How Many Carbohydrates Does the Body Actually Need?
In several other articles here I refer to carbohydrates (in line with nutritional science) as non-essential . That means they don’t have to be supplied to the body through food. Yet dietary guidelines still recommend that more than 50% of daily energy intake should come from carbohydrates.
A common justification is that carbohydrates are the body’s preferred fuel (an argument I’ve already pushed back on), and that the brain alone requires 120g of carbohydrates (glucose ) per day.
The Preferred Fuel
How does that square with a ketogenic metabolism, where considerably less than 50g of carbohydrates are consumed per day?
Let’s look at the numbers in detail:
The Absolute Carbohydrate Minimum
There are very few cells in the body that are absolutely dependent on glucose for energy. Among them: the brain and red blood cells.
The brain, however, does not need 120g/day. It gets along just fine with 20-30g when ketone bodies are present - that is: when the body is running in solid fat-burning mode, ketosis .
Red blood cells need around 20g/day. Since they have no mitochondria (“cellular power plants”), they depend on glucose for energy and burn it anaerobically to lactate (rather than to carbon dioxide and water).
On top of that, there’s glucose demand from cells in the eye, the kidneys, and the immune system. These add up to 3-7g/day - an imprecise number, but low enough that it doesn’t tip the scales much.
Taking the maximum: 57g are needed, that’s about 14 teaspoons per day. As a reminder: a healthy blood sugar level means roughly 1 teaspoon of glucose is circulating in the blood at any given moment. Even double that amount, sustained over time, is toxic to the body.
But if the body’s need for glucose is non-negotiable, why are carbohydrates still non-essential? Where does the glucose come from when it doesn’t come from food?
The body can generate glucose from various sources - especially when only such small amounts are needed:
Fat Burning
Fat exists as triglycerides - picture an E-shaped molecule: a glycerol molecule as the vertical backbone, with 3 longer fatty acids forming the horizontal arms of the E.
When fat is burned, these glycerol molecules are released and converted to glucose by the liver.
As a rough estimate (and depending on the chain length of the fatty acids involved), about 10% of fat mass becomes glucose: 100g of fat yields 10g of glucose. A well-adapted organism with a daily energy need of 2,500 kcal requires 277g of fat for that - yielding around 27g of glucose, nearly half of the absolute requirement.
Lactate Recycling
Red blood cells need glucose, but leave behind lactate, which the liver can convert back into glucose(1).
Want the details? Here goes: red blood cells have no mitochondria, so they draw their energy from anaerobic processes. They don’t fully combust glucose to carbon dioxide and water - instead, they convert it to lactate while harvesting energy. The lactate is fully recyclable - practically the ultimate closed-loop system.
In this way, the full 20g of daily glucose needed by red blood cells can be covered by a constant cycle between glucose and lactate.
Stored Reserves
The liver stores up to 100g of glycogen (the storage form of glucose), from which glucose can be released very quickly without needing to synthesise any new supply. So if carbohydrates are consumed through food - despite their non-essential status - they aren’t wasted. And anyone who’s been keeping count will have noticed there’s no meaningful gap left to fill.
Gluconeogenesis
If you indeed consume absolutely zero carbohydrates through your food, still nothing is lost: Any remaining small shortfall can be covered by a process called gluconeogenesis . In it, glucose is synthesised from proteins.
Given how little additional glucose is actually needed, this is no issue at all - a few grams of protein are always available through cellular turnover.
Cross Check
After nearly 14 years of ketogenic eating, including almost 2 years with virtually no carbohydrates at all, I appear to have not collapsed yet. Which I take as personal confirmation that the body supplies itself with the small necessary amount of glucose just fine.
Evidence-based biochemistry, confirmed by my n=1 experiment (and many others), in agreement with nutritional science and biochemistry, but in stark contradiction of dietary guidelines. 🤷
To set it straight: The body’s demand for exogenous carbohydrates is exactly 0 (in words: Zero)!
