Calories and Your Brain

Your brain is about 2% of your body weight but burns roughly 20% of your daily calories. It runs almost entirely on glucose, which is why slashing carbs hard can leave you foggy for the first few days.

When you cut calories sharply, your brain gets first claim on fuel. To protect it, your body dials down everything else — lowering body temperature, slowing digestion, and quieting reproductive hormones.

That's why crash diets feel miserable. Feed someone who needs 2,400 calories only 1,200, and the body flips into emergency conservation mode.