Newsletter #309: Are There “Non-Responders” to Aerobic Exercise?
Recently, there has been a surge in interest in moderate, steady-state aerobic exercise, even among people who aren’t into endurance sports. This is popularly known as zone 2 training, because it usually targets the second training zone, at around 60-70% of your maximum heart rate.
This training zone is widely perceived as a sweet spot, because it’s at just the right intensity to drive mitochondrial biogenesis, as well as beneficial alterations in the heart muscle itself, while incurring very little strain on the body. Thus, you are able to maximize your recovery while accumulating enough training volume to elicit the physiological adaptations that make you able to go faster and longer.
However, this training approach has also received some pushback.
I’ve heard some people suggest that a substantial portion of the population, as much as 20-40%, are “non-responders” to zone 2 training, meaning that they fail to significantly improve their cardiorespiratory fitness by adhering to this sort of regimen.
Considering the amount of time that traditional aerobic exercise involves, this is a particularly demoralizing idea. Imagine diligently running or biking every day, hoping to boost your fitness (and health), yet failing to get a payoff for your hours of toil.
But is this even true? Where does this notion come from?
Well, first off, responses to endurance exercise interventions do vary considerably between individuals. In other words, if you make a bunch of people perform the exact same training program on a bike, they will each experience different levels of improvement in cardiorespiratory fitness. The reasons for this are complex, and are still being elucidated. We do know that an estimated 47% of VO2max responsiveness is heritable. So, due to a combination of lifestyle and genetic factors, no single person will respond to aerobic exercise in exactly the same way.
The concept of “non-responders,” in this context, seems to emanate at least in part from a large exercise trial known as HERITAGE. In that study, 481 adults engaged in supervised endurance training at 55-75% of their max heart rate, ultimately working up to 150 minutes per week. After five months, exercise testing revealed a broad range of responses, with a significant portion showing little improvement.
So, it seems that some people do not respond to that relatively modest weekly dose of moderate aerobic exercise. But does that necessarily mean that they are non-responders to zone 2 training as a whole?
Maybe not. When exercise trials expose groups to different amounts of exercise, and then measure subsequent performance improvements, the number of individuals who can be classed as non-responders goes down as the dose of exercise increases. This suggests that perhaps people who seem to be non-responders need to train a bit longer, or more frequently, in order to experience the same beneficial adaptations. However, methodological issues in these studies have made it difficult to say for sure whether simply increasing the duration/frequency of training would resolve the phenomenon of non-response.
Fortunately, researchers in Switzerland designed a very clever trial that answers this question.
This Week’s Research Highlight
In “non-responders” to aerobic exercise, increasing total training volume results in unanimous improvements in cardiorespiratory fitness.
To examine whether apparent non-response to exercise could indeed be explained by inadequate exercise stimulus, Swiss researchers decided to collect a group of non-responders, and then expose them to a higher dose of exercise. To that end, they recruited 78 healthy young male volunteers from the University of Zürich, and put them through a two phase endurance exercise intervention using a stationary bike.
The first phase was designed to capture all of the men in the group who would meet the criterion of a non-responder to aerobic exercise. In this phase, all of the participants were split into five groups, which differed only in the number of training sessions per week.
- Group 1 — One exercise session per week (60 min total)
- Group 2 — Two exercise sessions per week (120 min total)
- Group 3 — Three exercise sessions per week (180 min total)
- Group 4 — Four exercise sessions per week (240 min total)
- Group 5 — Five exercise sessions per week (300 min total)
The guys stuck to this schedule for 6 weeks. Importantly, all of the cycling sessions were supervised, ensuring perfect adherence.
Let's take a look at the results from this phase. The vertical axis represents the percent change in maximal power output on the bike (which indicates improvements in fitness).
Okay, so one thing that jumps out right away is that the magnitude of response varied across all subjects, and even within the groups. This reinforces that two people, assigned to the exact same exercise regimen, will not see the same fitness gains. Life can be unfair like that.
