The Randle cycle is the body’s built-in competition between fat and sugar as fuel: when cells burn more fat, they burn less sugar. It gave researchers one of the first explanations for why people with high levels of fat in their blood often respond poorly to insulin. Decades of work have since recast it as one piece of a larger story rather than the whole cause. The idea now reaches well beyond diabetes, because it touches everyday questions about fasting, high-fat diets, and why some bodies switch fuels better than others.
How Fat and Sugar Compete for Fuel
The idea is named after British scientist Philip Randle, who described it with colleagues in a 1963 paper in The Lancet.1PubMed. The glucose fatty-acid cycle. Its role in insulin sensitivity and the metabolic disturbances of diabetes mellitus They called it the glucose fatty-acid cycle, a two-way relationship between the body’s two main fuels. When more fat is available in the blood, cells burn more fat and burn less sugar.2Diabetes/Metabolism Reviews. Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years Extra fat also gets stored, and cells may store more sugar as glycogen if those reserves aren’t already full.3Diabetes/Metabolism Reviews. Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years
After a carbohydrate-heavy meal, a healthy body shifts toward burning sugar and lets fat burning fall. Between meals or overnight it shifts back to fat and saves the sugar for the brain. The Randle cycle is not a disease in itself. Trouble starts when fat stays high at times the body should be switching to sugar.
The fat in question is called free fatty acids. They mostly come from fat tissue releasing its stored fat, and some comes from food. Your lab report may not list them, since standard tests show triglycerides, a related but different blood fat. Levels naturally rise during fasting and fall after insulin is released.
Fat use dampens sugar use, and sugar availability dampens fat use. After a carbohydrate meal, rising insulin shuts down fat release from fat tissue, so less fat is on offer and sugar burning takes over. The 1963 work was done on heart muscle and diaphragm muscle in the lab, so it began as a basic question about how tissues choose fuel, not as a theory of diabetes.
Insulin’s job after a meal is to help muscle, liver, and fat cells take up and use sugar from the blood. Insulin resistance means cells respond weakly to insulin, so blood sugar stays higher than it should. If fat is crowding sugar out as fuel, cells may need less sugar and take up less of it, which can look like insulin resistance.
So sugar can linger longer after a meal while cells are busy burning fat. Insulin may be present in normal or even high amounts, with cells simply taking up less sugar than expected.
How the Randle Cycle Can Lead to Insulin Resistance
Obesity sets the stage. There is more fat tissue releasing fat into the blood,4Clinical Research in Diabetes and Obesity. Mechanisms of Insulin Resistance in Obesity and in many people with obesity, insulin no longer shuts off that release as well as it should.5Clinical Research in Diabetes and Obesity. Mechanisms of Insulin Resistance in Obesity Fat levels then stay high even after meals, the conditions in which fat and sugar compete most.6Clinical Research in Diabetes and Obesity. Mechanisms of Insulin Resistance in Obesity
The timing is the key detail. In a healthy person, the insulin rise after eating switches off fat release, so blood fat drops just when sugar needs to be used. When that switch works poorly, the body tries to burn fat and sugar at the same moment.
The routes to high blood sugar differ, though. Blood fat held high for a long time blocks sugar storage in muscle when glycogen stores are already full.7Diabetes/Metabolism Reviews. Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years But in type 2 diabetes, sugar storage is blocked even though muscle glycogen stores are low.8Diabetes/Metabolism Reviews. Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years So the simple fuel competition doesn’t fully explain what goes wrong in diabetes.
The liver adds its own contribution. It makes new sugar during fasting, and burning fat helps power that process.9PubMed Central. Requirement of hepatic pyruvate carboxylase during fasting, high fat, and ketogenic diet When fat burning stays high, the liver can keep releasing sugar even when it should stop. High fat burning in the liver may therefore push blood sugar up from the supply side, not just by blocking uptake.10PubMed Central. Requirement of hepatic pyruvate carboxylase during fasting, high fat, and ketogenic diet
Muscle and liver resistance show up differently. Muscle is the main place sugar goes after a meal, so muscle resistance raises after-meal blood sugar. Liver resistance shows up more as high fasting blood sugar in the morning. Both can be present at once, and the Randle cycle touches each differently. Because the two organs are affected to different degrees, two people with the same diagnosis can have different glucose patterns.
