The effect of fasting diets on medication management

To the Editor: Fasting diets have been used by humans for millennia for religious and medical purposes and are now gaining popularity for wellbeing and weight loss purposes. With increasing use of short- and long-term courses of medication to manage a multitude of conditions, a question that needs to be asked is will fasting diets impact on medication regimens?

The 5 : 2 diet, where calorie intake is unrestricted 5 days a week and limited to 500 calories for women and 600 calories for men 2 days a week, is becoming increasingly popular due to widespread publicity. In humans, there is some evidence that intermittent fasting (mainly alternate day fasting rather than the 5 : 2 regimen) could lead to weight reduction, decreased insulin resistance and prevention of type 2 diabetes.1,2

It is possible that patients who are taking medication and intermittently fasting each week could encounter adverse effects or therapeutic failure. Medications of concern generally fall into two categories: those for which absorption may be significantly altered by administration on an empty stomach, and those for which increased gastrointestinal3 or other4 adverse effects may result when taken on an empty stomach (Box).

For example, the bioavailability of telaprevir when taken while fasting is 27% of that when taken with a standard meal.5 As telaprevir needs to be taken three times a day for 12 weeks for treatment of chronic hepatitis C, treatment failure may result with intermittent fasting.

On a similar note, while the absorption of warfarin is not adversely affected by fasting, it is possible that an altered diet (particularly a diet that is high in vitamin K-containing foods) in patients taking warfarin may lead to volatility in international normalised ratio.6

Caution is warranted for patients with diabetes who wish to embark on these fasting diets, despite the appeal in terms of weight loss and reduced insulin sensitivity.2 Glibenclamide, glimepiride and insulin carry a high risk of hypoglycaemia if continued as normal when fasting.7

We urge all health professionals to consider the possible impact of fasting diets on medications and investigate further where required. Information regarding potential clinical significance of the diet may be evaluated via the full product information for individual medications and through community and hospital pharmacies.