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Fast absorbing proteins vs. slow proteins - What's the difference?


If you’re looking for a protein supplement to help you achieve specific fitness goals — such as building muscle or losing weight — it’s important to take a supplement’s protein absorption rate into consideration.

A fast digesting protein is a protein that is typically digested within 1-2 hours; a slow digesting protein is a protein that is typically digested and utilized over a period of 4+ hours. But which is better?

It all depends on your overall goals. Are you looking for a protein powder to assist with fast recovery after a workout? Do you need a protein powder to help keep you feeling full and ward off hunger during a calorie deficit? Or are you making changes to your diet and just need a protein powder to up your protein intake?

Because people use protein powders for different purposes, there are a number of different types on the market. Some are quickly absorbed by the body, while others feed protein to the muscles very slowly.

What are fast absorbing proteins?
Fast absorbing proteins are proteins that can be absorbed within a few hours. When a protein is absorbed quickly, that means your body is able to utilize it faster to aid in processes like muscle protein synthesis — a naturally occurring process in which protein is produced to repair muscle damage caused by intense exercise. 

Protein has been shown to be particularly effective at increasing muscle growth when it’s consumed as close to the time of a workout as possible: A 10-week study examined the effects of resistance training combined with protein supplementation and found improvements in muscle performance, as well as more effective muscle protein synthesis, in participants who consumed fast absorbing protein like whey. Because muscle protein synthesis is usually maximized in the time period after training, you can see why someone who weight-trains or engages in rigorous workouts would want their protein to be absorbed right away. 

So then what is the fastest releasing protein? Whey protein is the most popular fast release protein. It’s also one of the most widely-studied proteins, proving itself decade after decade after decade in various studies as the standard for fast absorbing protein. Whey’s absorption rate has been estimated at roughly 10 grams per hour. At this rate, it takes just 2 hours to fully absorb a 20 gram-dose of whey. A commonly recommended protein powder dosage is 1–2 scoops (around 25–50 grams), so if you consumed 20 grams of whey protein around the time of a workout, that means your body would be able to utilize that protein within two hours of exercising — an especially fast rate of absorption compared to other proteins.

A possible downside: Whey protein contains lactose and is a by-product of cheese production. This is a problem for people who are vegan or lactose intolerant. Luckily, there are a few fast absorbing protein options for those with dietary restrictions. Of those most common vegan or plant-based proteins (pea protein, soy protein, brown rice protein, hemp protein and other seed, nut, or legume proteins), pea protein has the fastest absorption rate. 

Although it’s not quite as fast absorbing as whey, pea protein has proven it can deliver similar results when it comes to muscle performance: In a 2015 study published in the Journal of the International Society of Sports Nutrition, male participants between the ages of 18 and 35 followed the same 12-week upper-body lifting program. One group supplemented with 25 grams of pea protein twice a day, and another took the exact same amount of whey protein. Each 25 grams of protein from pea protein delivered around two grams of leucine, one of the 3 branched chain amino acids (BCAAs) that stimulates protein synthesis. Whey delivered 2.65 grams of leucine, but both the pea protein and the whey protein fell right in the middle of the optimal leucine range. Researchers recorded biceps muscle thickness at the beginning and end of the study, and at the end of the 12 weeks, both groups showed identical increases in biceps muscle thickness. Both pea protein and whey protein types also showed greater results than placebo. As long as a person supplementing with pea protein gets sufficient amounts of leucine, pea protein digests and can help build muscle in a comparable way to whey.

What are slow digesting proteins?
While a fast absorbing protein is digested in just a few hours, a slow digesting protein is a protein that is typically digested and utilized over a longer period of time. (Think 4+ hours.)

Casein is a slow digesting protein. During cheesemaking, special enzymes are added to heated milk, which cause the casein in the milk to coagulate, or change to a solid state. The curds of casein are then able to be used in food products or dietary supplements. 

Although casein and its counterpart, whey protein, are both derived from milk, the body absorbs them very differently. Your body breaks down protein into amino acids, which stay in your bloodstream until they’re absorbed. When a person consumes casein, levels of these amino acids stay elevated in the blood for about 4-5 hours. With whey, however, these levels are elevated in the blood for only about 90 mins. That’s because casein forms curds once exposed to the acids in your stomach (just as it does in cheesemaking). These curds actually lengthen your body’s digestion and absorption processes.

By taking longer to digest, casein protein is able to provide the body with a slow and steady release of amino acids. Slower digesting proteins are ideal for people who want to maximize muscle growth throughout the night. A 2018 study published in the journal Nutrients found that consuming proteins like casein right before bed can help a person maintain healthy muscles — so that means you can actively work on your muscles, even while asleep.

