The ketogenic diet, or keto diet, has gained popularity as a weight-loss strategy despite several health concerns, especially with long-term use. While everyone makes personal choices about their diet, it is essential to be aware of the potential short- and long-term side effects and risks associated with the keto diet. An informed decision also involves evaluating whether the restrictive nature of the keto diet is sustainable for you over time.

This evaluation aims to support that decision.

What Is the Keto Diet?

In general, the ketogenic diet involves significantly reducing carbohydrate intake and increasing fat consumption, similar to the Atkins diet popularized in the 1970s. Although the concept may seem new, a low-carbohydrate, high-fat diet was originally introduced in the 1920s to help reduce seizures in children with epilepsy. It later gained popularity as a weight-loss tool.

“Keto” is a restrictive diet that limits carbohydrate intake to 5–10% of total calories, which typically amounts to no more than 50 grams per day (about one cup of white rice). Protein intake is usually around 10–20% (from foods like fish, chicken, eggs, cheese, pork, and beef), while fats make up 70–80% of the total calorie intake in the diet (from sources such as nuts, seeds, avocados, olive oil, and olives).

Because carbohydrates are found in many foods, the diet eliminates bread, cereal, and other grains, and significantly limits fruit and starchy vegetable consumption. The goal is to induce a state called nutritional ketosis, which helps reduce appetite and increase fat burning.

What Is Ketosis?

Ketosis is a metabolic state that occurs when the body does not have enough glucose (from carbohydrates) for energy. In response, the liver begins to break down stored fat, producing ketones (or ketone bodies): beta-hydroxybutyrate, acetoacetate, and acetone. These molecules serve as an alternative energy source, even for the brain, which typically relies on glucose.

As a result, the blood has elevated ketone levels during ketosis.

What Are Safe Levels of Blood Ketones?

Ketosis is a natural and safe metabolic process that differs from ketoacidosis, a dangerous condition that can occur in people with type 1 diabetes.

Normal ketone levels fluctuate due to factors like fasting and exercise, typically remaining below 0.6 mmol/L. On a ketogenic diet, levels often range from 0.5 to 3.0 mmol/L. Some sources suggest that the optimal range for weight loss is between 1.0 and 3.0 mmol/L.

However, in some individuals, ketone levels between 1.6 and 3.0 mmol/L may increase the risk of ketoacidosis. Ketone levels above 3.0 mmol/L can indicate ketoacidosis, which requires prompt medical attention - especially in individuals with diabetes. In diabetic ketoacidosis, the body produces excessive ketones due to a lack of insulin, causing the blood to become dangerously acidic. This condition can be life-threatening without urgent treatment.

How Long Does It Take to Enter Ketosis?

With a daily carbohydrate intake of 20–50 grams, most people enter ketosis within 2 to 4 days. However, for some, it may take a week or longer. Factors influencing this timeframe include:

• Age
• Metabolic rate
• Physical activity level
• Sleep patterns
• Stress levels
• Previous carbohydrate intake

People who consumed a high-carb diet before starting keto may take longer to deplete their glucose stores.

Intermittent fasting may help you reach ketosis more quickly. This approach focuses on when you eat rather than what you eat. For example, you might eat during an 8-hour window and fast for the remaining 16 hours each day or eat only one meal on two non-consecutive days per week. Many different fasting schedules exist.

Potential Benefits of the Keto Diet

• Rapid weight loss
  Initial weight loss is often due to glycogen and water depletion. Many people report feeling less hungry than on other diets.
• Enjoyment of high-fat foods
  The diet allows consumption of popular foods that some people enjoy, such as red meats, fatty fish, cheese, nuts, and butter while still promoting weight loss.
• Therapeutic effects
  Originally developed to manage epilepsy in children, the keto diet is now being researched as a dietary therapy for other neurological, neurodegenerative, and metabolic diseases. In addition, it is used by endurance athletes and bodybuilders for the rapid reduction of body fat. 

Potential Short-Term Side Effects

Some individuals may experience:

• "Keto flu": fatigue, headache, nausea, vomiting, and reduced tolerance for exercise as the body transitions from using carbs to fat for energy
• Bad breath: due to high ketone production
• Insomnia
• Low blood sugar (hypoglycemia): particularly risky for people with diabetes, especially when combined with medication. This can lead to dizziness, shakiness, and heart palpitations.
• Low blood pressure: reduction in blood volume and changes in the fluid balance can decrease blood pressure. While it is beneficial in people with hypertension some individuals may experience dizziness, lightheadedness or fainting, especially when standing up quickly.

Potential Long-Term Health Issues

• Nutrient deficiencies
  The restrictive nature of keto can lead to insufficient intake of vitamins and minerals like B1, B6, C, E, folate, calcium, magnesium, and potassium. Some of these are already commonly deficient in individuals with obesity, and keto may worsen those deficiencies. Inadequate magnesium, vitamin C and potassium can contribute to muscle cramps, fatigue and weakened immune function.
• Elevated cholesterol
  A diet high in saturated fats may raise LDL ("bad") cholesterol. To protect heart health, prioritize unsaturated fats from sources like olive oil, avocado, and fatty fish.
• Kidney issues
  High fat and protein intake may burden the kidneys, potentially leading to kidney stones and other issues, especially in people with pre-existing kidney conditions.
• Low bone density (osteopenia)
  The acid load of the diet can cause the body to leach calcium from bones to buffer the acidity. The keto diet may also lower bone-building hormones, increase bone breakdown accompanied by deficiencies in nutrients like calcium, magnesium, and vitamin D.
• Digestive problems
  Low fiber intake may lead to chronic constipation and high fat intake can cause digestive discomfort. Fiber is needed for microbial production of beneficial colonic short-chain fatty acids, which enhance nutrient absorption, stimulate the release of satiety hormones, improve immune function, and have anti-inflammatory and anti-carcinogenic effects
• Reduced athletic performance
  Some athletes report reduced endurance and strength, especially during high-intensity workouts.

Considerations Before Starting the Keto Diet

Before beginning the keto diet, particularly if you have any health conditions, consult with a qualified healthcare provider. Nutrient supplementation may be necessary to avoid deficiencies.

While many people report rapid weight loss due to water loss and glycogen depletion, long-term fat loss depends on maintaining a calorie deficit and adapting metabolism. Be aware that once people stop following the keto diet, they often regain some or most of the weight lost. Many find the diet difficult to maintain due to its restrictive nature.

Also, ketosis can be challenging to achieve consistently—much like a light switch: it is either on or off. Regular food tracking improves your chances of staying in ketosis. However, the only reliable way to confirm ketosis is with a blood test.

Are There Alternatives for Long-Term Weight Loss?

Yes. Many health professionals recommend more balanced approaches, such as:

• The Mediterranean diet
  Focuses on plant-based eating and includes fruits, vegetables, whole grains, legumes, healthy fats, and moderate protein.
• Intermittent fasting
  Promotes mild ketosis without dietary restrictions. During fasting periods, the body shifts to burning fat for fuel. During eating windows, you can enjoy a variety of nutrient-rich foods, which makes this dietary pattern easier to sustain over time.

These alternatives support metabolic health, weight management, and cellular repair, and are often easier to maintain long-term.

Final Thoughts

No single diet works for everyone. The best approach is to make consistent, small changes that align with your health goals. These gradual changes often lead to more sustainable and impactful results than extreme or yo-yo dieting.