Graham Beaton BHSc, ND
Doctor of Naturopathic Medicine

Fasting for short periods of time is common in our society. People may fast for religious reasons, to prepare for blood work (lab tests) or surgery, for weight loss or to control weight, as part of a detox, or due to skipping meals because of lack of time. But what effects does fasting have on subsequent food choices? A literature search using Pubmed revealed the following recent studies on the subject:

  • A study by Wansink and colleagues in the June 25th, 2012 edition of the Archives of Internal Medicine. In this study 128 participants were recruited and randomly assigned to two groups. The first group was instructed not to eat or drink for 18 hours prior to the study – a lunch which took place at 12 pm. The second group was told not to fast and continue to eat their normal diet. A buffet consisting of 2 starches (dinner rolls and French fries), 2 proteins (chicken and cheese), 2 vegetables (carrots and green beans) and a beverage was presented to small groups of participants. Participants served themselves and the food that they chose was recorded by video cameras and weighed. Any food left over on participants’ plates was weighed at the end of the meal. In addition to the scales and video camera, participants completed a questionnaire after lunch in which they provided information on their food consumption prior to lunch (i.e. if they had fasted), and on the order that they had first tasted or eaten the foods that they had for lunch.

    The researchers found that those who fasted started their meals more often with high calorie foods (e.g. starches and protein) than those who did not. Also, participants tended to eat more of the food that they started with. Thus, if this was a starch, they ate more starches than if they had started with a different food.

  • A study by Goldstone et al. in the European Journal of Neuroscience. In this study 20 non-obese adults participated in two testing sessions. In one session, they came in to the lab after fasting approximately 16 hours, while in a second session they came in after having just eaten one to two hours previously. In both sessions, functional magnetic resonance imaging (fMRI: which looks at oxygenated blood flow in the brain) was used to scan their brains. During the fMRI scan, subjects were shown pictures of high calorie vs. low calorie foods.

    The researchers found that after fasting, participants showed a greater response to pictures of high calorie foods compared to after eating. In particular blood flow increased in areas of the brain that control emotion, behaviour, motivation and areas that function as reward centres. Fasting thus enhances the appeal of high calorie foods more than low calorie foods.

As these two studies have shown, fasting influences the appeal of high calorie foods and food choices made in subsequent meals. This means that we are more likely to choose foods that are less healthy (i.e. filling up on bread prior to a meal, eating cookies, ice cream, candy or other desserts before supper) and eat more of them.

Thus, fasting is clearly not a successful strategy for weight loss as it predisposes a person to make unhealthy and counterproductive choices for their next meal. Additionally if you are fasting for religious reasons, for a medical test, surgery, or if you skipped a meal because you were busy at work, remember to be conscious that fasting will influence your subsequent food choices. It may be beneficial to take steps to ensure that your subsequent meal is healthy and nutritionally balanced.

Graham Beaton is a Naturopathic Doctor in Ottawa. For more information on fasting, weight loss, nutrition, Naturopathic Medicine, or to book an appointment, please call 613-290-6115.