Does Insulin Stimulate Muscle Growth? - Nutrition Tactics

You probably heard about insulin before.

It’s the hormone that’s released when you eat those tasty carbohydrates and that makes you fat. At least, that is what all your diet books have told you right?

But if insulin is so bad for your physique, then why do bodybuilders inject insulin?

Well I’m glad you asked.

Insulin 101

One of the main functions of insulin is too keep your blood sugar under control. Your body can become resistant to insulin and this results in diabetes.

In recent years, insulin has gotten a bad name as ‘’the evil storage hormone’’.

Insulin is demonised as a hormone that stores the food you eat as body fat and prevents your body from using body fat as a fuel. This is principle where most low carb diets are based upon: eating less carbs results in less insulin release, which results in less body fat.

While that sounds nice in theory, things are nowhere near as simple as that. A topic I’m sure I’ll be talking a lot about in the future.

So a lot of people are afraid of insulin because they think it’s bad for their health and weight. Yet many dedicated gym rats intentionally try to increase insulin levels through food strategies or even injections because they think it will help them build muscle mass.

But are they right or wrong?

Insulin and building muscle

Chances are if you have the guts to read one of my research reviews, you’ve been reading about fitness for a while. In that case, I’m wondering what you currently believe the effect of insulin is on building muscle? Is it extremely effectively, totally useless, somewhere in between?

Second question: how sure are you of your answer?

Because not all scientists agree with each other on this question. Simply because there’s quite a few studies saying it’s working, but just as many who say it does nothing.

And that’s why I conducted a systematic review on this topic.

Specifically, we looked at the effect of injected insulin, rather than insulin release in the body in response to food.

We did this because eating does not only results in insulin release, but also provides energy and provides nutrients that turn on all kind of processes in the body. This makes it really hard to say if it’s the insulin that is causing the muscle growth or one of these other effects of food.

How does insulin affect muscle protein synthesis?

Before we go to the juicy results of the review, it’s important to know some background information to make sense of it all.

We muscle physiologist often measure a process called muscle protein synthesis. It’s the process of building new muscle tissue.

There’s two ways how insulin is thought to influence muscle protein synthesis:

The first is that insulin directly stimulates the cellular pathway in the muscle that regulates muscle growth. Basically, it goes to a muscle cell and turns its muscle building machines up a few notches.

The second is that insulin increases blood flow to the muscle. Thereby, more nutrients, hormones and the like can get into the muscle which can stimulate muscle protein synthesis and provide the building blocks for growth.

The effect of insulin in combination with high doses of amino acids

After I indexed the whole scientific literature, I found the results where a coin flip. About half of the studies reported that insulin increased muscle protein synthesis, while the other half reported that insulin had no effect.

So I started to look if there were differences in the studies that might explain the different results.

A lot of studies didn’t just inject insulin, but also injected high doses of amino acids (the building blocks of protein and muscle).

All these studies found that the combination of insulin and a high dose of amino acids increased muscle protein synthesis. However, these studies cannot conclude that insulin stimulates muscle growth, because infusion of high doses of amino acids can stimulate muscle growth on their own.

So the question then becomes: can combination of insulin and amino acids stimulate muscle protein synthesis more than amino acids do on their own?

And that is exactly what was investigated in a very nice study. In this study four groups of subjects where infused with a high dose of amino acids. All four groups received insulin as well, but all four at a different dose from very low to very high. And no matter what dose of insulin was infused, muscle protein synthesis was equal in all groups.

So the first conclusions from my review are of my review is:

When amino acids are high in the body, insulin does not further increase muscle protein synthesis.

The effect of insulin on amino acids

Of course, there are also a lot of other studies that infused insulin without infusing amino acids.

There’s one problem with that though.

Insulin lowers the amount of amino acids in the blood. And since amino acids stimulate muscle protein synthesis, these studies basically shoots themselves in the foot.

So to no surprise, insulin did not stimulate muscle protein synthesis in any of the studies in which it lowered the amount of amino acids in the blood.

Therefore, the second conclusion of the review was:

Insulin infusion into the circulation lowers the amino acids in the blood which obviates any possible effect on muscle protein synthesis.

What is the effect of the insulin dosage?

An obvious difference between all the studies was the dose of insulin infused.

The dose actually didn’t seem to matter much, except when extremely high doses were injected that increased the amount of insulin in the body to levels that the body cannot produce itself. We call this supraphysiological levels.

These supraphysiological doses have not been studied a lot, partly because they are very dangerous. But in all those studies, insulin did increase muscle protein synthesis.

Therefore, the third conclusion of the review was:

Supraphysiological doses of insulin stimulate muscle protein synthesis.

