Few diets have inspired as much passionate debate as keto. Advocates claim it transformed their energy, cleared brain fog, and melted fat effortlessly. Critics call it unsustainable and potentially risky. The reality, as with most things in nutrition, is more nuanced than either camp admits. This guide gives you the honest, research-backed picture so you can make an informed decision.
What Is Ketosis? The Biochemistry in Plain English
Your body runs on glucose by default. Every carbohydrate you eat — whether it's a banana, a bagel, or a bowl of lentils — is broken down into glucose, which fuels your brain and muscles. Under normal circumstances, your body prioritises glucose above all other fuel sources and stores any excess as glycogen in the liver and muscles, or as body fat.
Ketosis is a metabolic state your body enters when glucose becomes scarce, a process described in detail by the U.S. Dietary Guidelines. When carbohydrate intake drops below approximately 20–50g per day, liver glycogen depletes within 24–72 hours. At this point, the liver begins converting stored fat into molecules called ketone bodies — specifically beta-hydroxybutyrate, acetoacetate, and acetone — which the brain and muscles use as alternative fuel. This fat-burning metabolic state is what keto practitioners are after.
"The ketogenic diet is a legitimate therapeutic intervention with well-documented efficacy for epilepsy and promising evidence for metabolic disorders. It is not a magic bullet, but it is a real tool for people who implement it correctly." — Dr. Dominic D'Agostino, University of South Florida Department of Molecular Pharmacology
How the Ketogenic Diet Works
The keto diet achieves ketosis through severe carbohydrate restriction. Most people enter ketosis by keeping net carbs (total carbs minus fibre) below 20g per day, though some individuals can sustain ketosis at up to 50g. To compensate for the removal of carbs as a calorie source, fat becomes the primary macronutrient, making up 65–75% of total daily calories.
Once you're in ketosis, your body becomes highly efficient at mobilising and burning fat. Appetite also tends to decrease — likely because ketones themselves suppress ghrelin and stabilise blood sugar, eliminating the energy crashes associated with high-carb eating. Many people report reduced appetite as one of keto's most significant practical benefits.
Keto Macros: The Exact Ratios
Standard keto macros are approximately 70% fat, 25% protein, 5% carbohydrates. On a 2,000-calorie diet, that looks like:
- Fat: 156g per day (1,400 calories)
- Protein: 125g per day (500 calories)
- Carbohydrates: 25g net carbs per day (100 calories)
These ratios are intentionally high in fat because fat is your primary fuel source. Protein is kept moderate rather than high — excess protein can be converted to glucose via gluconeogenesis, potentially disrupting ketosis.
What You Can and Cannot Eat on Keto
Keto-Friendly Foods
- Meat: beef, pork, lamb, chicken, turkey, bacon
- Fish and seafood: salmon, sardines, mackerel, shrimp, tuna
- Eggs (any preparation)
- Dairy: butter, heavy cream, hard cheeses, full-fat yogurt (small amounts)
- Non-starchy vegetables: leafy greens, broccoli, cauliflower, courgette, peppers, asparagus
- Fats and oils: olive oil, coconut oil, avocado oil, lard
- Nuts and seeds: macadamia nuts, walnuts, almonds, chia seeds, flaxseeds
- Avocado
- Berries (small amounts): raspberries, blackberries, strawberries
Foods to Avoid on Keto
- All grains: bread, pasta, rice, oats, quinoa, cereals
- Sugar and sweets: soft drinks, fruit juice, sweets, cake, chocolate (standard)
- Most fruit: bananas, apples, oranges, grapes, mangoes
- Starchy vegetables: potatoes, sweet potatoes, corn, parsnips
- Legumes: lentils, chickpeas, black beans, kidney beans
- Low-fat products (usually have added sugar to compensate)
The First Week: Keto Flu Explained
Most people who start keto experience what is colloquially known as the "keto flu" during days 2–7. Symptoms include headaches, fatigue, brain fog, irritability, nausea, and muscle cramps. It is temporary — but it is real, and many people abandon keto during this window believing it isn't working.
