Fasted Training: What the Evidence Really Says
Fasted training is currently a pretty polarized topic in the fitness world, with some arguing that it can provide meaningful improvements for fat loss, and others claiming it will ruin your performance potential or cause hormonal dysfunction in women. Like most of these things, the truth of most of these claims lies somewhere in the middle. This is a pretty broad topic, so I am going to try and focus on some of the more common claims I have seen recently and hope that in addressing those, I will provide a decent overview of the topic. So here are the main claims I want to look at:
Fasted training improves fat loss
Fasted training ruins training adaptations
Fasted training does not work for women
Throughout this article, I will be referring to fasted training as training after a period of fasting 8 hours or longer.
Fasted Training Improves Fat Loss
I think of this one as the OG fasted training claim, bodybuilders have been using fasted training as a fat loss tool since we were in the caves, with the belief that it will get them leaner than training in a fed state. The idea here is that when we are fasted, insulin levels are low, glycogen is depleted, and therefore we are more likely to use fat as fuel during exercise which will then lead to greater overall fat loss. The first part is true, fasted cardio does acutely increase fat oxidation during the training itself. However, this does not seem to lead to a greater overall loss of fat. Our bodies are pretty damn good at shifting energy sources around so that, in the long term, only a net caloric deficit impacts whether fat loss occurs or not.
I like to think about it like balancing a budget. Let’s say I have $100 per day to use, and I can spend that on either a credit card, debit card, or with cash. Is there a difference between these two spending scenarios?
$20 cash, $40 debit, $40 credit
$0 cash, $60 debit, $40 credit
In either scenario, you are using the entire $100. Spending cash and putting something on credit might feel different, but in the end you are still spending the same money. You will simply have to shift things around in your bank account to ensure balances are not negative. The same thing is happening in your body, the source of energy during your workout does not make an impact on overall weight management as long as the total daily calorie intake is accounted for.
Fasted Training Ruins Training Adaptations
For the more performance-oriented folks reading this, I am afraid the answer is a little bit more complicated. For this discussion, we will need to talk about endurance-based adaptations and resistance training adaptations separately.
Lifters
There are two main fasting-related factors that have the potential to influence lifting performance and long-term adaptations. The first is the availability of glycogen (stored carbohydrate in the liver and muscle). The second is being hungry for your lifting session.
Traditional resistance training is going to primarily rely on carbohydrates as fuel, so it makes sense to think that being low on carbohydrates going into a session (via a fast) will decrease your performance, and therefore your long-term gains. And we have some decent evidence to back up this claim. A 2022 meta-analysis of 22 studies found that pre-exercise carbohydrate ingestion had a significant effect on resistance training volume performed when the fast was over 8 hours and the training session was longer than 45 minutes(or over 4 sets to failure per muscle group). One thing to note here is that the confidence interval for this effect is quite wide, spanning from 0.11 to 1.11, with a point estimate of 0.61. What this means is that our estimate of the effect of pre-exercise carbs on lifting volume is 0.61 (moderate effect), but it is just as likely to be anywhere from 0.11 (trivial effect) to 1.11 (large effect). This is simply a reflection of how varied the outcomes in each of the individual studies included in the meta-analysis are. In summary, we can be pretty confident that there is SOME positive effect here, but the magnitude of that effect could either be very small or very large, and we would need more research to get a more precise estimate.
From the research we have at the moment, the chances of a pre-workout feeding improving your performance increases the longer you fast and as the length and volume of a session increases. However, I want to point out that this meta-analysis looks specifically at single session performance. While it would make sense that this would eventually translate into better gains in the long-term, we cannot make that conclusion from this data specifically.
Interestingly, the effect size of pre-exercise carbohydrate in this meta-analysis (SMD = 0.61) is larger than what we see with pre-exercise caffeine (SMD = 0.20). So if we were to take the effect size at face value, and you are doing a high volume lifting session in the morning, it may benefit you more to grab a banana over a coffee. Or you could do both!
Endurance
It is no question that consuming carbohydrates prior to prolonged (>60 minutes) aerobic training will enhance performance in that session. However, maximizing objective performance during each individual session does not automatically mean better long-term adaptations. I understand this is confusing at first, but think about zone 2 training as an example. If I were to go out and try to do those easy running sessions as fast as I could in an attempt to attain the best possible outcome as measured by my average pace, it would run counter to the entire point of zone 2 training. When we are doing zone 2 training, we are seeking a very specific set of adaptations (improving fat oxidation, increasing mitochondrial density, increased capillary density, etc.).
