What is Intermittent Fasting?

Intermittent fasting (IF) is an eating plan that alternates between fasting (no nutritional intake) and eating on a regular schedule. The two main regimens of IF are alternate day fasting and time restricted eating. During periods of feeding/eating, food may be consumed un-restricted or it may be consumed in conjunction with other dietary interventions. For the most part, IF determines when you eat, not what or how much.

Most IF guidelines follow one of four plans:

Time restricted eating:

• 8:16—eating during 8 consecutive hours of the day and fasting for the remaining 16 hours
•  4:20—eating during 4 consecutive hours of the day and fasting for the remaining 20 hours

Alternate day fasting:

• 24-hour fast on alternative days of the week

Modified fasting:

• 5:2—eating ‘normally’ for five days of the week, and for two non-consecutive days, limit nutritional intake to 500kcal or less

Why is intermittent fasting promoted?

Some research has suggested IF may have the potential to reduce blood pressure, blood lipid and blood glucose levels, inflammation and other metabolic markers. However, the most common reason IF has become popular is related to weight loss. Studies have demonstrated that IF can result in weight loss for some study participants. At this time, the research is quite heterogeneous, meaning that the research methods vary from study to study.

For example, some studies look at modified or alternate day fasting, while others look at time restricted feeding; some studies include activity while others do not; some studies modified nutritional intake during periods of feeding while others did not; participants with different body weight classifications were used between studies; and there was variation between studies related to the inclusion of males and females.

For this reason, some researchers suggest there may not be enough data to determine the optimal fasting regimen, including the length of the fasting interval, the number of fasting days per week, the degree of energy restriction needed on fasting days, and recommendations for dietary behavior on non-fasting days to confidently recommend a specific IF regimen.

Much of the research around IF comes from studies in animals (e.g., male rodents). Though some of it appears promising, the results cannot be confidently extrapolated to humans. Human studies are somewhat limited to observational studies of religious fasting, cross-sectional studies of eating patterns associated with health outcomes, and experimental studies with small sample sizes.

As with most diets, the main aspect of IF that may contribute to weight loss is calorie/energy restriction. In several studies, the overall metabolic benefits of fasting regimens did not appear superior to those of general caloric restriction.

What are the concerns?

Intermittent fasting can disrupt general well-being as a consequence of its rigidity. It can influence opportunities for social interactions such as later dinners that fall outside of fasting hours. It can impact energy levels throughout the day and during activities. It can complicate eating when away from home.

For many, IF can feel restrictive. The potential for ‘last supper’ eating during the feeding hours can lead to a preoccupation with food, a tendency to ignore feelings of fullness and the potential for an increase in emotion related to food and eating.

Like most diets, IF is an external system of messages to the body, which means disconnecting from the body’s internal cues. This can disrupt the food relationship and diminish trust with the body.

Feelings of restriction and a disrupted relationship with food can lend itself to disordered eating and the potential to exacerbate pre-existing eating disorder behaviours such as binging. These should be considerations for anyone with a history of eating disorders.

During periods of fasting individuals may develop symptoms such as headache, nausea, irritability, constipation and lethargy. To lessen the impact of these effects, it would be helpful to engage in fasting that allows time to eat every day.

The other potential concern with intermittent fasting is that it may be dangerous for people with particular medical conditions, such as diabetes or for those that take certain medications. Long periods without food, in conjunction with medication can cause imbalances in necessary minerals such as sodium, potassium and magnesium.

Is intermittent fasting right for me?

An important question to consider is whether following a regular, intermittent fasting regimen is feasible and sustainable for you. Most studies on IF range from 2 weeks to 3 months, with very few lasting as long as 6 months. Dropout rates can be as high as 25% in some of the studies due to the rigidity of the diet. Large-scale randomized trials of intermittent fasting regimens in free-living adults that last for at least a year would be helpful to properly assess whether behavioral and metabolic changes from IF are sustainable and whether they have long-term effects on health.

Intermittent fasting is no different from other diets in that it can result in weight loss, but the sustainability of this regime should be a critical consideration. The obstacle most people run in to when following a restrictive eating pattern is the challenge of ‘sticking to it’. At some point, it becomes too hard and eventually the diet is stopped. Weight regain after dieting happens in most cases. This experience of weight loss followed by regain is called weight cycling – something research says has its own health consequences.

So, consider carefully when you’re making decisions about any diet – is this a sustainable lifestyle for me? Am I getting balanced nutrients? And is this really best for my long-term health?

Nutrition is unique to every individual, and there is no one size fits all. To find a way of eating that is health-promoting, and brings you joy and connection, consider reaching out to a weight-inclusive dietitian for support.