated. 

How Do Fat, Carbohydrates and Fibre Fit into This

A major issue with the research previously mentioned is that it is focused on protein consumed in isolation after fasting. This makes it far less relevant to how people are often consuming protein. 

We care about what happens when consuming protein in a non-fasted state. We also care about what it would look like as part of a mixed meal containing carbohydrates, fat and/or fibre.

Research on the topic has identified that when carbohydrates or fats are added to the mix, the differences between the digestion and absorption rates of whey and casein pretty much disappeared.

study that added 60g of carbohydrate to a shake containing 20g of casein showed that it slowed digestion in comparison to 20g of casein by itself.

It makes sense that adding fibre, fat and/or carbohydrate into the mix would slow down digestion and absorption. It would also, therefore, be safe to say that most whole food protein sources would be slower to digest and absorb as well. 

Acute vs Long-Term Data on Muscle Growth

An important aspect to cover here is that most of the support for slow-release protein before bed typically comes from acute data. 

Often the studies cited on the topic are measuring muscle protein synthesis and/or muscle protein breakdown over a period of hours. 

If you read closely, you will notice that one of the main studies I mentioned earlier did not even measure muscle-protein synthesis, it measured whole-body protein synthesis. There is a lot of overlap, but they are not the same thing.

This information can be useful to a certain extent. From another perspective though, people wondering about this topic likely just care about actual muscle growth or retention.

A great review summarising this topic well was done by Brad Schoenfeld and Alan Aragon in 2018. 

If you focused on a few acute studies, you would start to think there was no benefit to consuming more than ~30g of protein in a single sitting. But this review highlighted that when looking at longer-term studies in addition to acute data, having >30g of protein in a sitting can have benefits at times. 

The summary from that review was to optimise muscle growth you should aim for 0.4-0.55g/kg of protein per meal, over 4-6 meals, for a total range of 1.6-2.2g/kg/day. Having a minimum of 1.6g/kg/day was an important aspect, which might not be possible for larger individuals if they limited themselves to a maximum of 30g of protein per sitting. 

The reason I have mentioned that review is mostly to highlight that when looking at muscle growth and retention, we also want to factor in longer-term data, not just short-term data.

Muscle Growth over time. Man doing bicep curls.

What Does the Data on This Specific Topic Show?

One study on the topic found no difference in lean mass gains following 8 weeks of either whey or casein before and after training.

Another study compared whey vs casein in different ratios and combinations, also finding no noticeable difference.

Both studies were based on timing around the workout, not prior to sleep. But it still goes against the hypothesis that whey protein would be superior post-workout, which partly factors into this discussion. 

whey vs casein muscle growth outcomes

Some studies make it look more positive though. For example, a 2020 systematic review identified that 40-48g of casein 30 minutes before bed showed a lot of positive outcomes.

This seems impressive in isolation, until considering that most research on the pre-sleep protein of a variety of kinds results in similarly positive outcomes.

And something that might come as a bit of a surprise to people, the research on intermittent fasting has been surprisingly positive in terms of muscle mass retention during calorie deficit phases. 

The reason I say “retention” and “calorie deficit” is just because there has been far less research done during calorie surpluses since it is harder to get into a surplus while intermittent fasting, which is partly why it is a bit rarer to be studied. 

We know that spreading protein intake out across the day is important. But these intermittent fasting studies make it clear that total protein intake is more important, even though both aspects matter. 

As a bit of a nail in the coffin though – if casein is superior to other protein forms prior to bed because of its slower digestion rate, shouldn’t a mixed meal option be even better? Adding a carb/fat/fibre source to it would slow things down even more. Part of why this concept is not discussed often is because it does not seem to provide any additional benefits.

salmon, rice, peas, beans, cherry tomato and spinach

Practical Summary

Part of why casein prior to sleep often looks beneficial is due to sub-optimal protein protocols utilised in studies. 

Often, researchers will be adding casein in a way that increases total protein intake from a sub-optimal amount to closer to the optimal amount. Obviously, that will consistently improve results.

Other times, they might match the total protein amount, but because the amount or timing of the rest of the protein is sub-optimal, factors like digestion and absorption rates might matter more.

Alternatively, sometimes people might just be focusing a lot on acute data focused on muscle protein synthesis and muscle protein breakdown. Since casein reduces muscle protein breakdown for far longer than whey, it looks superior for that.

But as mentioned earlier, mixed meal protein sources are digested and absorbed even slower than well-known slow-release protein sources like casein. Based on this logic, they should outperform even more in terms of results. But that doesn’t seem to be the case.

Having a decent amount of protein within a few hours prior to going to bed is certainly not a bad idea. It might help muscle growth a little bit and it comes alongside no major downsides. Some people would question whether it impacts sleep quality or quantity. You could argue that large meals prior to bedtime would do that, but a moderate amount of protein (20-40g of protein) in isolation does not appear to have any issues like that.

Focusing on aiming for a sufficient total protein intake spread across the day, coming from a variety of protein sources seems to optimise protein intake. I would not specifically be focusing on having slow digesting and absorbing protein before bed specifically, although there is no downside to it either. 

By Aidan Muir

Aidan is a Brisbane based dietitian who prides himself on staying up-to-date with evidence-based approaches to dietetic intervention. He has long been interested in all things nutrition, particularly the effects of different dietary approaches on body composition and sports performance. Due to this passion, he has built up an extensive knowledge base and experience in multiple areas of nutrition and is able to help clients with a variety of conditions. One of Aidan’s main strengths is his ability to adapt plans based on the client's desires. By having such a thorough understanding of optimal nutrition for different situations he is able to develop detailed meal plans and guidance for clients that can contribute to improving the clients overall quality of life and performance. He offers services both in-person and online.