Intermittent fasting is a term used to describe a pattern of eating, cycling between periods of fasting (with no food or a significant calorie reduction) and periods of unrestricted intake. Intermittent fasting can take different forms and follow different patterns, but the lure of these strict eating schedules is the same, regardless of the approach.
As with many fad diets, the claims associated with intermittent fasting centre around promises of weight loss and fat loss, as well as a reduction in inflammation, improved blood pressure and cholesterol levels and a "slowing of the aging process."
Alternate day fasting, modified fasting and time-restricted fasting are three common methods of fasting that differ primarily in terms of eating schedule, but not necessarily in the total amount of food/calories consumed.
Alternate day fasting refers to rotating days of eating with days of fasting. On fasting days, foods and beverages with calories cannot be consumed and calorie-free drinks, including water, black coffee and tea are allowed. On non-fasting days, you can eat whatever you want, with the recommendation to choose healthier options.
This schedule of eating assumes that an individual's intake will remain unchanged on those non-fasting days, leading to a substantial reduction in the overall amount of calories consumed in a week, resulting in weight loss.
The side effects of this fasting pattern include hunger pangs and decreased concentration on fasting days, and may be worse if you're an active person or have higher caloric requirements. Some studies have shown that hunger can overwhelm people on fasting days and cause them to eat more than usual on non-fasting days, leading to an increase in overall intake. It's also important to note that the long-term effects of this pattern of eating are not yet known.
Modified fasting involves eating very few calories on fasting days, approximately 20-25 per cent of normal intake or 500 calories per day. A modified fast has also been referred to as a 5:2 fast, with five days of "normal" eating and 2 days of fasting, with no intake.
Although this method of eating is endorsed as a means for improving blood sugar control and blood pressure, there is currently not enough evidence to know if this eating pattern is effective at promoting an improvement in either, or if it's even a safe method for weight loss, with studies showing conflicting results.
Time-restricted fasting may be one of the most common methods of intermittent fasting, with caloric intake limited to 8-12 hours per day and the majority of the fast occurring while you sleep. This style of fasting may be more popular because it's an easier transition for people who are already used to skipping breakfast or not snacking after dinner.
As with a modified fast, evidence to support a time-restricted fast is not strong enough to warrant an evidence-based recommendation of this diet and more research is required. Although I've seen this diet promoted as a "solution" to improving blood sugar control in individuals with diabetes, that claim would be a direct contradiction to the recommendations being made by Diabetes Canada (DC), which publishes the practice standards for the management of diabetes in Canada. DC recommends eating three meals per day at regular times, spacing meals no more than six hours apart and eating breakfast to help keep blood sugars stable throughout the day.
Reviewing the research behind current fad diets is something I have to do on a regular basis. Sometimes I find weak evidence to support claims, sometimes there's conflicting research and other times there's overwhelming evidence against the claims being made. Intermittent fasting falls somewhere between weak evidence and insufficient evidence.
Although there is evidence from animal studies that calorie restriction can increase lifespan and improve tolerance to certain metabolic stressors, there is insufficient evidence to show that those results are equaled in humans.
Arguments for intermittent fasting sometimes include the claim that the stress caused by fasting will prompt an immune response that repairs cells and produces positive metabolic changes including a reduction in triglycerides, cholesterol, blood pressure, weight, fat mass, and blood glucose levels.
Unfortunately, different studies compare different methods and schedules of intermittent fasting, which makes it difficult to make a sweeping statement with regards to the efficacy of this style of eating. There needs to be more research, especially regarding any negative side effects.
We do know that extended periods of minimal caloric intake can actually lead to physiological changes that can cause the body to adapt to the calorie restriction and prevent further weight loss. In other words, if you restrict your caloric intake for long enough, the body will learn to function on that reduced intake and you will not lose any more weight, plus you'll have to eat less calories just to maintain your weight.
Intermittent fasting carries with it the risk of nutrient deficiencies and the potential to constantly be overriding feelings of hunger on fasting days and fullness on non-fasting days. Intermittent fasting does not allow for the flexibility that comes from listening to your hunger and fullness cues.
If you get up in the morning, hungry for breakfast, but you can't eat until 2 p.m., does it make sense to go through most of the day lethargic and light-headed while you wait to break your fast?
Unfortunately, the number of books on intermittent fasting and how it can be incorporated into our daily lives seem to far outweigh the number of trials examining whether fasting should be encouraged at all. As with many fad diets, more research is required before evidence-based claims can be made.
-- Kelsey Leckovic is a registered dietitian with Northern Health working in chronic disease management.