What is nutritional ketosis?
With a carbohydrate-rich diet containing foods such as potatoes, cereals, fruit and pulses, glucose is an important fuel. The brain normally relies heavily on glucose, although it can also use ketone bodies; long-chain fatty acids do not readily cross the blood–brain barrier.
Glycogen stores are limited. When carbohydrate intake is sharply reduced, liver glycogen falls and other pathways become more prominent, including gluconeogenesis, the production of glucose within the body, and ketogenesis, the production of ketone bodies. Increased circulating ketones in this context are called nutritional ketosis.
What are ketone bodies?
Ketone bodies are small molecules produced mainly in liver mitochondria from fatty acids. The 3 usually described are acetone, acetoacetate and beta-hydroxybutyrate. Their concentrations and use change with the metabolic situation.
Ketones cross the blood–brain barrier and can supply energy alongside glucose. During prolonged fasting or adaptation they may supply a substantial share of brain energy, sometimes approaching the 70% figure cited in the original; this is not a fixed result after just a few days.
How can nutritional ketosis be reached?
Ketosis can arise through fasting or a very-low-carbohydrate, higher-fat approach known as a keto diet. Neither is automatically suitable for everyone. The carbohydrate threshold varies, and a nutritionally adequate plan needs careful assessment.
Foods commonly restricted in such plans include:
- Cereals and cereal-based foods
- Pulses
- Carbohydrate-containing dairy foods such as milk; skimmed milk also contains lactose
- Larger portions of carbohydrate-rich fruit, dried fruit, fruit in syrup and juices, including apple, pear, kiwi, pineapple, mango, grapes and banana
- Portions of blueberries, plums, cherries, watermelon, oranges, peaches, grapefruit and lemon also contribute to the carbohydrate total. Their inclusion depends on quantity rather than a simple permitted/forbidden list.
- Larger portions of vegetables such as carrots
- Some nuts, including cashews
Foods often included are:
- Lower-carbohydrate vegetables such as spinach and tomatoes
- Avocado, olives and measured portions of berries or coconut
- Meat and fish
- Eggs
- Fats and oils, favouring suitable unsaturated sources
- Nuts
- Lower-carbohydrate dairy foods such as brie; lower carbohydrate does not make unrestricted saturated fat desirable
Why is ketosis discussed in sports nutrition?
Nutritional ketosis is not for everyone and can have adverse effects. Research explores several changes relevant to sport:
With an energy deficit, it can help reduce body fat, but ketosis itself does not guarantee greater fat loss than another suitable diet.
Over time, keto-adaptation increases the capacity to use fat and reduces reliance on glucose at some exercise intensities. This does not necessarily improve exercise economy or performance, particularly when intensity is high.
Reduced glycolytic flux may mean less lactate production at a given workload, but lactate is also a useful fuel and is not simply the cause of muscle acidity or fatigue.
Ketosis does not guarantee protection against low blood glucose or “hitting the wall”. Appropriate fuelling remains essential.
These are general concepts. A qualified professional should assess medical suitability, training demands, dietary quality and the individual response. Diabetes medication, pregnancy and breastfeeding require particular caution.
Here are some lower-carbohydrate recipe ideas:
Broccoli and grated cheese omelette
Ingredients
- Raw egg 120 g
- Broccoli 60 g
- Olive oil 5 g
- 0% fat Cheddar-style cheese 15 g
- Table salt 1g
Method
- Boil the broccoli for around 10 minutes until tender, drain and rinse under cold water.
- Beat the eggs in a bowl with a pinch of salt.
- Heat olive oil in a frying pan and add the eggs.
- As they begin to set, add the broccoli and grate over the cheese.
- Cook through, adjust the salt and serve.
Figures per serving
Calories: 249
Protein: 30% | Carbohydrates: 5% | Fat: 63%
Spinach salad with avocado, tuna, egg and olives
Ingredients
- Spinach 40 g
- Avocado 50 g
- Tuna canned in water 60 g
- Hard-boiled egg 60 g
- Olive oil 5g
- Black olives 10 g
- Table salt 1 g
- Dried oregano 1g
Method
- Boil the egg for around 10 minutes, then cool and peel.
- Wash and drain the spinach and arrange on a plate.
- Top with sliced avocado, black olives, drained tinned tuna, hard-boiled egg, olive oil, a pinch of salt and oregano.
Figures per serving
Calories: 249
Protein: 31% | Carbohydrates: 9% | Fat: 61%
Smoked salmon salad with mozzarella, tomato and mango
Ingredients
- Romaine lettuce 30 g
- Red-leaf lettuce 30 g
- Mozzarella 50 g
- Salad tomato 80 g
- Olive oil 5 g
- Table salt 1 g
- Smoked salmon 50 g
- Mango 50 g
- Lemon juice 20 g
Method
- Wash and chop the lettuces, tomato and mango.
- Cut the salmon and mozzarella, combine on a plate and dress with salt, lemon and olive oil.
Figures per serving
Calories: 249
Protein: 27% | Carbohydrates: 20% | Fat: 53%
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For a personalised sports nutrition plan, explore our sports nutritionist service.
Further reading
Keto-adaptation and endurance exercise capacity
Nutritional and physiological aspects of low-carbohydrate diets
Efficacy and safety of low-carbohydrate diets: a systematic review
