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Keto flu explained: why it happens and how to fix it
The fatigue, headaches, and brain fog some people get starting keto aren't really about ketones — they're mostly about sodium and water.
What causes it
Cutting carbs sharply depletes glycogen, and each gram of stored glycogen is bound to roughly 3 grams of water — so in the first few days of a very low-carb diet, you lose a real amount of water weight fast. That water carries sodium and potassium out with it, and low-carb eating also naturally cuts many of the foods (fruit, grains, legumes) that are typical sources of potassium and magnesium.[1] The resulting electrolyte drop is what produces the classic fatigue, headache, irritability, and brain fog cluster often called "keto flu," usually appearing 2-7 days in.[1]
How to actually fix it
Deliberately replacing sodium (bouillon, broth, or adding salt to food), and getting enough potassium and magnesium (leafy greens, avocado, nuts — all carb-light), resolves most symptoms within 24-48 hours. Staying well-hydrated matters too, since the water loss itself is part of the mechanism.[1] If symptoms persist past a week or two, that's a signal to reassess whether a very low-carb approach is the right fit for you rather than pushing through indefinitely.
Keep reading: The ketogenic diet explained · Water weight vs. fat loss · Sports drinks vs. water