The ketogenic diet is not a benign, holistic or natural treatment for epilepsy; as with any serious medical therapy, there may be complications. These are generally less severe and less frequent than with anticonvulsant medication or surgery.
Common but easily treatable short-term side effects include constipation, low-grade acidosis, and hypoglycaemia if there is an initial fast. Cholesterol may increase by around 30%. Long-term use of the ketogenic diet in children increases the risk of retarded growth, bone fractures, and kidney stones. Supplements are necessary to counter the dietary deficiency of many micronutrients.
About 1 in 20 children on the ketogenic diet will develop kidney stones (compared with one in several thousand for the general population). A class of anticonvulsants known as carbonic anhydrase inhibitors (topiramate, zonisamide) are known to increase the risk of kidney stones, but the combination of these anticonvulsants and the ketogenic diet does not appear to elevate the risk above that of the diet alone.
The stones are treatable and do not justify discontinuation of the diet. Johns Hopkins Hospital now gives oral potassium citrate supplements to all ketogenic diet patients, resulting in a sevenfold decrease in the incidence of kidney stones. However, this empiric usage has not been tested in a prospective controlled trial. Kidney stone formation (nephrolithiasis) is associated with the diet for four reasons:
- Excess calcium in the urine (hypercalciuria) occurs due to increased bone demineralisation with acidosis. Bones are mainly composed of calcium phosphate. The phosphate reacts with the acid, and the calcium is excreted by the kidneys.
- Hypocitraturia: the urine has an abnormally low concentration of citrate, which normally helps to dissolve free calcium.
- The urine has a low pH, which stops uric acid from dissolving, leading to crystals that act as a nidus for calcium stone formation.
- Many institutions traditionally restricted the water intake of patients on the diet to 80% of normal daily needs;
this practice is no longer encouraged.
In adults, common side effects include weight loss, constipation, raised cholesterol levels and, in women, menstrual irregularities including amenorrhoea.
The ketogenic diet is usually initiated in combination with the patient's existing anticonvulsant regime, though patients may be weaned off anticonvulsants if the diet is successful. There is some evidence of synergistic benefits when the diet is combined with the vagus nerve stimulator or with the drug zonisamide, and that the diet may be less successful in children receiving phenobarbital.
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