Episode 88 – Low Carb Diets: Effects on Strength Training

Defining what ‘low carb’ actually is  

  • There are some pretty loose definitions floating around. Sometimes research classifies anything <45% of calories coming from carbs as a low carb diet. But that’s obviously not what we are talking about. That is still relatively high or ‘normal’ 
  • We are talking about intentional restriction of carbohydrates. For the sake of this episode, we will be referring to <20% of calories coming from carbs as a low carb diet. 
  • We will also be referencing a lot of research specific to keto diets. Although that is specific to very low carb diets, the same concepts can also apply to kind of low carb diets in some cases. 
  • Keto diets are often thought of as <50g of carbs. But depending on size, activity and other factors, it could be significantly higher or lower than that number. 

Why might low carb diets be sub-optimal for muscle growth: 

Part 1

  • Assuming that calories and protein intake are equal, there are still a few things that could theoretically be issues. 
  • The first one is that insulin is anabolic, but insulin will be lower on a low carb diet. Personally I think this a bit of a non-factor. While we clearly see insulin being used as a performance enhancer when it is used by enhanced bodybuilders via injections. From the research I’ve seen in natural athletes, the stuff we do to intentionally increase insulin production in our body doesn’t seem to make much difference. This is because injections often involve far higher levels of insulin than will be naturally produced. 
  • See blog post from Nutrition Tactics below which goes deep on this topic.  

Part 2

  • The second issue is that low carb diets will likely involve lower glycogen stores than an optimised higher carb diet 
  • Lifting weights burns through less glycogen than endurance stuff, but it might still be relevant.
    • The most commonly cited study involved 6 sets of leg extensions to failure, which resulted in a 38% decrease in glycogen of the quads.  
    • Something that is often overlooked in that study is that it depleted glycogen more in type II fibres than type I, which means some fibres were depleted over 50%. 
  • This raises a few questions – 1) What if you are doing more than 6 sets of a muscle group in a session. 2) What if you are coming from a place of sub-optimal glycogen stores? 3) We know performance decreases as you get lower in glycogen. But how much does this effect your performance in your individual case? 
  • And then an important question – although this likely means a low carb diet will result in slightly worsened performance at the end of sessions – does that really matter much for muscle growth? If you are taking sets to a similar proximity of failure, it might not even matter much. 

Does ketosis change anything about this? 

  • There is an argument to be made that going into ketosis and using ketones as fuel will improve performance vs a low carb diet without ketosis 
  • We clearly see drops in performance when people transition to ketosis and then it usually picks back up after the transition phase. 
  • So arguably it is better than a low carb diet with just enough carbs not to enter ketosis. But we will discuss research on whether it is better than a higher carb diet. 

Looking at the Research: 

  • Before discussing individual studies we will address two logistical challenges 
  • The first one is that in research, and in the real world, the vast majority of people attempting low carb diets end up in a calorie deficit. 
  • We know muscle growth occurs a lot more easily in a surplus, so this is a huge confounding variable outside of tightly controlled conditions. 
  • The second logistical challenge is that in the real world and in a lot of studies, protein intake often increases when people switch to low carb diets – which can also influence muscle growth outcomes 

Obviously there are heaps of studies we can talk about, but we have just picked a few that are interesting. 

  • The first study we will talk about is a great example of both of these challenges. 
  • It involved a group of women who did resistance training 2x per week for 10 weeks. One group did a keto diet with 22% of calories coming from protein and the other group had a higher carb diet with 17% of calories coming from protein. 
  • Already that’s a red flag, because if the keto diet outperformed we couldn’t tell if it was because of ketosis or because of the protein difference
  • In terms of outcomes, the keto group lost 5kg and had no change in lean mass. The high carb group gained 800g and had a 1.6kg increase in lean mass. 
  • Clearly the keto group ended up in a decent sized deficit, which made this study pretty irrelevant for what we care about. 
  • In a perfect world, we would want studies to have the same calories and protein intake.   

Keto vs. Western Diet – Bodybuilders 

  • A 2021 study had – 19 offseason natural bodybuilders randomly assigned to keto or standard western diet groups for 2 months.  
  • Body fat significantly decreased in the keto group, while lean mass only increased in the western group. Max strength increased similarly in both groups.  
  • Both diets were designed to be the same calories and similar protein. Bodybuilders are well known for compliance with nutrition, but even in this case, they appeared to end up in a deficit on the keto diet.  
  • The authors speculated that this could be because of an increase in energy expenditure due to a spontaneous increase in activity on a keto diet.
    • I speculate that it is because of lower calorie intake, but there is no way to know for sure. Either way, it continues the theme of a lack of muscle growth, which is seen in almost all low carb studies. 

Keto vs. Western Diet – College-aged men 

  • The most promising keto study I’m aware of involved 25 college-aged men, where they compared a keto diet to a western diet with the same calories and protein for 10 weeks, followed by adding carbs in week 11 to match for some variables. 
  • At the end of week 10, both groups increased lean mass by 2.4% and 4.4% respectively. 
  • If you looked at that without context, you would assume the western diet outperformed.  
  • But in week 11, carbs were added to the keto diet and it ended up being a 4.8% increase in lean mass from the baseline. 
  • This is because adding carbs increases lean mass in the form of water and glycogen.  
  • Both groups dropped similar amounts of fat mass, with the keto group losing 2.2kg of fat, and the western diet group losing 1.5kg. 

Practical Thoughts: 

  • A low carb approach seems to be fine for building muscle assuming you are able to consume sufficient calories.  
  • Is it optimal? To be honest we don’t have enough of a sample size of people doing it in research to even have a good comparison.  
  • I would speculate that it is slightly sub-optimal due to the glycogen aspect, but even then I would assume results would not be too dissimilar.  
  • It is likely just that the logistical challenges of consuming enough calories, for most people, which would be the bigger factor. 

Relevant Links/ Resources

Studies Mentioned:

Leg extensions and muscle glycogen

Ketosis and resistance training in women aged 20-40

Bodybuilders and ketosis

CrossFit and ketosis

Tightly controlled ketosis study with calories and protein matched

Relevant Blog Posts:

Insulin and Muscle Growth