Does age play a role in the effect of insulin?

The 3 previous categories provided pretty clear conclusions as to when insulin works and when it doesn’t. But we’re were still left with a lot of studies that not fall in the previous catogories and reported different results.

All of the remaining studies solved the problem of amino acids dropping in response to the insulin infusion in one of two ways.

The first option is to infuse a small amount of amino acids to keep the amino acids at normal levels (not high enough to stimulate muscle protein synthesis on their own).

The second option is to infuse insulin only locally in a muscle. For example, infuse insulin only into the circulation of one leg. That way, the insulin has no effect in the rest of the body and the blood continually delivers the leg with fresh blood that has normal levels of amino acids.

The last step we did was to separate the studies based on age of the subjects.

We did this because there is some indication that the elderly are less responsive to the effects of insulin.

We found that 3 studies found no increase in muscle protein synthesis in the elderly, but one study did. Interestingly, that study used the highest dose of insulin of all four studies.

Therefore, our fourth conclusion was:

Insulin may be less effective to stimulate muscle growth in elderly subjects

So does it work or not?

So in the end, we took these 4 factors into account: – whether amino acids were being infused at high doses along the insulin – whether amino acids in the blood dropped – whether the concentration of insulin in the blood was normal or supraphysiological – whether the subjects where young or old

This allowed us the get the best possible picture of what insulin does without a lot of other variables that might influence the results.

Despite that, it still wasn’t entirely clear what the effect of insulin was.

We were left with 21 studies, of which 8 concluded that insulin improved muscle growth, but 13 concluded it did not.

Therefore our fifth conclusion was:

The majority of evidence suggests that insulin does not increase muscle growth in healthy young adults

Practical Implications

So how can we apply what we’ve learned.

Let’s go back to the bodybuilders who inject insulin. They inject insulin at incredibly high doses that cannot be produced in our body naturally, and yes that approach seems to be effective to increase muscle protein synthesis. Warning: it’s extremely dangerous and NOT recommended.

So what about regular fitness people who use dietary approches to manipulate circulating insulin levels?

In the fitness world it’s common advice to eat carbohydrates after resistance training to stimulate insulin production which is supposed to stimulate muscle growth.

However, the majority of the scientific literature suggests that it won’t help.

It’s much more important to get your protein after resistance training.

A protein shake or meal provides you with the amino acids needed to stimulate muscle protein synthesis. And as my review pointed out, there’s not a single study that shows that insulin has any benefit for muscle protein synthesis when you have enough amino acids in your blood, which is the case if you drank your protein shake.

In line with these insulin injection studies, the addition of carbohydrates to protein also has no effect of muscle protein synthesis (1).

What about muscle breakdown?

Now some of you will scream: but muscle protein synthesis is only one side of the coin! What about muscle protein breakdown? I’ve heard insulin slows down muscle protein breakdown!?

Indeed, it’s pretty well established that insulin reduces muscle protein breakdown.

However, this effect occurs at low levels of insulin with no further effects at higher levels. Eating a decent protein meal will already increase insulin levels high enough for the maximal effect (2).

What about muscle glycogen?

So carbohydrates and insulin are not directly helping you to build muscle.

But what about carbohydrates to recover muscle glycogen (storage form of carbohydrates in the muscle), which in theory could help increase training volume?

Muscle glycogen repletion is one of the most important factors in recovery from endurance-type exercise.

However, resistance training with moderate volume (6-9 sets) only depletes ~36-39% muscle glycogen (3) (4) (5). While some athletes have considerable larger training volumes, it usually comes at the expensive at training frequency. For example, body part splits train muscles with very high volume, but only train each muscle group once or twice a week.

Ironically, unless you’re on a low carb diet, muscle glycogen should always be replenished between resistance training sessions for the same muscle group.

Bottom line: there’s really no need to focus on manipulating insulin through dietary means if you want to build more muscle.

However, don’t mistake this as a ”avoid carbohydrates” message. Carbs are absolutely fine, and if they fit your macronutrient plan, you might as well have some surrounding your training.

Now here’s what I want you to do…

I would love to hear from people who heard that it’s essential to eat carbohydrates after resistance-training ASAP because the insulin release is crucial for optimising the growth response in this ”anabolic window”.

Leave a comment.

Eur J Endocrinol. 2015 Feb 2. pii: EJE-14-0902. MECHANISMS IN ENDOCRINOLOGY: Exogenous insulin does not increase muscle protein synthesis rate when administrated systemically: a systematic review. Trommelen J, Groen B, Hamer H, de Groot LC, van Loon LJ.

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