Choosing your main fuel: carbs or fat

The human body runs on two main energy sources: glucose, which comes mostly from carbs, and fatty acids, which come from dietary fat and from fat stores. Protein can fill in, but that is not its job: the body would rather keep it for building and repairing.

Both pathways exist at all times and run together. What varies is the proportion. And that proportion is not a free setting: it is largely decided by what you have just eaten.

The reason comes down to one hormone. When blood sugar rises, the pancreas releases insulin, whose job is to move glucose into cells. But insulin does a second, less known thing: it blocks the release of stored fat. While it is high, the body cannot draw on its reserves. It burns what arrives.

Glucose is a fast fuel. It mobilizes quickly, feeds the brain and muscles without delay, and powers intense effort. That is what makes it the fuel of choice for high-intensity sport.

Its limit is storage capacity. The body holds roughly 400 to 500g of carbs as glycogen, in the liver and muscles, about a day's worth. Once those stores are full, the surplus goes to fat.

Its second limit is the rhythm. A hit of fast carbs raises blood sugar, insulin follows, the glucose is put away, and blood sugar comes back down, sometimes below where it started. That is the mid-afternoon dip, with its craving. The page What happens after a meal goes into that cycle, and Glycemic index and glycemic load explains why not all carbs trigger it to the same degree.

None of this makes carbs bad. Lentils and a chocolate bar are both carbs, with unrelated effects. The form, the fiber that comes with it and the quantity matter more than the category.

Fat is a slow, dense fuel. One gram supplies 9 kcal against 4 for a gram of carbs, and the reserves are on another scale entirely: even a lean person carries tens of thousands of kilocalories of fat, against a day's worth of glycogen.

Burning fat does not require insulin. Quite the opposite: reserves start moving when insulin is low. Hence the sensation, often reported by people who cut carbs sharply, of steadier energy without the dips, and of hunger that is less insistent.

When carbs become very scarce, the liver makes ketone bodies from fatty acids. The brain, which cannot burn fatty acids directly, can use those ketones for a good share of its needs. This is nutritional ketosis, a normal physiological state, not to be confused with diabetic ketoacidosis, which is a medical emergency and comes from an entirely different mechanism.

Fat's limit mirrors the carbs' one: energy is supplied more slowly. In a short, very intense effort, the body calls for glucose, and a system used to going without produces less peak power.

The more interesting question may not be "which of the two", but "can I switch between them". This is what is called metabolic flexibility: the ability to move cleanly from glucose to fat according to what is available.

A flexible metabolism goes several hours without eating without it becoming unpleasant, because it switches to its reserves. A rigid one, used to carbs every three hours, demands the next hit as soon as the last is spent. Hunger is then not a need for energy, it is a need for that particular fuel.

This reading explains why two people on the same calorie intake experience such different days. And it moves the goal: rather than picking a side, regaining the ability to move between them.

The debate is heated, and what is established has to be separated from what is not.

One point deserves flagging because advocates of fat often leave it out: in a minority of people, a very high fat diet raises LDL sharply. The phenomenon is real, it mostly concerns lean, athletic profiles, and what it means for cardiovascular risk is debated. That is not a reason to give up, it is a reason to get a lipid panel before and after.

The benefit to expect is not the same depending on where you start.

• Type 2 diabetes, prediabetes, metabolic syndrome: this is where cutting carbs has the clearest and best documented effects.
• Cravings and slumps: the gain is often fast and noticeable within days.
• Endurance sport: worth considering, with an adaptation period of several weeks during which performance drops before recovering.
• Explosive sport, very intense effort: little benefit, possibly a handicap.
• Someone metabolically healthy, with no symptoms: no physiological urgency. The benefit lies elsewhere, in willpower: a flexible metabolism does not demand its next carb hit three hours after the last, which removes much of the craving and snacking that otherwise has to be fought with discipline.

Many people meet a flat refusal when they raise this in an appointment. It is tempting to read that as bad faith. The reality is more interesting, and part of that reserve is well founded.

There is also, over time, what Thomas Kuhn described: a scientific paradigm does not turn over because it has been refuted, but because the generation that held it departs. Nutrition is a textbook case, with a particularly slow cycle, since observational studies take decades.

• Both fuels work. The body uses whichever it is given first, and insulin arbitrates.
• While insulin is high, the fat stores stay shut.
• The soundest goal is not to pick a side, it is to be able to move between them.
• The benefit of cutting carbs is clear and documented on the metabolic side, type 2 diabetes above all. It is far more debatable for someone already healthy.
• The medical profession's reserve is not only inertia: long-term safety is not established, and the harms are real.