The whole thing can feed itself. Weak insulin action lets fat tissue keep releasing fat, and more fat in the blood further crowds out sugar use in muscle. Higher blood sugar pushes the pancreas to make more insulin to compensate. Over years, that extra demand can wear down the pancreas and lead to type 2 diabetes.
Why Scientists Now Think Fat Build-Up Matters More
Randle’s group proposed that fat slowed sugar use by blocking steps in how cells break sugar down.11PubMed Central. Lipid-induced insulin resistance: unravelling the mechanism Work over the past few decades shows the bigger problem in muscle is that sugar doesn’t get into the cell in the first place.12PubMed Central. Lipid-induced insulin resistance: unravelling the mechanism The reason is that fat by-products called diacylglycerols build up inside muscle and liver cells and switch on enzymes that interrupt insulin’s signal.13PubMed Central. Lipid-induced insulin resistance: unravelling the mechanism Both fuel competition and blocked signaling can happen, but blocked insulin signaling seems to be the larger problem.
Diacylglycerol is a fat molecule cells make as a normal step in building or breaking down stored fat. It is only harmful when too much collects inside the muscle or liver cell, not in the blood around it.
Evidence in people supports this. When fat levels were raised in people’s blood, their muscle became insulin resistant within hours.14PubMed Central. Role of diacylglycerol activation of PKCθ in lipid-induced muscle insulin resistance in humans The resistance appeared just as the fat by-product built up and insulin’s signal inside the cell weakened.15PubMed Central. Role of diacylglycerol activation of PKCθ in lipid-induced muscle insulin resistance in humans A similar fat build-up drives insulin resistance in the liver.16PubMed Central. Diacylglycerol Activation of Protein Kinase Cε and Hepatic Insulin Resistance The old and new ideas overlap: both start with too much fat reaching muscle and liver, one focusing on fuel choice and the other on blocked signals.
Too much fat burning can also backfire. In obesity-related insulin resistance, muscle can actually burn fat too fast, overloading its energy-making machinery.17PubMed. Mitochondrial overload and incomplete fatty acid oxidation contribute to skeletal muscle insulin resistance The fat is only partly burned, leaving leftover fragments that cause cell stress.18PubMed. Mitochondrial overload and incomplete fatty acid oxidation contribute to skeletal muscle insulin resistance19PubMed Central. Acylcarnitines: potential implications for skeletal muscle insulin resistance In lab-grown muscle cells, these fragments cut insulin’s effect by about 20 to 30 percent, an effect antioxidants reversed.20PubMed Central. Acylcarnitines: potential implications for skeletal muscle insulin resistance Still, the fragments don’t always track with insulin resistance in people, so they are likely one marker among several.21PubMed. Fish oil omega-3 fatty acids partially prevent lipid-induced insulin resistance in human skeletal muscle without limiting acylcarnitine accumulation
Metabolic Flexibility and Why It Fails
Metabolic flexibility is the body’s ability to burn mostly fat when fasting and switch to sugar after eating.22PubMed Central. Relationships between Mitochondrial Function and Metabolic Flexibility in Type 2 Diabetes Mellitus People with type 2 diabetes burn less fat while fasting and switch less well to sugar after meals.23PubMed Central. Relationships between Mitochondrial Function and Metabolic Flexibility in Type 2 Diabetes Mellitus In one comparison, the shift toward sugar burning was about half as large in people with type 2 diabetes as in similar people without it.24PubMed Central. Relationships between Mitochondrial Function and Metabolic Flexibility in Type 2 Diabetes Mellitus
Across a day, a flexible body burns mostly fat overnight and before breakfast, then burns more sugar after a meal as insulin goes up. By the next fast it swings back toward fat. In type 2 diabetes this swing is flattened, so the body stays closer to the middle, using a mix of fuels without shifting clearly either way. The problem is poor switching in both directions: muscle burns less fat during fasting and also fails to take up and burn sugar after eating.
