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Ketogenic Diet Guide

What Is Ketosis?

Ketosis is a metabolic state where the body burns fat for fuel instead of glucose. When carbohydrate intake drops below approximately 20-50g per day (depending on individual), glycogen stores are depleted within 1-3 days. The liver then converts fatty acids into ketone bodies (β-hydroxybutyrate, acetoacetate, acetone) which become the primary fuel for the brain and muscles. Blood ketone levels above 0.5 mmol/L indicate nutritional ketosis. Keto is distinct from diabetic ketoacidosis (DKA), a dangerous complication of type 1 diabetes involving very high blood sugar and ketone levels together — a genuine medical emergency, unlike the mild, controlled nutritional ketosis a keto diet aims for.

What's the key thing to understand about Keto Macro Ratios?

Strict ketogenic diet macros: 5-10% calories from carbs (20-50g/day), 20-25% from protein, 70-75% from fat. Fat is used to meet calorie requirements, not as a 'free food' to eat unlimited amounts of. Protein is kept moderate — excess protein can be converted to glucose (gluconeogenesis) and disrupt ketosis at high intakes. Net carbs = total carbs − fibre − half of sugar alcohols. Fibre is not digested or absorbed so does not affect blood glucose or ketosis. Total carbs shown on UK food labels include fibre in the total, which is why net carbs (used for tracking ketosis) and total carbs (shown on the label) are two different figures.

What do I need to know about The Keto Adaptation Period?

Keto flu — the adaptation period (days 3-7 after starting): fatigue, brain fog, headaches, muscle cramps, and irritability. Caused by electrolyte loss (sodium, potassium, magnesium are excreted more rapidly when carbs are low) and the body's switch from glucose to ketone metabolism. Remedy: increase sodium intake (salt food liberally, bone broth), supplement potassium and magnesium, stay very well hydrated. Full keto-adaptation (fat-burning efficiency) takes 4-8 weeks — athletic performance typically improves after this initial adjustment period, as the body becomes more efficient at using fat and ketones for fuel.

What's the key thing to understand about Evidence for Keto?

Strong evidence: ketogenic diet is the gold standard treatment for drug-resistant childhood epilepsy (reduces seizures by 50%+ in approximately 50% of cases). Consistent evidence: keto produces equal or greater short-term weight loss than low-fat diets (mainly due to calorie reduction from food restriction). Moderate evidence: improves triglycerides, HDL cholesterol, and blood glucose control in type 2 diabetes. Weak/limited evidence: superior long-term weight loss vs other diets — adherence is the main factor, not the macronutrient composition itself. People who can stick to keto long-term do well, but so do people who stick to other well-designed diets.

What should I know about Risks and Who Should Avoid Keto?

A ketogenic diet isn't appropriate for everyone, and some groups should avoid it without medical supervision. People with type 1 diabetes face a real risk of diabetic ketoacidosis (a dangerous build-up of ketones and blood acidity that differs from the mild nutritional ketosis this diet targets) and need specialist guidance before attempting it. Anyone with a history of pancreatitis, liver disease, or certain rare metabolic conditions affecting fat breakdown should also avoid keto, as should pregnant or breastfeeding women, where energy and nutrient needs change and the diet hasn't been adequately studied for safety. Gallbladder conditions can also be aggravated by a very high-fat intake. Even in generally healthy adults, initial side effects ('keto flu' — fatigue, headache, irritability) are common in the first one to two weeks, and very low carbohydrate intake sustained long-term can make it harder to meet fibre, and certain vitamin and mineral, targets without careful planning. Anyone with an existing health condition or on regular medication should discuss starting keto with a GP or dietitian first, rather than treating it as a general-purpose diet.

What should I know about Keto Flu?