So yes, consuming some carbohydrates before an easy run will let you run a faster pace at the same effort level, but is that better for long term performance? Maybe not. We know from the mechanistic research that pre-training carbohydrate blunts the expression of adipose tissue messenger RNA (mRNA) and protein expression of key enzymes involved in lipid metabolism. These are all responses that we want from our aerobic training that eventually allow us to become more fit. This has caused some to theorize that we should complete our low intensity sessions in a fasted state (low glycogen availability) and complete our higher-intensity sessions in a fed state (high glycogen availability), so that we can get the best of both worlds. I wrote about this extensively here. To summarize the main points, the research here is a bit inconclusive. We do see better cellular responses from training when in a fasted state, but it is unclear if that eventually leads to better performance in the long run.
For high-intensity sessions, you should absolutely be fed in order to get the most out of those sessions. For easier sessions, training in a fasted state might be beneficial, but the jury is still out on that one.
Fasted training does not work for women
It is true that women tend to be underrepresented in exercise science, a fact that has lead many to suggest that the current exercise guidelines that are based on that research cannot be used to make evidence-based recommendations for women. While there is a grain of truth to this, several prominent exercise science influencers(most notably Dr Stacy Sims) have used this, along with questionable mechanistic research, to claim that fasted training does not work for women(that it increases fat storage), or that fasted training causes lean mass loss. So is there any truth to these claims?
This topic has been covered in great depth recently by both Dr. Lauren Colenson-Semple and Dr. Alyssa Olenick, so I would recommend checking out their recent posts on this, but I will attempt to synthesize some of the main points.
The central idea behind these claims is that there are unique aspects of female physiology that cause different responses to fasted training when compared to men. This claim usually relies on mechanistic and/or acute responses to fasting and training. For example, here is an excerpt from a Stacy Sims article outlining the rationale for this claim:
“When the brain perceives a drop in nutrient availability—especially carbohydrates—significant changes within neuropeptide production and expression happen, which stimulates greater conservation of energy stores. Add exercise stress into the mix and additional hormone and neuropeptide changes occur. Acutely, a physiological drive to increase food intake appears to be greater in women than men. Longer term, if energy demand is unmet, thyroid activity drops, menstrual cycles can become irregular, and an increase in body fat is often observed.”
Sounds pretty convincing, and you can even find some evidence supporting some of those claims about post exercise hormonal changes. So if this line of reasoning holds up, we should expect to see fasted training meaningfully impact the outcomes we care about, like muscle growth and fat loss, right? Well, when we look at these outcomes that doesn’t seem to pan out. It turns out, that as long as overall daily calories are sufficient, being fasted or fed does not have a negative impact on muscle growth or fat loss (one, two, three, four). While I must concede that we are limited to looking at a handful of individual studies at the moment, and therefore cannot conclude anything too confidently, the idea that you could claim unequivocally that fasted training causes all these negative downstream effects on women is simply not supported by the evidence.
What is happening here is something that happens all around the science communication side of social media. Prominent influencers will weave very compelling storylines about mechanisms or acute hormonal responses to something (many times in animal models), and make assumptions about how those changes will affect the outcomes people care about. However, in doing this they completely ignore the existing research that looks at the outcome itself. They get so lost in the story they’ve built for themselves (and their brand on) that they cannot reconcile it with what is happening when people actually do the thing in real life.
The graveyard of exercise science research is filled with compelling mechanistic rationales of cellular pathways that did not pan out. For example, early studies suggested that limiting resistance training rest periods to ~45 seconds would maximize hypertrophy because it elicited the largest growth hormone and testosterone response in the post-exercise period. I even remember seeing this confidently claimed in many textbooks I read through my academic years. Despite my personal experience telling me that performing sets of deadlifts 45s apart is a fate worst than death, I had to go with what the ‘science’ said. Yet, subsequent longitudinal research (looking at the actual outcomes of muscle growth and strength) demonstrated that these acute hormonal surges bore little relationship to actual muscle growth, and in fact, longer rest periods (2-3 minutes) often enhanced hypertrophy by allowing greater training volume.
In practice, even the most elegant and confidently espoused biochemistry often crumbles under the weight of randomized trials, reminding us that physiology in theory rarely survives contact with training in the wild.
Written by: Chris Bonilla, PHD(C), Black Iron Performance Coach
