Metabolic Flexibility: The Holy Grail of Weight Loss

“Burn more fat!”

It’s the goal of every weight-loss program and the promise of every infomercial. And no wonder. Who doesn’t love the visual image of unwanted belly, hip and thigh fat being incinerated into oblivion?

But what does “fat burning” actually mean? And why is it so hard to accomplish?

First things first. Your body uses two primary sources of fuel: fats and carbs. Under normal, resting conditions, most of your energy comes from fat. The mix changes as soon as you start to move. If you’re sitting on the couch, you’re burning about one calorie per minute, most of which comes from fat. But if you get up from the couch and walk to the kitchen to get some chips, the ratio starts to change slightly. The percentage of the calories you’re burning from fat goes down and the percentage of calories you’re burning from carbs goes up.

If, instead of walking to the kitchen, you decide to sprint 15 minutes to and from the corner store to get those chips, then things change a lot. The number of calories you burn goes way up (compared to sitting on the couch), the proportion of those calories coming from fat goes way down, and the proportion of those calories coming from sugar goes up considerably.

How do we know this? Because we can measure it, using a metric known as respiratory quotient, which tells you how many of the calories you’re burning during exercise are coming from fat. In the early days of Equinox, we had a metabolic lab, where I worked with the world-renowned ultramarathoner and exercise physiologist Stu Mittleman. When we put clients on higher-fat, lower-carb diets, we’d see measurable changes in their respiratory quotient. They just became better at the metabolic skill of burning fat. (We’ll return to this in a minute.)

The best endurance athletes can go a long time without running out of sugar because they’re so good at burning fat. They’ve got the ATM code to their fat stores, so their bodies aren’t as dependent on a constant influx of sugar.

And that’s exactly what you — and I and every client I’ve ever worked with in my life — want. A body that’s got the ATM code for our fat deposits, so it can go in there and make withdrawals! That’s what we mean by a fat-burning metabolism, or, more correctly, a “flexible” metabolism. A healthy, flexible metabolism can handle anything. It’s the brass ring when it comes to health and weight loss.

Metabolism 101

Every moment of every day, you’re burning calories. You need calories for every activity, from thinking to square dancing to meditation to moonwalking. You even burn calories when you’re sleeping. And these calories come from food.

But think about this for a minute. At mealtime, we ingest a large number of calories, but where do they go? Clearly, we don’t need all those calories at the exact moment we decide to have dinner. If our bodies didn’t have a way to store those calories for future use, we’d be extinct.

Fortunately, that’s not the case. We store carbohydrates in the liver and muscles as something called glycogen. We store fat as something called triglycerides (which your doctor measures via a blood test). Triglycerides get packed into our fat cells — and to a certain extent, within the muscle cells themselves — where they wait patiently until the body needs them for fuel.

As mentioned earlier, these two sources of fuel — fat and sugar — are the primary source of energy (calories) on which our bodies run. Our bodies can use protein as a significant source of fuel, but it’s a really bad idea, like burning the sails on a sailboat in order to stay warm. Under normal conditions, the body gets about 15% of its cellular fuel from protein, which is good.

Your body can only store 1,800 to 2,000 calories’ worth of sugar (glycogen), but it can store a gazillion calories of fat (triglycerides) in your fat tissues. That’s because sugar is only intended to be used by the body sparingly — in emergencies. Sugar is the perfect fuel if you need a quick burst of energy lasting under 30 seconds because the body can grab that sugar instantly, while it takes up to 20 minutes for the body to mobilize a significant amount of fat.

Sugar is great in a pinch — but if you want sustained energy, you’re much better off using fat as your primary fuel. Nature knew what she was doing when she gave you (and me) a virtually endless supply of it.

Fat is precisely and exactly the perfect fuel to power our cellular machinery. It’s what we want our cells to run on. The question is, how do we access these storage tanks of fat that seem to pile up in the last places we need them on our bodies? And the answer is simple. If you want to burn fat, you’ve got to eat fat.

To Burn More Fat, Remove the Alternative!

Parents have asked me time and again: “How do I get my kid to eat healthier?” The answer is always the same: Take away the alternative. If all you have in your fridge is good food, your kid will complain for a couple of days, and then eventually they’ll eat it!

And so it is with fat burning. We have trained our bodies to be very good at running on sugar. The metabolic pathways that are involved in breaking down sugar and using it for energy are well-worn, like popular hiking trails in the Delaware Water Gap. We train these metabolic pathways to be more (or less) efficient, just as sure as we train our biceps to either get bigger by performing biceps curls or get flabby from lack of stimulation.

However, we have not done a very good job of training our bodies’ fat-burning pathways. We — and I include myself here — taught people for years to eat “mini meals.” To graze. To eat every two hours. To avoid fat and load up on carbs. To never ever skip breakfast. We all had sugar-burning metabolisms then, and we’d be crawling the walls if we didn’t eat something every two hours. But now we know better.

Clinicians I interview — and I’ve interviewed all the greats — routinely speak of getting their patients into what we call a “fat adapted” state, where they are more easily able to access (and “burn”) their fat stores. These folks are always surprised to find that they actually don’t need an energy bar every two hours once their body has dusted off the metabolic pathways needed to burn fat.

The recent craze over keto diets is all about fat-burning metabolisms. If you reduce your carbs enough, the body has no choice but to use the metabolites of fat burning, called ketones, just like your kid has no choice but to eat the “good” food in the fridge once you get rid of the cookies. Keto basically forces your body to go into fat-burning mode because ketones are only produced when you are actually burning fat.

What’s Keto-Friendly?

The keto diet is great for many people. But you don’t have to go keto to make your metabolism more flexible. Instead, consider going “keto-friendly.”

You can cut out sugar and processed carbs and you can increase good fats like extra-virgin olive oil, avocado oil, whole eggs and grass-finished butter. You can stick to low-sugar fruits like berries, start snacking on nuts instead of pretzels, and go heavy on the olive oil when you eat all those vegetables you should be eating in any case. A recent article in the Journal of Sports Medicine defined a fat-burning diet as one including 65% or so of calories from fat and 25% – or less – from carbs.

Remember, if you give your body mostly cereal, pasta and energy bars, it will get very good at using these quick-acting sugars for fuel. But you won’t like the way you feel – or the way you look.

You can eat a higher-fat, whole-foods diet whether you’re a vegan, a carnivore or — like most people — somewhere in between. All it takes is following one single mantra: Eat. Real. Food.

To the extent that you can cut off your body’s supply of toxic and useless processed food, you will also be reducing your metabolism’s dependence on glucose and will be teaching it how to use the most wonderful and abundant fuel it has available: fat. And that’s a very good thing — for how you feel and for how you look.