What Is a Ketogenic Diet?

A ketogenic diet is a very low-carbohydrate, high-fat eating pattern designed to shift the body's primary fuel source from glucose to ketone bodies. When carbohydrate intake drops below approximately 20–50g of net carbs per day, liver glycogen depletes within 2–4 days and the liver begins producing ketones (beta-hydroxybutyrate, acetoacetate, acetone) from fatty acids.

Nutritional ketosis is confirmed by blood BHB ≥0.5 mmol/L. The metabolic shift has been studied for epilepsy treatment since 1921, and more recently for weight management, type 2 diabetes, metabolic syndrome, and neurological conditions.

Standard Keto Macros (Volek & Phinney 2011)

The standard ketogenic diet (SKD) is the most studied and most commonly practised form. Macronutrient targets are personalised using the Mifflin-St Jeor BMR equation multiplied by an activity factor (Harris-Benedict scale) to derive TDEE, then adjusted per goal:

• Fat Loss (−20%): protein 1.8 g/kg · net carbs 20g · fat = remainder
• Maintenance: protein 1.6 g/kg · net carbs 25g · fat = remainder
• Muscle Preservation: protein 2.0 g/kg · net carbs 25g · fat = remainder
• Athletic Performance: protein 2.2 g/kg · net carbs 35g · fat = remainder
• Therapeutic: protein 1.4 g/kg · net carbs 15g · fat = remainder

ISSN 2017 Position Stand confirms protein 1.4–2.0 g/kg is appropriate for active individuals on ketogenic diets without disrupting ketosis, as gluconeogenesis is demand-driven not supply-driven.

Glucose-Ketone Index (GKI)

Introduced by Thomas Seyfried and colleagues in 2012, the GKI provides a single numerical score to quantify the degree of metabolic ketosis:

GKI = blood glucose (mmol/L) ÷ blood ketones (mmol/L)

To convert blood glucose from mg/dL to mmol/L: divide by 18.016. The zones are:

• GKI <1: Optimal therapeutic — deepest ketosis, used for GBM and refractory epilepsy
• GKI 1–3: Moderate therapeutic — cancer metabolic therapy and neurodegeneration
• GKI 3–6: Light therapeutic / weight loss — general metabolic health
• GKI 6–9: Minimal ketosis — borderline
• GKI >9: Outside ketosis — glucose dominates metabolism

Classical Keto Ratio Therapy (Freeman 1998)

The classical ketogenic ratio diet, developed at Mayo Clinic by Russell Wilder in 1921 and validated clinically by Freeman and colleagues at Johns Hopkins in 1998, expresses macronutrients as a ratio of fat to combined protein and carbohydrates:

fat_g = ratio × (protein_g + carbs_g)
total_kcal = (protein_g + carbs_g) × (9 × ratio + 4)

Common ratios and their typical applications:

• 4:1 — Severe paediatric epilepsy; ~90% fat calories; deepest ketosis
• 3:1 — Standard clinical protocol; most studied ratio
• 2:1 — Adults and moderate conditions; more sustainable
• Modified Atkins (~1.5:1) — Higher protein; shown equivalent efficacy in adults

⚠ Classical ratio diets require supervision by a registered dietitian trained in ketogenic therapy. Nutrient deficiencies, growth delays in children, and hypoglycaemia are documented risks.

Keto Electrolyte Needs (Phinney & Volek 2011)

Ketogenic diets suppress insulin, reducing renal sodium reabsorption. This causes rapid urinary excretion of sodium (and water) in the first 1–2 weeks — the primary cause of "keto flu."

Sodium targets by adaptation stage:

• Initial (Week 1–2): 3,500–4,500 mg sodium/day — aggressive supplementation required
• Adapting (Week 3–6): 2,500–3,500 mg/day — kidneys begin reabsorbing sodium
• Adapted (6+ weeks): 1,800–2,500 mg/day — approaches normal dietary recommendations

Potassium (2,500–3,500 mg/day) and magnesium (310–520 mg/day, as glycinate or malate) complete the electrolyte picture. Both are lost in increased urine output and found in lower concentrations in grain-free diets.

Net Carb Budget (ADA 2019)

The American Diabetes Association (2019) Consensus Report recognises four carbohydrate restriction levels as evidence-based dietary patterns:

Keto Weight Loss Timeline (Phinney Glycogen Model)

Weight loss on a ketogenic diet occurs in two distinct phases:

Phase 1 — Glycogen Depletion (Days 1–14):

The liver and muscles store ~400–500g of glycogen, each gram bound to ~3–4g of water. Complete depletion releases 1.5–2.5 kg of glycogen-bound water. This is not fat loss and should not be used as a baseline.

Phase 2 — Fat Loss (Week 2 onwards):

True fat loss follows the energy balance model: fat loss rate (kg/week) = caloric deficit (kcal/day) × 7 days ÷ 7,700 kcal/kg. A 500 kcal/day deficit produces approximately 0.45 kg/week of fat loss in the long run. Ketogenic diets may enhance fat oxidation, but total energy balance remains the primary driver of body composition change.

Frequently Asked Questions