"Keto flu" describes the cluster of flu-like symptoms — headache, fatigue, irritability, nausea, muscle cramps and brain fog — that commonly appear in the first few days to two weeks after sharply cutting carbohydrates to start a ketogenic diet. It isn't caused by ketones themselves but mainly by rapid shifts in fluid and electrolytes: cutting carbs lowers insulin levels, which causes the kidneys to excrete more sodium and water, taking potassium and magnesium with it. This is the same mechanism behind the early, dramatic "water weight" loss often seen on keto. Symptoms are usually manageable by increasing salt intake, eating potassium- and magnesium-rich foods, and drinking enough water to replace what's being lost, rather than restricting fluids further. Most people find symptoms ease within one to two weeks as the body adapts to using fat and ketones for fuel. Keto flu that is severe, persistent, or accompanied by irregular heartbeat, severe dizziness or fainting warrants medical review, since significant electrolyte imbalance can occasionally require attention, particularly in people on blood pressure or diabetes medication.

Worked Example: Converting a Calorie Target Into Keto Macros

A standard ketogenic ratio aims for roughly 70-75% of calories from fat, 20-25% from protein, and only 5-10% from carbohydrate — low enough to keep the liver producing ketones as an alternative fuel source, which is the defining metabolic feature of a ketogenic diet.

Worked example: for an 1,800 kcal target on a strict 75/20/5 keto split, fat gets 1,350 kcal, which divided by 9 kcal/g gives 150g of fat per day. Protein gets 360 kcal, divided by 4 kcal/g gives 90g. Carbohydrate gets only 90 kcal, divided by 4 kcal/g gives about 22g — roughly the carbohydrate content of a single small banana and a slice of bread combined, which illustrates how little room a strict ketogenic approach leaves for starchy foods, fruit, and sugar across an entire day. Many people on a less strict "moderate keto" approach instead target 20-50g of carbohydrate per day rather than under 25g, trading slightly slower ketone production for a more sustainable and varied diet.

What Is the Difference Between Keto and Low-Carb Diets More Generally?

Ketogenic diets are a specific, stricter subset of low-carbohydrate eating, deliberately restricting carbohydrate low enough (typically under 50g per day, and often under 20-30g for strict versions) to shift the body into measurable ketosis, whereas "low-carb" is a much broader, less precisely defined term that can include anything from moderate carbohydrate reduction (say 100-150g per day) up to the stricter ketogenic range. A diet can be meaningfully lower in carbohydrate than a typical Western diet without being ketogenic at all, and many of the general benefits associated with reducing refined carbohydrate and added sugar intake are achievable at carbohydrate levels well above the ketogenic threshold. This distinction matters because strict ketogenic eating carries the specific considerations around keto flu, electrolyte balance and, for some people, medical suitability discussed above, which don't apply in the same way to a more moderate carbohydrate reduction.

Does Exercise Performance Change Differently on Keto for Different Sports?

The impact of ketogenic eating on exercise performance tends to differ depending on the type of activity, because carbohydrate and fat are used preferentially at different exercise intensities. Keto diets appear to be reasonably well tolerated for sustained, lower-intensity aerobic endurance activity once an athlete is fully fat-adapted (a process that itself can take several weeks beyond the initial keto-adaptation period described above), since fat oxidation can supply sufficient energy at lower exercise intensities. Performance in high-intensity, carbohydrate-dependent efforts — sprinting, heavy strength training near failure, or repeated high-intensity intervals — is more consistently shown in research to be impaired on a strict ketogenic diet, because these efforts rely heavily on rapidly available glycogen in a way that fat oxidation cannot fully replace. This is one reason competitive strength and power athletes are generally more cautious about strict ketogenic approaches than endurance athletes tend to be.

Net Carbs in Common Keto-Friendly Vegetables

Net carbs (total carbohydrate minus fibre) are what actually count toward a daily keto carb limit, and fibre-rich vegetables can be eaten in larger volume than their total carb figure alone would suggest.

This is why leafy greens and cruciferous vegetables dominate keto meal plans while starchy vegetables like sweet potato and parsnip are avoided even though they're nutritious foods generally — the net-carb cost of a starchy vegetable can use up most of a day's entire carb allowance in a single serving.