Those issues can be part of what's known as the “keto flu,” Warren says. Other side effects of the keto diet, all of which are tied to carb withdrawal, can include lightheadedness, nausea, mental fog, cramps, and headaches, in addition to tiredness. Luckily, the keto flu doesn't usually last more than a week—which is coincidentally about when people start to see the number on the scale go down, says Warren.
Aude, Y., A. S, Agatston, F. Lopez-Jimenez, et al. “The National Cholesterol Education Program Diet vs a Diet Lower in Carbohydrates and Higher in Protein and Monounsaturated Fat: A Randomized Trial.” JAMA Internal Medicine 164, no. 19 (2004): 2141–46. doi: 10.1001/archinte.164.19.2141. jamanetwork.com/journals/jamainternalmedicine/article-abstract/217514.

In terms of weight loss, you may be interested in trying the ketogenic diet because you’ve heard that it can make a big impact right away. And that’s true. “Ketogenic diets will cause you to lose weight within the first week,” says Mattinson. She explains that your body will first use up all of its glycogen stores (the storage form of carbohydrate). With depleted glycogen, you’ll drop water weight. While it can be motivating to see the number on the scale go down (often dramatically), do keep in mind that most of this is water loss initially.
Disclaimer: In this weight-loss guide, we recommend a low-carb diet, as it appears to be more effective than other diets. While a low-carb diet has many proven benefits, it’s still controversial. The main potential danger regards medications, especially for diabetes, where doses may need to be adapted. Discuss any changes in medication and relevant lifestyle changes with your doctor. Full disclaimer
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