The lavender shaded section encompasses a response of zero change in maximal power output, plus the typical error of measurement (4%). That means that any dot that falls within this section represents a non-responsive participant. (Some unlucky subjects appear under that bar, indicating that their performance somehow got worse.)
From group 1, 69% (11 out of 16) were non-responders; in group 2, 40% (6 of 15) were non-responders; finally group 3 had 29% (4 of 14) non-responders.
An obvious pattern emerges here, as we alluded to previously — less exercise volume is associated with greater likelihood of being a non-responder. Meanwhile, there were zero non-responders found among those who underwent four or five exercise sessions per week, or an accumulated weekly total of 240-300 minutes.
Through this initial phase, the researchers were able to identify 21 individuals who would normally have been labeled as non-responders in aerobic exercise trials.
Next, we move to the second phase.
The researchers had all of the non-responders go through another 6-week exercise intervention, except this time they were assigned two additional training sessions, thus increasing their total weekly exercise volume by 120 minutes. So group 1 increased to 3 sessions, group 2 increased to 4 sessions, and group 3 increased up to 5 sessions.
Here’s what their performance looked like after another 6 weeks of training.
Again, the lavender shaded section represents non-response. And since we are looking exclusively at the non-responders, every dot starts out in that area or below it.
But after the 6 weeks of increased volume, every single one of them finished above the shaded bar. Boosting the amount of exercise completely abolished non-response.
To put it another way, none of these guys were truly non-responders. They just didn't achieve the right stimulus to spur adaptations.
Indeed, it's likely that there are no true non-responders to aerobic exercise.
As the researchers summarize: "The potential for [cardiorespiratory fitness] improvement may be present and unveiled with appropriate exercise training stimuli in healthy individuals without exception."
Random Trivia & Weird News
Many gyms have literally built their business model around a majority of their members not showing up.
In a lot of businesses (think mobile games), the 80/20 principle can be observed, wherein around 80% of profit comes from a small portion of highly engaged customers.
But with commercial gyms, the opposite is often the case — most of the profit comes from people who are hardly ever in the building.
For instance, Planet Fitness reportedly has an average of 6,500 members per gym, but each individual gym is only capable of holding about 300 people! In other words, they need to attract thousands of customers who are aspirational enough to keep paying their fees, but not so engaged that they actually participate on a regular basis.
As you can imagine, this drives some very strange incentives at such venues.
(On the other hand, it also helps control the cost of belonging to these gyms. If you stop and think about it, those of us who do show up every day at commercial gyms should be quite grateful that our memberships are, in effect, being subsidized by masses of donors.)
Podcasts We Loved This Week
- Iñigo San Millán: Zone 2 training — the science between peak metabolic health. Via The Proof.
- David Bishop: Volume vs intensity. Via Inside Exercise.
Products We Like
MRM Super Foods Matcha Green Tea Powder
Matcha is a powder made from finely grinding green tea leaves. When you use it, you are consuming the whole plant, rather than just an infusion like when you brew green tea. As a result, you wind up getting far more of the bioactive catechins, like EGCG.
I like this brand in particular because it has been third-party lab-tested for quality, safety, and flavonoid content. They also offer it in capsules, but I think the the powder is easy enough to use; you can even add it to juice, smoothies, yogurt, etc.
humanOS Catalog Feature of the Week
The How-to Guide to the Mediterranean Diet
In this reference sheet, we go over the fundamental principles of the Mediterranean diet, what components of the diet make it healthy, and what sorts of foods and beverages you should consume in order to achieve the best possible version of this dietary pattern based on the current scientific literature.
Great if you’re looking for a basic, efficient guide to how to Mediterraneanize your eating habits. For a little bit of a deeper dive, you can refer to our full Mediterranean Program, which delves into the background of the Mediterranean diet as well as the clinical research.
Wishing you the best,
