A keto diet can lead to both expected and unexpected medical complications. In most cases they’re not serious, and can be avoided or minimized.

The Keto Flu

The most predictable effect of a ketogenic diet has earned the nickname “the keto flu.” It’s caused when the body switches its energy source from glucose to ketones and is entering ketosis.

That big change in metabolic fuel often leads to symptoms very similar to the actual flu: tiredness and exhaustion, headache, cramps, nausea and vomiting. Frequent urination is also common, as the body loses water weight and tries to excrete excess ketones.

The keto flu generally lasts only a few days, from the start of a keto diet until the body reaches full ketosis. It’s possible to shorten that time period with increased hydration and by eating foods rich in nutrients like potassium, sodium and magnesium. Supplements can be a faster and easier way to replace those lost nutrients, as long as they’re not in electrolyte drinks which contain sugar.

The duration of the keto flu can also be lessened with sufficient rest and sleep, and avoiding serious exercise for a few days.

When the flu eases, that’s good news for two reasons. Not only do you feel better, but it means your body is in ketosis and you’re ready for serious weight loss.

Unexpected Complications

Those who rigorously follow the keto diet, the Atkins diet or other low-carb diets eliminate many important sources of nutrients, such as legumes, fruits and whole grains. That can create deficiencies in important micronutrients like magnesium, selenium, citrate, vitamins A, B7 and B9, D, and E. (10)

Some elements of the keto diet can also cause other problems. For example, eating a lot of high-fat animal foods can cause urine and blood to become too acidic. A drastic reduction in carbohydrates can limit dietary fiber. 

Some of the medical issues which can develop due to those issues include kidney stones (11), osteoporosis and bone loss (12), flare-ups in patients with gout (13), digestive problems and constipation (14). Most keto dieters don’t experience any of them, but they are certainly possible.

Consulting a nutritionist or dietitian, who can help you avoid those deficiencies and issues, is always a good idea before making any major dietary changes. At the very least, some vitamin and mineral supplements should be considered.

One other risk of a keto diet isn’t literally a health risk, but it’s common: when people go back to their “normal” way of eating, weight gain is typical. In fact, when some end their keto diet they regain most or all of their lost weight. A maintenance diet of some sort is usually required to keep the weight off, whether it involves staying with keto-friendly meal plans or switching to a healthy, balanced diet.

The side effects of keto are either transient or largely preventable, so keto is safe for nearly everyone. Some serious medical conditions and illnesses don’t play well with keto, however.

Ketosis changes how many of the body’s systems function. If those systems aren’t healthy to begin with, a keto diet could be problematic or even dangerous.

• Chronic kidney disease: Keto can lead to increased acid in the urine. Weakened kidneys aren’t able to efficiently remove the acid, and that could cause kidney disease to worsen.
• Gallbladder disease: Those whose gallbladder has been removed or is compromised will usually not do well on keto, since the gallbladder is needed to help digest the higher fat intake of a keto diet.
• Thyroid disease: The keto diet can lower the levels of thyroid hormones that the body produces, a problem for those with hypothyroidism. (15)
• Diabetes and heart disease: Type 1 diabetics are usually advised to avoid a keto diet because of the increased risk of diabetic ketoacidosis or hypoglycemia. Ketogenic eating can help those with type 2 diabetes lose weight and improve their glycemic control, but increased fat consumption may lead to an increase in “bad” cholesterol. That can worsen the risk of serious heart issues in diabetics and heart patients. Diabetics and those with heart disease should speak with their doctor before beginning keto. (16)

Experts also caution against a keto diet for those with eating disorders, and women who are pregnant or nursing.

Our discussion on the safety of the keto diet has so far centered on its use for weight loss over a period of several months. And evidence clearly shows that it is safe for most people over the short term.

The majority of medical professionals recommend three months as the outside limit for a keto diet. Some say do say, though, that cyclical keto program (for example, three months on and one month off), can safely be followed for longer periods.

The reason that keto is not recommended for permanent dieting and maintenance is that the long-term results and health risks of a ketogenic diet are still up for debate.

Some concerns center around the possible nutritional deficits mentioned earlier. The longer people remain on keto, the greater the chance that they could suffer complications involving dehydration, electrolyte imbalances and even hypoglycemia. (17)

There are also more serious concerns. An extensive survey of 25,000 Europeans presented in late 2019 reviewed the participants’ diets and their medical outcomes. One of the conclusions was that those who followed regular, very-low-carbohydrate diets ran the highest risk of dying from diseases ranging from stroke and heart disease, to cancer.

Even more telling was the fact that low-carb dieters who were not obese were more than twice as likely to die, than those who were obese. In other words, low-carb diets were much safer for people who needed to lose weight. They were more dangerous for those who weren’t obese. (18) That argues strongly for limited use of the keto diet.

A somewhat-similar study had a similar outcome. This one followed more than 15,000 Americans for 25 years. It found that the highest death rates occurred among those following low-carb or high-carb diets. Those who ate moderate amounts of carbohydrates didn’t die as often. Just as importantly, death rates among the low-carb dieters decreased when the carbs were replaced with plant-based foods. But deaths increased when carbs were replaced by animal fat and protein – the same prescription followed by keto dieters. (19)

Some keto advocates point to research claiming there are positive long-term effects to ketogenic eating. However, the studies they cite usually define “long-term” as periods of six months or so. (20) To date, there has been no major research supporting the use of keto for longer periods, or contradicting the two studies summarized earlier in this section.

Bottom line: there’s no firm proof either way, but it’s likely that the risk of a keto diet increases over time, particularly among those who are not obese and don’t need to lose weight.

Probably not. You shouldn’t be scared of a short-term keto diet unless you have one of the medical risk factors we’ve mentioned – or unless you’re scared of putting aside potatoes, cake and fast food and replacing them with avocados, leafy green vegetables, dairy, protein and more olive oil and coconut oil.

Unlike paleo, vegan and Mediterranean diets, keto has been used as a medical treatment for decades; it’s definitely not just a “fad diet.” And the many benefits of a ketogenic diet plan have been well established and documented.

After you’ve entered the state of ketosis, low-carb/high-fat dieting should lead to improved cholesterol levels and lower blood pressure, as well as lower blood sugar and triglyceride levels. Those are all good indicators of future heart health. (20) Keto may even lower the risk of Parkinson’s and Alzheimer’s disease. (21) It will probably reduce your appetite, too.

Then there’s the benefit most keto dieters are most interested in: weight loss. Keto may not be the best diet to follow for a lifetime – but there’s no doubt that most people who follow keto guidelines for a few months will lose substantial amounts of weight. (22)