The Ketogenic Diet: How It Works, What to Eat, and What to Know

The ketogenic diet is a high-fat, moderate-protein, very-low-carbohydrate eating plan designed to shift the body's primary fuel source from glucose to fat. By drastically reducing carbohydrate intake, the body enters a metabolic state called ketosis, where it produces molecules called ketones from fat stores in the liver. These ketones then serve as the primary energy source for the body and brain.

The diet was originally developed in the 1920s by Dr. Russell Wilder at the Mayo Clinic in a clinical context. It has since been adopted more broadly by people pursuing body composition goals, metabolic health, and endurance performance. This guide covers how the diet works, the different approaches, what to eat, and what to consider before starting.

How Ketosis Works

Under normal dietary conditions, the body converts carbohydrates into glucose, which serves as its preferred energy source. When carbohydrate intake drops low enough (typically below 20-50 grams per day), glucose availability decreases and the body shifts to burning fat for fuel. The liver breaks down fatty acids into ketone bodies, which are then used by cells throughout the body, including the brain.

The brain is worth noting specifically. It requires a substantial amount of energy and typically relies on glucose. However, it can efficiently use ketones as an alternative fuel source. This metabolic flexibility is what allows the ketogenic diet to function: rather than the body breaking down muscle tissue for glucose during carbohydrate scarcity, it shifts to ketone production from fat.

Entering ketosis typically takes 2-7 days of sustained carbohydrate restriction, though the timeline varies by individual. Ketone levels can be measured using urine strips, blood meters, or breath analyzers.

Types of Ketogenic Diets

Not all keto diets follow the same structure. The four main approaches are:

Standard Ketogenic Diet (SKD): The most common and most studied version. Approximately 70-75% fat, 20-25% protein, and 5-10% carbohydrates.

High-Protein Ketogenic Diet: Similar to SKD but with increased protein. Approximately 60% fat, 35% protein, and 5% carbohydrates. May be preferable for those focused on maintaining or building muscle.

Targeted Ketogenic Diet (TKD): Allows small amounts of carbohydrates around workouts. Typically used by athletes and active individuals who need glucose availability for high-intensity training.

Cyclical Ketogenic Diet (CKD): Alternates periods of strict keto with higher-carb refeeding days (e.g., 5 keto days followed by 2 higher-carb days). Often used by bodybuilders and strength athletes.

The SKD and high-protein versions have been the most extensively studied. TKD and CKD are primarily used by athletes and individuals with specific performance goals.

What to Eat on a Keto Diet

The foundation of a ketogenic diet is whole, minimally processed foods that are high in healthy fats, moderate in protein, and very low in carbohydrates.

Healthy fat sources: Avocados, olive oil, coconut oil, avocado oil, nuts (almonds, walnuts, macadamia, pecans), seeds (chia, pumpkin, sunflower), butter and ghee (preferably grass-fed), and MCT oil (medium-chain triglycerides, which are easily digested and quickly converted to ketones).

Protein sources: Eggs (omega-3 or pasture-raised when possible), fatty fish (salmon, mackerel, tuna, trout), poultry, beef, pork, and full-fat dairy (cheese, Greek yogurt, cream). A whey or plant-based protein powder can also help meet protein targets without adding carbohydrates. FitWhey+ provides 21g of protein per serving with minimal carbohydrate content, and FitPlant+ offers a dairy-free alternative.

Low-carb vegetables: Leafy greens (spinach, kale, arugula), cruciferous vegetables (broccoli, cauliflower, Brussels sprouts), bell peppers, zucchini, asparagus, mushrooms, and tomatoes (in moderation).

Other: Herbs and spices, unprocessed cheeses (cheddar, mozzarella, goat cheese, blue cheese), dark chocolate (85%+ cacao, in moderation), and olives.

What to Limit or Avoid

Any food high in carbohydrates needs to be restricted or eliminated to maintain ketosis.

Sugars: Candy, soda, fruit juice, ice cream, cake, and other sweetened foods.

Grains and starches: Bread, pasta, rice, cereal, and wheat-based products.

Most fruits: Fruit is generally too high in sugar for keto, with the exception of small portions of lower-sugar options like berries.

Root vegetables and tubers: Potatoes, sweet potatoes, carrots, and parsnips.

Beans and legumes: Lentils, chickpeas, kidney beans, and peas.

Processed foods: Anything with added sugars, unhealthy seed oils, or hidden carbohydrates. Read labels carefully, as sugar and starch appear in unexpected products, including sauces, dressings, and "diet" foods.

Alcohol: Many alcoholic beverages are high in carbohydrates. Dry wines and spirits are lower-carb options, but alcohol can also slow ketone production. For more on alcohol and body composition, see our article on the effects of alcohol on muscle growth..

