If, when you hear the term “metabolic flexibility,” you wonder whether it has something to do with stretching or yoga, you’re probably not alone. Spoiler alert: it doesn’t. Over the past few years, researchers have attached this name to a feature of our physiology that has incredibly important consequences for our long-term health. So, what’s it all about -- and how to channel metabolic flexibility to be a force for good in your life? Here’s my in-a-nutshell guide:

Metabolic flexibility stretches way, way back.

Let’s start at the beginning. Over the course of our development as a species, we evolved to run on both the carbs and the fat our bodes store for fuel. Because our hunter-gatherer ancestors didn’t know where their next meal was coming from, or what it would be, or even if it would be, they had no choice but to be “flexible” when it came to their sources of energy. Our bodies found a way to ‘make it work’ with whatever came its way.

Modern diets impact metabolic flexibility.

Fast forward a few thousand years to the modern world, and scarcity is considerably less of a problem. But all too often, we can, and do find ourselves eating three carb-heavy meals a day with carb-y snacks in-between, one big consequence being that we’ve become fairly well hooked on the stuff. As a result, our bodies get less and less good at tapping into fat and burning it for energy, so much so that when we burn through our body’s carb stores – and it doesn’t take very long— it doesn’t readily switch to fat. We become literally weak with hunger. It's as if your backup generator doesn’t kick on when the power goes out.

Got the ‘hangries’?

Maybe you’re the person who can’t go more than a few hours without eating something to forestall the dreaded “hangries.” Or maybe you’re a recreational aerobic athlete who “bonks” after an hour or two on the bike unless you gobble a Clif Bar. If so, you (and too many of us) are likely metabolically “inflexible.” And that’s not just an inconvenience – there are bigger downsides particularly if you’re on the sedentary side. Eating an abundance of fast-digesting, high-sugar carbs, especially when you’re not burning them off with exercise, keeps both insulin and blood sugar levels high. Over time, that sets the stage for weight gain, prediabetes and even diabetes, and a dramatically increased risk of heart disease.

Think ‘clean’ energy…

What’s the solution when the carb stores are depleted and your body is running on fumes? Flip the switch and start burning fat! In much the same way as a Prius hybrid switches back and forth between burning electricity and gas depending on the demands of the drive, the body of the metabolically flexible person switches over to fat-burning when carb stores are low. And fat is a “cleaner” (no insulin or blood-sugar spikes) and much longer-lasting fuel, which begs the question, how to flip the switch?

…Now, burn it!

Though “improved fat-burning” may sound like a Madison Avenue come-on, really, it’s just basic physiology. Simply put, your body will burn fat more effectively when you train it to do so, either through diet or exercise. Looking for the most impressive results? Then combine the two. Here are a few methods to help you stoke the flames of metabolic flexibility:

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1. Go low on carbs: While there is no “one size fits all” diet that’s perfect for everybody, and you may need to experiment a bit, when it comes to keeping carbs low, the 50 -100 grams per day range is a reasonable baseline – and far less than the 250 grams or more many people eat each day. The fastest way to cut back? Keep sugar and processed foods off your plate. Why go low? Because that means, when your body burns through the relatively modest amount of carbs you’ve recently consumed, or stored in your muscle cells as glycogen, it will turn to the dietary fat you’ve eaten or the fat stored in your fat cells for energy. This flexible, Prius-like switch-over is great for keeping you energized, your blood sugar stable and weight in a better place, so it’s a great place to start. From their Steer clear of processed foods and sugar.
2. Intermittent fasting and time-restricted eating: Mildly stressing the body to keep generating energy with little or no food coming in, is another way to train your body to burn fat. Two time-sensitive approaches to eating: intermittent fasting, or IF, and time-restricted eating or, TRE. In practice, they mostly overlap but both are about lengthening your fast – and shortening your daily eating ‘window.’ The easiest way to do it, without having to go to extremes, is simply to lengthen the day’s natural fast, the time between dinner and the aptly named “break-fast.” A traditional rule of thumb is to allow twelve hours between the two meals, for instance dinner at 7 p.m. and breakfast at 7 a.m. But that’s really a minimum. Work towards expanding that daily fast, up to 16 hours, three to four times a week, with an earlier dinner and/or a later breakfast. The pay-off will be a re-set of your now more efficient, and flexible, metabolism – think, lower blood sugar and insulin levels, less fat build-up.  
3. A keto diet:  It’s the more extreme version of a low-carb diet, and in my view, a decent choice for those who can stick with it. But here, you’re burning ketones for energy, so that’s different than switching back and forth between fat and carbs which is the hallmark of conventional metabolic flexibility. Some health coaches recommend “cycling keto,” where you might eat a high carb diet for one day a week, so your body doesn’t have to completely forgo carbs as an energy source and can handle them without digestive upset.
4. Go long, and slow(ish): It’s no secret that I’m a big fan of working more movement into your daily routine, multiple times throughout the day. It might be walking more, doing the stairs a few times during the day, some yoga poses between meetings, etc. – but not overdoing it with intense daily work-outs that can leave you feeling washed-out and, counterproductively, stimulate the release of the stress hormone cortisol. But bolstering your everyday doses of frequent movement with longer bouts of steady exercise – it could be a long, brisk walk, or a jog, cycle, or swim -- is a great way to burn through your glycogen and tap into your fat stores. Not only are you burning up the excess sugar in your system and keeping your blood-sugar levels nice and low, your muscles stay more sensitive to insulin for a full day after the work-out is over. Your body does more with less insulin which is exactly what you want.  Whether it’s an hour-long-walk or a low-carb diet (or even better, both), the great thing is that you’re mildly stressing the body in a healthy way (“hormesis”) that promotes metabolic flexibility.
5. Support metabolic flexibility with bioactive compounds: No matter what fuel mixture you’re burning at any given moment, keep in mind that the “internal combustion” takes place in the tiny power plants inside the muscle cells, the mitochondria. Keep them happy – and the power ‘on’ – with plenty of magnesium. Get yours from dark leafy vegetables and nuts and seeds; omega 3 fatty acids from small fatty fish like salmon and anchovies are tops; and polyphenol-rich veggies (eat the rainbow!)