The Dangers of Mixing Sugar and Fat: Bart Kay on the Randle Cycle
Bart Kay, also known as the “Nutrition Science Watchdog,” is a former university lecturer in cardiovascular and respiratory physiology, exercise physiology, nutrition, and research methods. He has gained attention for breaking down complex nutrition science into plain language. One of his strongest warnings is about the dangers of eating sugar and fat together, explained through the Randle cycle.
What Is the Randle Cycle?
The Randle cycle, also called the glucose–fatty acid cycle, describes how the body chooses which fuel to burn: glucose or fat. Both cannot be burned efficiently at the same time. When you eat carbohydrates, glucose metabolism blocks fat oxidation. When you eat fat, fatty acid oxidation suppresses glucose use.
In nature, humans rarely had access to high sugar and high fat foods together. Fruit may have been seasonal, and animal fat was eaten without refined carbohydrates. The modern food environment has changed that. Today, sugar and fat are packaged together in nearly every processed food.
Why Sugar and Fat Together Are a Problem
Bart Kay points out that in today’s diet, sugar and fat are constantly paired. Think doughnuts, pizza, fries, ice cream, and even “healthy” granola bars. These foods activate the Randle cycle every time you eat them.
According to Kay, here’s what happens when sugar and fat collide:
Cells receive both glucose and fat but cannot process them efficiently.
This metabolic conflict forces the body to prioritize storing energy instead of burning it.
Over time, this constant activation drives insulin resistance, metabolic syndrome, and weight gain.
The result is poor metabolic flexibility, meaning your body loses the ability to switch fuels smoothly.
In his words, “By choosing to eat both energy sources daily, you activate the Randle cycle. Cells will self-preserve by blocking the efficient use of fatty acids and glucose.”
What We Need to Do
This is where awareness makes all the difference. When you understand what’s happening inside your body, you can change the way you eat to protect your health.
Stop eating sugar and fat together. When you eat both, your body is caught in a metabolic tug-of-war. Glucose blocks fat burning, fat slows glucose use, and your body turns to storage. This is the foundation of obesity and insulin resistance.
Choose one primary fuel. If you want to eat carbohydrates, keep fat low and rely on whole-food carbs like fruit, root vegetables, or rice. If you want to eat a higher fat diet, keep carbs minimal and let your body use fat as its main energy source.
Cut processed foods. Almost all ultra-processed foods are designed to exploit the Randle cycle. Chips, baked goods, fast food, and snack bars combine sugar, fat, and salt in a way that makes you overeat and disrupts normal fuel use.
Prioritize whole foods. Meat, eggs, seafood, vegetables, fruit, and dairy (if tolerated) give your body clean energy. Whole foods do not create the same metabolic confusion as processed combinations.
Give your body clarity. The body thrives when it knows which fuel to use. Either burn glucose or burn fat, but do not force your metabolism into a constant state of conflict.
Why This Matters
Understanding the Randle cycle is not just about science—it is about protecting your health. Each time you eat a meal loaded with both sugar and fat, your body is pushed toward fat storage, inflammation, and insulin resistance. Over time, this adds up to weight gain, fatigue, and higher risk for chronic disease.
Your body is designed for efficiency. When you fuel it properly, it rewards you with stable energy, mental clarity, and better metabolic health. By removing the constant sugar-and-fat combination, you give your metabolism a chance to work the way it was meant to.
Bart Kay’s perspective on the Randle cycle is a reminder of how far modern diets have drifted from human biology. Sugar and fat are not the problem on their own. The real danger comes when they are combined and consumed daily in processed foods. To restore metabolic health, choose meals that give your body clarity: fuel with carbs or fuel with fat, but do not overload both at once.
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.