The Adaptation Phase and "Keto Flu"

The transition into ketosis is not always seamless. During the first few days to two weeks, some people experience a cluster of symptoms commonly called the "keto flu," including fatigue, headaches, nausea, dizziness, irritability, and difficulty concentrating. These symptoms are largely driven by fluid and electrolyte shifts that occur as the body adjusts to lower insulin levels and reduced glycogen stores.

Practical strategies for easing the transition:

Maintain electrolyte intake. Sodium, potassium, and magnesium are the three electrolytes most affected during keto adaptation. Many people find that supplementing with electrolytes significantly reduces keto flu symptoms. FitBoost provides all three alongside B vitamins for energy metabolism support, with zero sugars and zero net carbs.

Stay hydrated. The keto diet increases fluid loss, particularly in the early stages. Drink water consistently throughout the day.

Reduce carbohydrates gradually. Rather than dropping from a high-carb diet to under 20 grams overnight, some people find a gradual reduction (over 1-2 weeks) eases the transition and reduces symptom severity.

Prioritize sleep. Poor sleep amplifies fatigue and cravings during adaptation. Aim for 7-9 hours per night. For more on the relationship between sleep and performance, see our sleep and well-being guide.

Potential Benefits

Research and anecdotal reports suggest several potential benefits of the ketogenic diet. It is important to note that individual responses vary significantly, and the research base is still evolving for some of these areas.

Body composition: The combination of reduced carbohydrate intake, increased satiety from fat and protein, and metabolic shifts may support body composition goals. Some studies have found keto to be effective for this purpose, though adherence remains the most important factor in any dietary approach.

Satiety and appetite regulation: Many people report reduced hunger on keto, which may be related to the satiating effects of fat and protein, as well as the appetite-suppressing properties of ketones themselves.

Mental clarity and focus: Some individuals report improved cognitive focus on keto, potentially related to the brain's efficient use of ketones as fuel and more stable energy levels compared to glucose-dependent energy swings.

Endurance performance: Research on athletes in endurance activities suggests that extended ketosis allows for an adaptation where fatty acids become the preferred fuel source, potentially reducing reliance on glycogen stores during prolonged exercise.

Potential Risks and Considerations

The ketogenic diet is not appropriate for everyone, and it carries risks that should be understood before starting.

Nutrient gaps: Restricting entire food groups (fruits, grains, legumes) can create deficiencies in vitamins, minerals, and fiber. A high-quality multivitamin and intentional vegetable intake can help address this. FitNutrients+ provides a broad-spectrum vitamin and mineral profile to support micronutrient needs.

Electrolyte imbalances: As noted above, the shift in fluid balance during keto adaptation can deplete sodium, potassium, and magnesium. This is manageable with supplementation and dietary attention, but should not be ignored.

Saturated fat intake: The high fat intake on keto can include significant saturated fat, depending on food choices. Prioritizing unsaturated fat sources (olive oil, avocado, nuts, fatty fish) and monitoring bloodwork with a healthcare professional is advisable.

Reduced high-intensity performance: During the adaptation phase, and potentially longer for some individuals, performance in high-intensity, glycolytic activities (sprinting, heavy lifting, HIIT) may decrease. This is less of a concern for endurance activities.

Not suitable for everyone: People with certain health conditions, those who are pregnant or breastfeeding, and individuals taking medication that affects blood sugar or blood pressure should consult a healthcare professional before starting a ketogenic diet. Some conditions may require careful medical supervision or may be incompatible with a very-low-carbohydrate approach.

Practical Tips for Getting Started

Track your macros. Especially in the beginning, tracking your macronutrient intake helps ensure you are actually hitting the ratios needed for ketosis. A food tracking app makes this significantly easier.

Focus on food quality. A keto diet built on processed meats, cheese, and butter will look very different from one built on salmon, avocado, olive oil, and leafy greens. The latter supports both ketosis and overall nutritional quality.

Plan your meals. Keto requires more meal planning than most diets, particularly in the early stages when you are learning which foods fit and which do not. Our keto meal plan provides a full week of structured meals to get you started.

Be patient. The adaptation phase takes time. Give your body at least 2-4 weeks before evaluating whether keto is working for you. Initial water weight changes and keto flu symptoms are not indicators of long-term results.

Consult a healthcare professional. This is especially important if you have pre-existing health conditions, take medication, or have concerns about how a high-fat, low-carbohydrate diet may affect your individual health profile.

Conclusion

The ketogenic diet is a well-established dietary approach that fundamentally changes how the body produces and uses energy. For those who adhere to it consistently, it can support body composition goals, satiety, and cognitive focus. But it requires planning, adjustment, and an understanding of its limitations and risks. Not every dietary approach works for every individual. The most effective nutrition plan is one that is sustainable, meets your nutritional needs, and aligns with your goals. If keto interests you, start with the fundamentals covered here, plan your meals with our sample keto meal plan, and consult a healthcare professional to ensure it is appropriate for you.