The mechanism: when you deplete glycogen, your kidneys excrete significantly more sodium. Sodium takes water with it, and potassium and magnesium follow. This rapid electrolyte loss is the primary cause of keto flu symptoms. It is not your body rejecting fat — it's dehydration and electrolyte depletion.
Keto vs Low-Carb vs Carnivore
Low-carb diets typically restrict carbs to 50–150g per day — enough to reduce insulin and improve metabolic markers without necessarily achieving ketosis. More sustainable for most people. Keto is stricter — under 20–50g net carbs — with the specific goal of achieving and maintaining ketosis. Carnivore eliminates all plant foods, eating only animal products. It is the most extreme variation and lacks the long-term research base that keto has. Not comparable in terms of evidence.
Who Benefits Most from Keto
Epilepsy: This is keto's strongest evidence base. The ketogenic diet has been used to treat drug-resistant epilepsy in children since the 1920s and remains a first-line clinical intervention. Multiple randomised trials show 50%+ seizure reduction in approximately 50% of patients. Type 2 diabetes: A 2021 two-year trial in Diabetes Therapy found keto reduced HbA1c by an average of 1.3% and allowed 60% of participants to reduce or eliminate diabetes medication, a meaningful outcome given the burden noted by CDC obesity and metabolic health data. Weight loss: Short-term keto produces faster initial weight loss than conventional low-fat diets — though much of this is water weight from glycogen depletion. Long-term outcomes (12+ months) are comparable to other structured diets when adherence is equal.
Long-Term Risks and Considerations
Keto is not suitable for everyone. Long-term considerations include: potential increases in LDL cholesterol in some individuals (particularly with high saturated fat intake), possible kidney stone risk, concerns also highlighted by NIDDK weight management guidance (especially in those with a history), and reduced dietary fibre intake if vegetables are neglected. People with type 1 diabetes, liver disease, pancreatitis, or rare metabolic disorders should not attempt keto without direct medical supervision. The diet also excludes many nutrient-dense whole foods — legumes, whole grains, most fruit — so micronutrient supplementation and food quality become more important.
Week One Meal Plan (7 Days, Full Keto)
- Day 1: B: Scrambled eggs with butter and bacon | L: Tuna salad on lettuce wraps | D: Grilled salmon with asparagus | S: Macadamia nuts
- Day 2: B: Full-fat Greek yogurt with almonds | L: Chicken thighs with roasted broccoli | D: Beef mince bolognese over courgette noodles | S: Celery with cream cheese
- Day 3: B: Omelette with cheese and spinach | L: Sardines with avocado | D: Pork belly with sautéed green beans | S: Walnuts
- Day 4: B: Fried eggs with avocado | L: Caesar salad with grilled chicken (no croutons) | D: Lamb chops with cauliflower mash | S: Hard cheese slices
- Day 5: B: Bacon and eggs | L: Smoked salmon with cream cheese on cucumber | D: Chicken thighs with roasted courgette and peppers | S: Brazil nuts
- Day 6: B: Keto pancakes (almond flour, eggs, cream cheese) | L: Beef burger in lettuce wrap with cheese | D: Baked cod with butter and lemon, green salad | S: Avocado halves with salt
- Day 7: B: Full English — eggs, bacon, sausages, tomatoes, mushrooms | L: Chicken liver pâté with cucumber | D: Ribeye steak with garlic butter and broccoli | S: Pecans
The Bottom Line
Keto is a legitimate dietary approach with real, research-backed benefits — particularly for epilepsy, type 2 diabetes management, and short-term weight loss. It is not magic, and it is not suitable for everyone. The first week is the hardest. Managing electrolytes aggressively makes it considerably more bearable.
If you're considering keto, speak with your GP or a registered dietitian first — especially if you take medication for diabetes, blood pressure, or thyroid conditions, as keto can significantly change your medication requirements. If you're otherwise healthy and well-informed, a two-to-four week trial is a reasonable way to see how your body responds. You may also want to understand why calorie counting alone doesn't work and explore sustainable weight loss strategies that complement or follow a keto phase.
