When some people attempt a ketogenic diet, they experience the “keto flu” during the period of time their bodies are adapting to the diet. This is a period of feeling lightheaded, lethargic, irritable, and unmotivated. “It typically lasts one to three days, and may very well be just the result of dehydration. A drop in insulin can impact renal sodium retention such that sodium and fluids are excreted more readily. This also acts to lower blood pressure,” Goss says.
What's more, it's especially important to make sure your diet is well-planned when you're eating keto-style, because the foods you can choose from are limited. In addition to checking in with a dietitian if you're able, Stefanski recommends that you "talk to your doctor and make sure she or he is aware that you'll be starting a diet that completely changes how your body metabolizes energy." You might also want to check your most recent bloodwork levels for things such as cholesterol, vitamin D, and other indicators of health because these can change while on keto. That's because for some people, a prolonged keto diet can result in certain nutritional deficiencies or even high cholesterol. But most experts will tell you that the ketogenic diet is not a permanent lifestyle change (as could be the case for something like the 80/20 approach to eating or a Mediterranean eating style).
Christopher D. Gardner, PhD; Alexandre Kiazand, MD; Sofiya Alhassan, PhD; Soowon Kim, PhD; Randall S. Stafford, MD, PhD; Raymond R. Balise, PhD; Helena C. Kraemer, PhD; Abby C. King, PhD, “Comparison of the Atkins, Zone, Ornish, and LEARN Diets for Change in Weight and Related Risk Factors Among Overweight Premenopausal Women,” JAMA. 2007;297(9):969-977. http://jama.jamanetwork.com/art icle.aspx?articleid=205916.
Other kinesiologists think the keto diet could have dangerous effects on athletic performance, and it's tough to know what potential side effects a long-term high-fat diet might have for a healthy person, since we don't have any solid study results yet. Low-carb diets like keto can make it easy to neglect key nutrients like magnesium, calcium, and potassium found in fresh, high-carb foods like beans, bananas, and oats.
There are three instances where there’s research to back up a ketogenic diet, including to help control type 2 diabetes, as part of epilepsy treatment, or for weight loss, says Mattinson. “In terms of diabetes, there is some promising research showing that the ketogenic diet may improve glycemic control. It may cause a reduction in A1C — a key test for diabetes that measures a person’s average blood sugar control over two to three months — something that may help you reduce medication use,” she says.