The first few days of keto can bring some noticeable changes. You may find yourself going to the bathroom more often, feeling thirstier than usual, or dealing with headaches, fatigue, dizziness, or muscle cramps.
Most of that comes down to water and electrolytes. Here's what's happening and what you can do about it.
Why do you lose water when you start keto?
Because your body is using up stored carbohydrate, and that carbohydrate is stored with water.
When you cut carbs significantly, your body draws on its stored carbohydrate, called glycogen, for energy. Glycogen is kept mainly in your liver and muscles, and it holds a lot of water. Research suggests about 3 grams of water are stored with every gram of glycogen. As those stores go down, that water is released and leaves your body.
This is one reason the scale often drops quickly in the first week of keto. Some of that early change is water weight, not necessarily body fat.
Why does lower insulin mean more sodium and water loss?
Insulin affects how your kidneys handle sodium, so when insulin drops, your kidneys let more sodium go.
Eating fewer carbohydrates generally lowers insulin levels. As insulin falls, your kidneys tend to excrete more sodium, a process called natriuresis. Water follows sodium, so losing more sodium also means losing more water and urinating more often. This effect is usually most noticeable in the first several days of keto.
In short:
Fewer carbohydrates → lower glycogen stores + lower insulin → more sodium and water leaving the body.
Why do electrolytes matter when you start keto?
When you lose extra water, you also lose electrolytes, especially sodium and potassium. Those shifts in fluid and electrolyte balance may contribute to some of the symptoms often called the "keto flu," including:
- Headaches
- Fatigue or weakness
- Dizziness, especially when you stand up
- Muscle cramps
- Increased thirst
- Dry mouth
These symptoms are tied to the early adjustment period, not to ketosis itself. Research on keto initiation has found that sodium and potassium losses tend to be highest in the first several days, then ease as your body adapts.
Is drinking more water enough?
Not on its own. When you're starting keto, pay attention to both fluids and electrolytes.
European clinical guidelines for very-low-calorie ketogenic diets recommend at least 2 liters of fluids per day. Your own needs can vary a lot depending on your body size, activity level, climate, medications, health conditions, pregnancy or breastfeeding, and other factors.
Drinking large amounts of plain water doesn't replace the sodium and other minerals you're losing. Foods that fit most keto plans and help with fluids and electrolytes include:
- Leafy greens
- Avocado
- Nuts and seeds
- Meat and fish
- Eggs
- Broth
- Appropriately salted food
What does this mean for your keto journey?
That quick drop on the scale at the start of keto can feel exciting. Just know that some of it is your body releasing stored water, so don't be discouraged if the pace slows after the first week.
As your body adapts to using more fat and ketones for fuel, these early shifts in water and electrolytes usually settle down. Staying hydrated and paying attention to electrolytes can make the transition a lot more comfortable.
And remember: keto isn't about chasing the lowest glucose number or the highest ketone reading. It's about understanding how your body responds and using that information to make decisions about your nutrition and health. Testing your blood ketones and glucose shows you what's actually happening as you adapt.
Common questions about electrolytes on keto
Why do I pee so much when I start keto?
Two things happen at once. Your body uses up glycogen, which is stored with water, and lower insulin levels cause your kidneys to excrete more sodium, which takes water with it. Both effects are strongest in the first several days and ease as your body adapts.
Which electrolytes matter most on keto?
Sodium and potassium are the main ones lost when you start keto, and magnesium matters too. Foods like leafy greens, avocado, nuts, seeds, meat, fish, eggs, broth, and appropriately salted meals can help. If you have a heart, kidney, or blood pressure condition, talk with your healthcare provider before increasing sodium or potassium.
Is the weight I lose in the first week of keto just water?
Some of it is. As your body uses glycogen, it releases the water stored with it, and lower insulin increases sodium and water loss. That's why the scale often moves quickly at first and then slows down.
Sources
- Skartun O, Smith CR, Laupsa-Borge J, Dankel SN. Symptoms during initiation of a ketogenic diet: a scoping review of occurrence rates, mechanisms and relief strategies. Frontiers in Nutrition. 2025;12:1538266. doi.org/10.3389/fnut.2025.1538266
- Volek JS, Phinney SD, Krauss RM, et al. Alternative Dietary Patterns for Americans: Low-Carbohydrate Diets. Nutrients. 2021;13(10):3299. doi.org/10.3390/nu13103299
- Muscogiuri G, El Ghoch M, Colao A, et al. European Guidelines for Obesity Management in Adults with a Very Low-Calorie Ketogenic Diet: A Systematic Review and Meta-Analysis. Obesity Facts. 2021;14(2):222–245. pubmed.ncbi.nlm.nih.gov/33882506
- Dyńka D, Rodzeń Ł, Rodzeń M, et al. Ketogenic Diets for Body Weight Loss: A Comparison with Other Diets. Nutrients. 2025;17(6):965. doi.org/10.3390/nu17060965
- Burén J, Svensson M, Liv P, Sjödin A. Effects of a Ketogenic Diet on Body Composition in Healthy, Young, Normal-Weight Women: A Randomized Controlled Feeding Trial. Nutrients. 2024;16(13):2030. doi.org/10.3390/nu16132030
This article is for educational purposes only and is not intended as medical advice. If you take medications or manage a condition that affects blood pressure, blood glucose, kidney function, or electrolyte balance, talk with your healthcare provider before making significant dietary changes.