This poor switching is mostly a result of cells failing to take up sugar, not the root cause.25PubMed Central. Metabolic flexibility and insulin resistance So fixing sugar uptake, not just fuel choice, is what restores flexibility. Regular exercise makes muscle better at taking up and burning sugar. Losing excess body fat lowers the steady stream of fat into the blood. Better sugar uptake then brings back the normal shift after meals.
What This Means for Keto and High-Fat Diets
In mice, a short-term keto diet caused worse liver insulin resistance than a fattening high-fat diet.26PubMed Central. Short‐term feeding of a ketogenic diet induces more severe hepatic insulin resistance than an obesogenic high‐fat diet The keto-fed mice burned almost only fat, and the more fat they burned, the worse their blood sugar control.27PubMed Central. Short‐term feeding of a ketogenic diet induces more severe hepatic insulin resistance than an obesogenic high‐fat diet Their muscles, though, still took up and burned sugar normally.28PubMed Central. Short‐term feeding of a ketogenic diet induces more severe hepatic insulin resistance than an obesogenic high‐fat diet That shows insulin resistance can be specific to one organ, here the liver.
Burning mostly fat for longer than about 18 to 24 hours may signal to the body that sugar is running low.29PubMed Central. Short‐term feeding of a ketogenic diet induces more severe hepatic insulin resistance than an obesogenic high‐fat diet Insulin may then deliberately lose some power to stop the liver making sugar, to refill sugar stores.30PubMed Central. Short‐term feeding of a ketogenic diet induces more severe hepatic insulin resistance than an obesogenic high‐fat diet High blood sugar readings on a keto diet can therefore partly reflect this adaptation, though these findings come from mice.
If you eat low-carb, you may notice higher fasting glucose or a worse result on a sugar-drink test. Eating carbohydrates for a few days before an oral glucose test is commonly advised so results reflect normal eating, and it’s worth telling your doctor about your diet before testing. The mouse diets were fed for a short time, so they describe an early response, not what happens after years. Mice also handle fat and sugar differently from people, so the results don’t prove the same happens in humans. The Randle cycle concerns fat in the blood, much of which comes from body fat stores, not just the fat in a single meal.
Can Blocking Fat Burning Treat Insulin Resistance
Drugs that block fat from entering the cell’s energy factories have been proposed for type 2 diabetes for nearly two decades.31PubMed Central. Carnitine Palmitoyltransferase 1b Deficiency Protects Mice from Diet-Induced Insulin Resistance One such drug, etomoxir, improved blood sugar in patients by shifting cells back toward burning sugar.32PubMed Central. Carnitine Palmitoyltransferase 1b Deficiency Protects Mice from Diet-Induced Insulin Resistance Etomoxir is an experimental compound, not an approved diabetes drug, so the result is evidence for how the body works, not a sign that a treatment is close. In obese mice, a similar drug given for 4 weeks improved insulin sensitivity and lowered fat by-products in muscle.33PubMed Central. Inhibition of Carnitine Palmitoyltransferase-1 Activity Alleviates Insulin Resistance in Diet-Induced Obese Mice So the Randle cycle does operate in muscle, even if it isn’t the whole story.34PubMed Central. Inhibition of Carnitine Palmitoyltransferase-1 Activity Alleviates Insulin Resistance in Diet-Induced Obese Mice
The logic is that if fat burning crowds out sugar use, limiting it should let sugar use recover. The risk is that blocking fat burning everywhere can starve tissues that depend on it.
Blocking the way fat burning switches on the liver’s sugar-making could also help in type 2 diabetes.35PubMed Central. Requirement of hepatic pyruvate carboxylase during fasting, high fat, and ketogenic diet These approaches remain experimental. Blocking fat burning can affect the heart, which relies heavily on fat for energy, and safety concerns slowed human use. Much of the evidence is from animals, and approved diabetes drugs work through other routes, such as improving insulin action or sugar excretion.
Some things that work on the same problem are available now. Exercise helps muscles take up sugar even when insulin signaling is weak, and weight loss reduces the fat stream from fat tissue that drives the cycle. If you have risk factors for diabetes, ask your doctor about an A1c or fasting glucose test.