Keto flu and kidney stone risks prompt caution as more adopt low-carb diets

While the ketogenic diet gains popularity for weight loss, new insights highlight temporary side effects like ‘keto flu’ and increased risk of kidney stones, urging careful monitoring and medical guidance.

The “keto flu” is a real and common complaint among people who start a ketogenic diet, even though it is not an actual influenza infection. According to Everyday Health, registered dietitian Kelly Keatley says the body can feel sluggish during the shift away from carbohydrates because it is less efficient at producing energy while it adapts to burning fat. Symptoms often include fatigue, headache, nausea and irritability, and Harvard Health and the Cleveland Clinic say they usually appear within the first week or so and fade within about one to two weeks.

That transition can be uncomfortable, but it is generally temporary. Cleveland Clinic recommends hydration, electrolyte replacement, rest and, in some cases, eating more often to ease the adjustment. Healthline also notes that some people respond better than others, with symptoms ranging from mild to severe. For anyone considering keto, that means the early phase can be rough even when the diet is followed correctly.

Kidney stones are another concern worth taking seriously. A systematic review and meta-analysis published in the National Library of Medicine estimated the pooled incidence of kidney stones among people on ketogenic diets at 5.9 per cent, far higher than the rate seen in the general population. The review found uric acid stones were most common. Everyday Health notes that the risk may rise further when keto is paired with high intakes of red meat and too little water, making hydration and electrolyte balance especially important.

The diet may also affect menstruation. Everyday Health reports that some younger women on long-term, very low-carbohydrate diets have experienced irregular or missed periods, suggesting that women may be more sensitive to carbohydrate restriction than men. At the same time, the picture is not entirely one-sided: limited research in women with polyendocrine metabolic ovarian syndrome suggests keto may improve hormonal markers in some cases, including free testosterone and fasting insulin.

Weight-loss claims are another major reason people try keto, but the results are often tied to strict calorie restriction as well as carbohydrate cutting. Everyday Health says one pooled analysis found large early losses in people with obesity, though those studies combined ketosis with very low-calorie intake. The same report warns that many people regain weight once they reintroduce carbohydrates or return to their previous eating habits.

The diet can also change cholesterol patterns in both directions. Lower weight can improve blood lipids, but keto often increases saturated fat intake through foods such as butter, bacon, cream and coconut oil. Everyday Health says some studies have found higher total cholesterol and HDL, or “good” cholesterol, after keto, while other research suggests the type of fat matters, with monounsaturated fats such as olive oil and avocado looking more favourable. For people with diabetes, the diet may lower blood sugar in the short term, but it can also cause dangerous hypoglycaemia if medication is not adjusted and may pose a risk of diabetic ketoacidosis in people with type 1 diabetes, so medical supervision is important.