The Diet Prescription
Ketogenic dietary therapies are designed to cause a metabolic shift within the body, with fat becoming the primary fuel rather than carbohydrate and ketone bodies replacing glucose as an energy source for the brain. Although all diets are based around similar principles of low carbohydrate and high fat intake and will all be tailored to individual nutritional requirements, the way in which the different protocols are implemented and resulting diet prescriptions will vary as outlined below.
Traditional ketogenic diets
The classical ketogenic diet is very low in carbohydrate and high in fat with protein provided to meet necessary minimum requirements for growth. It is calculated in a ratio of grams of fat to protein plus carbohydrate. Diets are most commonly prescribed at a 3:1 ratio (3g of fat to 1g of protein plus carbohydrate combined, 87% of dietary energy as fat) or 4:1 ratio (4g of fat to 1g of protein plus carbohydrate, 90% of dietary energy as fat) or somewhere in-between; a lower 2:1 starting ratio is often used which is increased as tolerated. Fat is mainly from foods, such as cream, butter, oil and mayonnaise although products available on prescription can also be useful. Carbohydrate is usually limited to small servings of vegetables and/or fruits. A dietitian calculates an individual dietary prescription with all meals and snacks at the correct ketogenic ratio; recipes or exchange lists are provided and food must be weighed to ensure dietary accuracy.
The medium chain triglyceride (MCT) ketogenic diet allows considerably more carbohydrate and protein as the substitution of some of the fat with an MCT source will increase ketosis therefore total fat intake can be reduced. The amount of MCT will vary depending on tolerance and individual requirements but is usually between 40-60% of total energy intake. MCT is given in the diet as an oil or emulsion that are available on prescription and included in all meals and snacks. Although the dietitian calculates an individualised prescription and all food is weighed, the more generous carbohydrate and protein intake means that exchange lists are generally used to implement the prescription rather than the calculation of recipes.
Free foods are very limited on the traditional ketogenic diets and fluid is not restricted but an adequate intake encouraged to reduce risk of kidney stones. Energy intake is carefully controlled as excess will be stored as fat which can compromise ketosis and seizure control. At the outset of a diet calculation, the energy prescription will be individually calculated, taking into account current dietary intake, nutritional requirements, current weight and height, recent growth trends, activity and seizure level and any medications. Imaginative use of food combinations is important to accommodate the low carbohydrate and high fat content and computer calculation tools such as Electronic Ketogenic Manager (EKM) can help with recipe development. Full vitamin, mineral and trace element supplementation is necessary to avoid nutritional deficiencies; the dietitian will advise on this taking into account any provision from the foods allowed on the diet.
Modified ketogenic diets
More recently two types of diet protocol modified from the traditional ketogenic diets have been developed.
The modified Atkins diet restricts carbohydrates and encourages high fat foods, but does not limit or measure protein or total calories. The original protocol as developed at the Johns Hopkins Hospital USA initiates the diet with a very low 10-20g carbohydrate intake daily (depending on age); this is then increased after 1 month with the final prescribed amount dependent on seizure control. In the UK a higher starting carbohydrate is frequently used, e.g. 20-30g daily; this is then reduced if necessary within the first couple of weeks on the diet. Protein is allowed freely on the modified Atkins diet and high fat foods are encouraged and should be eaten at each meal/snack to promote ketosis. Some centres use exchange lists to ensure an adequate fat intake although the overall energy content of the diet is not usually prescribed but adjusted with on-going dietary advice as needed.
The low glycaemic index treatment is more generous in carbohydrate which at 40-60g daily (including fibre) provide approximately 10% of dietary energy, but only those with a glycaemic index of less than 50 are allowed. An even daily carbohydrate distribution is recommended and this should always be eaten alongside a protein and/or fat source to reduce the overall glycaemic index of a meal or snack. Protein, fat, and calories are monitored, but not as strictly as on a traditional ketogenic diet. Food is not weighed, but based on portion sizes.
The modified diets will usually also require nutritional supplementation as advised by the dietitian to ensure requirements of vitamins, minerals and trace elements are being met, and again adequate fluid intake is encouraged.
In practice many dietitians are adopting a more flexible, ‘patient-tailored’ approach to ketogenic therapy, especially when treating older children and adults. This may combine elements from one or more of the different types of ketogenic diet rather than sticking to a rigid diet protocol.
Dietary fine tuning
The aim of fine-tuning a diet is to establish the prescription for optimal efficacy and on-going dietary modifications are an essential component of the dietetic care. Regular home monitoring of ketone levels, weight and seizures are important. Although ketone levels can be useful as an indication of how the body has adjusted to the diet, and to ensure they are not excess, the level of ketosis associated with the best seizure control will vary between individuals. The amount of carbohydrate and fat in a prescription can influence the ketone levels; decreasing carbohydrate and increasing fat will usually act to raise levels and increasing carbohydrate and reducing fat will lower ketone levels. Replacing some of the long chain fat provided from food and supplement sources in the diet with MCT supplements will also increase ketone levels.
Energy intake and weight changes can also influence ketosis and seizure control. The energy prescription on ketogenic diets will need regular review and fine-tuning as the child grows and the diet becomes established. For example as seizure activity decreases, mobility may increase and with it energy needs. Regular weight checks will enable calorie modifications as needed.
As a child grows, the diet may need re-calculating to increase protein intake in line with increases in body weight. Meal and/or snack distribution may also need changing to fit with changes in lifestyle. Micronutrient intakes should always be checked by the dietitian if they are likely to be altered by dietary modifications, and supplementation reviewed as necessary.