Why Your Body Can’t Burn Sugar and Fat at the Same Time: Understanding the Randle Cycle
For decades, we’ve been told to eat a “balanced” diet rich in carbohydrates, to fear fat, and to treat sugar as an innocent source of energy as long as it’s eaten “in moderation.” Yet when we look at how the human body actually works, a very different picture emerges. The truth is, your body struggles to burn sugar (carbohydrates) and fat at the same time. This tug-of-war is explained by something called the Randle cycle.
Here is something worth considering. Many of us grew up with decades of nutrition advice that we now find difficult to rethink. It’s not easy to question what we’ve always been told, yet approaching this with curiosity and an open mind can be powerful. What if we paused and looked at the top ten metabolic diseases so common today and asked ourselves whether they truly have to be part of modern life? Dr. Eric Westman often frames it simply: do we need more medicine, or could the real solution be as straightforward as changing the food we eat?
What the Randle Cycle Actually Is
The Randle cycle, also known as the glucose–fatty acid cycle, is a natural mechanism in your metabolism. It describes how your body decides which fuel to burn: glucose (sugar) or fatty acids (fat). Here’s the key: when one fuel is being burned, the other gets suppressed.
If you eat a carbohydrate-rich meal, your body shifts into glucose-burning mode. Insulin rises, fat burning is turned off, and the excess energy is more likely to be stored as body fat.
If you go without carbs or lower them significantly, your body shifts into fat-burning mode. Fatty acids are released from your stores and used for fuel, while glucose burning takes a back seat.
This is metabolic efficiency at work, and it’s nothing short of fascinating. The human body is incredibly smart, it doesn’t waste energy trying to run two systems at once. Instead, it chooses one dominant pathway, almost like flipping a switch. This is not a flaw but a survival mechanism. For most of human history, food was scarce and unpredictable. Our ancestors needed a way to make the most out of whatever fuel was available, whether it came from the berries they gathered or the fat stored on their own bodies. By running one pathway at a time, the body conserves resources and ensures survival.
To me, it’s exciting to think about how intentional this design really is. Every bite we take sends a signal that determines which fuel system the body will lean on. Learning about this cycle makes me appreciate the human body as an incredible machine complex, efficient, and beautifully adaptive. It’s fascinating to realize that what I put into my body is not just “food” but information that triggers a specific physiological response.
Why This Matters for Your Health
In the modern world, most people stay stuck in glucose-burning mode. With constant snacking, processed carbs, and sugar everywhere, insulin levels stay high and fat burning stays locked away. Over time, this imbalance contributes to obesity, insulin resistance, and type 2 diabetes.
Someone recently shared with me, “I have a defective body,” while describing their struggles with obesity, diabetes, high blood pressure, and a host of other metabolic conditions. My heart sank, because unless there is a rare genetic disorder, the body is not defective. It’s doing exactly what it has been told to do with the fuel it’s given.
When we understand the Randle cycle, we realize that the problem isn’t that our bodies are broken. The problem is that we’ve chosen the wrong fuel. By encouraging glucose burning all day long, we’ve lost our natural ability to switch to fat as a clean and steady energy source.
The Human Body’s Incredible Survival Design
Our ancestors didn’t have grocery stores or snack aisles. Food wasn’t guaranteed, and fasting was part of survival. The human body adapted by developing the ability to switch effortlessly between glucose and fat burning depending on what was available.
Scientific studies on human remains going back thousands of years show clear evidence of a diet that was naturally low in carbohydrates and centered around animal foods. In other words, the proper human diet has historically been low to no carb. Carbohydrates were seasonal, occasional, and never the daily staple they have become in the last century.
Why the Old Paradigm Doesn’t Work Anymore
The mainstream nutrition message has long promoted carbohydrates as the “preferred fuel” for the human body. But if that were truly the case, why are rates of obesity, diabetes, and metabolic disease higher than ever? The evidence is all around us that the old paradigm doesn’t work. We’ve traded our metabolic flexibility for constant cravings, energy crashes, and chronic illness.
By shifting back toward fat as a primary fuel—through lowering carbs and eating in line with our biology—we can restore the body’s natural design. This isn’t a fad. It’s going back to what kept humans healthy and resilient for thousands of years.
An Invitation to Rethink
It’s fascinating to consider whether the body “prefers” one method over the other. Glucose-burning is quick and useful when fast energy is needed, but it’s limited you can only store so much glucose. What’s even more interesting is that your body is capable of making the glucose it needs for the few functions that truly require it, such as red blood cells and certain brain processes. So why would we need to consume it in excess every day?
Fat-burning, on the other hand, is steady and reliable, with virtually unlimited storage to draw from. When you are fat-adapted, your body can maintain a sustainable supply of energy without the constant ups and downs that come with glucose dependence. From a survival standpoint, it makes sense that fat-burning would be the dominant, sustainable pathway. That doesn’t mean glucose has no role, but relying on it as a primary fuel every day is a modern choice, not a biological necessity.
This content is never meant to serve as medical advice.
In crafting this blog post, I aimed to encapsulate the essence of research findings while presenting the information in a reader-friendly format that promotes critical thinking and informed decision-making.