What’s better: whey protein or casein protein?
As you know from above, one of the significant differences between casein and whey is how quickly your body absorbs them. But which milk protein is the better choice?

Of the two, whey protein is popular among athletes, bodybuilders or people who are looking to build muscle mass because it provides protein and amino acids that serve as building blocks for increased muscle growth and help prevent muscle breakdown. People who engage in regular workouts typically want their protein to be quickly absorbed, so the body is able to utilize the protein sooner. With its fast absorption rate, whey protein is particularly effective at increasing muscle growth when consumed right before a workout or post-workout, as muscle protein synthesis is usually maximized in the time period after exercise. Whey protein also increases the release of anabolic hormones that can stimulate muscle growth and is high in the amino acid leucine, which is known to stimulate muscle protein synthesis.

However, if the goal is to maximize muscle growth throughout a longer period of time, such as during the night, you might find that casein protein is a better fit for your needs. Try consuming casein protein before bed, as it provides your body with a steady release of amino acids throughout the night. Compared to whey protein, casein protein’s slow release makes it ideal before fasting situations, such as during sleep. Casein may also be a better option for you if you are looking to lose weight, as it is a more satiating protein option than whey. That said, muscles use amino acids for muscle recovery and muscle protein synthesis for up to 48-72 hours after exercise, so getting a wide variety of protein sources spread across your intake in the days following your workout is just as important as how quickly your post-workout protein shake is absorbed.

Is egg protein slow or fast digesting? Is rice protein fast or slow digesting? The absorption rate of other types of proteins and foods
We’ve covered whey protein, casein protein and pea protein, but what about others proteins? Which proteins fall in the “medium” digesting rate category? 

A medium digesting protein is typically defined as a protein that is digested within 3 to 3 ½ hours. (So slower than whey protein, which is digested within an hour or two, but faster than casein, which takes 4+ hours to digest.) In addition to pea protein, whole egg protein, egg white protein, brown rice protein and soy protein are all considered medium digesting proteins. Since most people wait 3 or so hours between meals, these types of proteins are great for rationing your protein intake or spreading it evenly throughout the day, as well as keeping full between meals.

As for the absorption rate of whole foods, It’s important to note that whole foods usually contain more than just protein (unlike protein powders, which have been broken down) as well as more than just one kind of protein. Also, people consume more than just one food at any given meal. When you eat a full meal — not only one food — this leads to slower digestion, no matter what specific foods you eat. Any fiber and fat in your food will also slow digestion in general.

There are foods, like cottage cheese, that are great for people with certain fitness goals, such as building muscle or weight loss, because they contain significant amounts of both fast and slow absorbing proteins. (In cottage cheese’s case, that’s casein and whey protein. Casein is a slow digesting protein that is beneficial for your muscles’ long term growth and recovery while whey is a bodybuilding protein that helps gain muscle mass and burn fat.)

But the fastest digesting proteins are the ones that have been separated from their whole food source and turned into protein powder. These protein powders are then consumed as a liquid without much fiber or fat, and, generally speaking, liquids digest faster than whole foods. So that means protein powder supplements typically have a faster digestion rate than whole foods. (However, the digestibility among different types of protein powders also varies, as mentioned above.) Whey protein hydrolysates are usually the fastest digesting of all supplements because hydrolysates are processed further than whey protein isolates and whey protein concentrates. This makes hydrolysates even easier to break down.

Find the best protein powder for you
If you’re still a little confused about protein absorption rate, or you don’t know where to start when it comes to finding a protein powder that’ll work best for you, don’t worry: Gainful is here to help. Through Gainful, you can create a custom-made protein powder that aligns with your goals, so you can get exactly what your body needs. 

First, head here to take our quiz. Together, we’ll figure out what type of protein best suits your body type and overall goals — whether that’s a fast absorbing protein or a protein blend that’s slower digesting. We’ll also help determine the amount of protein you need and add customizations based on your preferences, taking your lifestyle and any dietary restrictions into account. Then we’ll deliver everything you need in the perfect amounts, making sure you’re getting the right protein and those essential amino acids necessary to achieve your goals. (And don’t forget you have a dedicated Registered Dietitian on-hand as part of your Gainful subscription. Your R.D. is here to answer any questions about your daily nutrition, your protein source, fast vs. slow absorption — whatever is on your mind.)

Gainful wants to help you distinguish between fast and slow proteins and find the perfect protein powder for you.

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