How (and Why) to Boost Your “Metabolic Flexibility”

One of the biggest health stories of the 2010s was the idea that “sitting is the new smoking”—that is, that too much sedentary time throughout the day is bad for your health no matter how much you exercise. The epidemiological evidence was clear, but figuring out what to do about it—how often to stand up, for how long, whether you need to move around, and so on—turned out to be trickier. Even now, there’s no consensus about the best anti-sitting protocol.

But research continues, and a new study from researchers at the University of Turku in Finland offers some insights that intersect with a topic that’s of growing interest to endurance athletes these days: metabolic flexibility, which is the ability to switch seamlessly between burning carbohydrate and fats in different contexts.

What the New Study Found

The researchers recruited 64 sedentary adults between the ages of 40 and 65. All of them did less than two hours of moderate or vigorous physical activity per week, and had metabolic syndrome, meaning some combination of obesity, high blood sugar, high cholesterol, and high blood pressure. They all wore accelerometers for four weeks to determine their typical movement patterns and sedentary time, then half of them were assigned to reduce their sedentary time by one hour per day for the next six months.

That’s easier said than done, of course. Lots of studies have found that telling people to sit around less doesn’t improve their health, in the same way that telling them to exercise more or eat differently tends to have minimal effects. You have to make sure they’re actually doing it. In this case, there was no one-size-fits-all behavioral change recipe. The subjects had individual hour-long counseling sessions to figure out what tactics might work best in their lives, such as getting a standing desk, taking stairs instead of elevators, or going for light walks. The counselors then followed up with subjects multiple times over the following months to check progress and make adjustments if needed.

The results, which appear in the Scandinavian Journal of Medicine and Science in Sports, show that the subjects managed to reduce their sedentary time by an average of 41 minutes per day. That’s a victory in itself: it’s possible to change your habits! Reduced sedentary time improved their insulin sensitivity, which is a key marker of metabolic health and predictor of future problems like type 2 diabetes. More subtly, it also seemed to improve metabolic flexibility.

What Is “Metabolic Flexibility” Anyway?

In general terms, metabolic flexibility is the ability to switch between fuels—carbohydrates, fat, or even protein—depending on the circumstances. In an exercise context, the ideal scenario is that you burn mostly fat when you’re going easy, and mostly carbohydrates when you’re going hard. That’s because we have (no offense) a virtually unlimited supply of fat that can efficiently fuel moderate rates of exertion, compared to a very limited supply of carbohydrates that are ideal for short bursts of high intensity.

What you don’t want, if you’re an endurance athlete, is to rely mostly on carbs even when you’re going easy. That depletes your carb stores prematurely, leaving you unable to accelerate and more likely to bonk. Starting in the 1990s, sports researchers spent a lot of effort trying to figure out how to ramp up fat-burning to preserve precious carb stores. That’s the logic behind low-carb, high-fat keto diets for endurance athletes, since these diets significantly increase fat-burning.

The problem, though, is that this method of ramping up fat-burning seems to come at the expense of carb-burning, compromising your ability to throw in carb-fueled surges or climb steep hills. What you want, instead, is the best of both worlds: high fat-burning without losing the ability to draw on carbs when you’re going hard. This, in a nutshell, is metabolic flexibility for athletes.

How best to achieve this remains a topic of ongoing debate. Ultrarunner and coach Marco Altini has a great discussion of his personal experimentation on altering metabolic flexibility to improve his running performance. The approach that works for him involves periodized nutrition, modulating his fat and carbohydrate consumption based on his day-to-day training needs. If you’re interested in learning more, he recently gave an interesting interview on Niki Micallef’s podcast on this topic.

From a health perspective, metabolic flexibility isn’t just about burning enough fat. You also need to be able to ramp up carb burning after a meal, when blood sugar and glucose levels are high. If you can’t, that’s thought to be a precursor to insulin resistance and eventually type 2 diabetes.

How Do Sitting Time and Metabolic Flexibility Connect?

Reducing sitting time by (on average) 41 minutes per day was not a magic bullet for improving metabolic flexibility. The Finnish study had two different ways of measuring metabolic flexibility; neither of them was significantly better in the reduced-sedentary-time group compared to the control group. However, if you only look at subjects who successfully reduced their sitting time by at least 30 minutes (i.e. just half of what they were aiming for), this subgroup did manage to improve their metabolic flexibility.

What’s most interesting is to look at the mechanisms. The group that decreased sedentary time had lower lactate levels in their blood, and within that group bigger decreases in lactate corresponded to greater improvements in metabolic flexibility. According to an emerging school of thought associated with Spanish exercise physiologist Iñigo San Millán, lactate levels are a key marker of mitochondrial health, which in turn is an important determinant of metabolic flexibility.

San Millán’s recommendation to improve the health of your mitochondria (and thus boost your metabolic flexibility) is to do lots of “zone 2” exercise, an easy-to-moderate effort where conversation is just beginning to feel strained. That fits with the way most endurance athletes train. The new Finnish results support this picture of mitochondria influencing metabolic flexibility, and they suggest another way of improving it: not spending too much time sitting motionless at a desk or lying on the couch.

That still doesn’t give us a precise prescription for how much sitting we can get away with. Do we have to get up every 20 minutes? Every hour? Every few hours? Is standing up for 30 seconds enough, or do we have to do some deep knee bends or walk around for five minutes or stay standing for a full hour? I hope we’ll eventually get some good research-backed answers. For now, the fact that reducing your daily sedentary time by a mere half-hour produces measurable gains feels like good news: it suggests that any habit change you can sustain will help.

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