Metabolic Flexibility, Intermittent Fasting and the New England Journal of Medicine
So, there’s is A LOT to unpack in this post but I’m going to tackle, yet again, the benefits of intermittent fasting, explain “metabolic flexibility” and share some highlights from an exciting publication from the prestigious New England Journal of Medicine.
A little background on human energy sources:
Humans have been around quite a while. Humans need to eat food for fuel. Eating provides fuel for the body’s organs to work. The human brain is relatively small (about 2% of our weight) but also very complex and a bit of an energy pit, using about 20-25% of our energy resources…..which is a ton compared to most other animals, as human beings are capable of doing very complex processes. We need energy to feed these brains and the rest of our bodies.
Following so far? Nothing too complicated yet.
Most of the scientific community argues that glucose (aka carbs) is the brain’s preferred fuel source. Whether it’s “preferred” or not, the brain and body will preferentially use glucose over any other energy source (such as fat) when glucose is available. This is likely an innate survival mechanism as glucose is typically not available in high amounts in nature (unless you come across a honeycomb or something in which about 30% of it is glucose). Now, our hunter-gatherer ancestors would not have balked at eating carbs if and when they were available, and, in fact, they did eat carbs depending on their climate and other factors. They ate things such as fruit (which were much smaller, less sweet and packed w/ more nutrients than modern fruit), some tubers (potatoes) and the occasional treat of honey if/when they were lucky enough to find some. Hunter-gatherers also hunted game (both big and small), fished and ate various species of insects (all depending on their geographical area).
Now, hunter-gatherer groups were not always successful in the hunt (for those of you who hunt you can sympathize!). They also weren’t always all that successful at gathering (think droughts, ice/snow, insect infestation, etc), and one thing is for darn sure is that they weren’t pounding hundreds of grams of carbs a day to supply their brain and bodies with sufficient energy to not only stay alive, but thrive and reproduce. So how did ancient humans stay fueled in the absence of glucose (carbs)?
FAT. As in F-A-T.
Fat? Yes, fat. In the absence of carbs (or very little carbs) and when the body’s liver and muscles are mostly depleted of glycogen (the stored form of carbs in our liver and muscles), the body will convert its own bodyfat into energy in the liver by making ketones. This adaptation was essential to the survival of humans as food, especially energy dense foods, was not always readily available in the pre-agricultural era. Fast forward to 2020. In 2020, we (industrialized nations) are constantly in the FED state. We rarely go very long without eating, except for maybe when we sleep. Hungry? Walk to the kitchen and grab some chips. No food at home? No problem! Drive down to the Qwik-e-Mart and grab a snack or go through the drive through of a local fast food joint. Most Americans are following the Standard American Diet (SAD for short) and eating lots of refined carbs and sugars, unhealthy fats and not enough protein. Unlike our hunter-gatherer ancestors, we are always eating food sources which are energy dense but nutrient poor! This means the foods of the SAD diet are packed with calories but lacking in nutrition. This causes blood sugar swings and increases in leptin (the hunger hormone) and leaves us constantly hungry and needing to snack. Since these cheap, nutrient-poor foods light up the “reward” centers of our brain (once again, we are designed to WANT to consume energy dense foods as a primal survival mechanism from a time period when a plethora and never-ending supply of energy dense foods did not exist, as it does today) we don’t know how to stop eating them! If you are addicted to this endless cycle of hunger, snack food and processed, nutrient-poor foods, IT’S NOT YOUR FAULT. Yes, I said it, IT’S NOT YOUR FAULT! You are designed this way! These adaptations were designed for survival in ancient times of food scarcity. But, as you know, food is no longer scarce. So, how do you break this revolving door of high-carb, poor qualify food, blood sugar swings and non-stop hunger?
Keep reading and we will get there.
Metabolic flexibility. It has a nice little ring to it. It sounds sorta fancy yet kind of complicated. What exactly does “metabolic flexibility” mean?
Metabolic flexibility is the ability for humans to effortlessly go between glucose (carb)-based energy system and fat (ketone) based energy system. As I alluded to above, most Americans are stuck in the glucose-based energy system. And if they go too long without more carbs they feel tired, irritable and hungry, also known as HANGRY. Metabolic flexibility is a sign of metabolic health. It means you have adapted to utilizing various energy systems depending on what you are doing and what you are eating (or not eating). It means you are avoiding the constant carb/high glycemic roller coaster that most Americans are stuck on. It means you are limiting the number of unhealthy carbs in your diet and going longer periods of time without eating, allowing your body to tap into its fat reserves for fuel. Our bodies, at most, contain about 350-500 grams of glycogen (stored carbs) in the muscles and maybe an additional 100 grams in the liver. That’s a total of about 1400-2000 calories. Fat, on the other hand, is PLENTIFUL (some of us more than others) where even relatively lean people can have as many as 100,000 stored calories from fat! Absolutely astonishing! If you deplete your glycogen (carb) stores then your body will start using fat as fuel! If your glycogen stores are always full, you will NEVER lose body fat!! This is why fairly low-carb eating, fasting and moderate exercise are so darn effective! Keeping carbs on the low side (in comparison to SAD), going longer periods of time without eating and exercise all works to DEPLETE your glycogen stores and tap into your fat-burning potential!
Stay tuned for part 2!