top of page

Intermittent Fasting – all the rage or hype for naught?

Updated: Sep 18, 2019

Like many other diet trends, Intermittent Fasting has taken on an identity of its own. The media, non-expert influencers, and Dr. Google have exploded with information about the unlimited benefits of intermittent fasting. But are the claims true?

Below, I’m breaking down all the science-based details – from the types, to the potential benefits of Intermittent Fasting, as well as getting to the bottom of what you need to know for your health goals.

Types of Intermittent Fasting:

Alternate Day Fasting: Rotating days of eating and days of fasting. On fasting days, no foods or beverages with calories are consumed. Calorie-free drinks like coffee tea and water are allowed. On non-fasting days anything is allowed – although following healthy eating guidelines is encouraged.

Modified Fasting: Consuming very little amounts of food on fasting days. Some modified diets restrict to 20-25% of needed calories. Other advocates encourage limiting intake to 500 calories. The third type of modified intermittent fasting is 5:2 – fasting two days per week, with regular eating the remaining five.

Time Restricted Feeding: Limiting intake to only waking hours- with the goal being to fast 8-12 hours, with the majority of the fasting happening while you are sleeping. Often called the 16:8 regimen.

How it works:

Through cycles of fasting and unrestricted eating, intermittent fasting is believed to change body composition via loss of fat mass and weight, and to potentially improve markers of health, like blood sugar levels, blood pressure and cholesterol.

One claimed benefit of intermittent fasting is that it helps the body to avoid “starvation mode”. Prolonged energy restriction can cause the body to adapt to the lack of intake, and go into this “starvation mode,” which in turn prevents weight loss after a period of time. Some believe that intermittent fasting helps prevent the adaptation by regularly alternating between consumption and restriction.

What the science says:

At this time there are very few human studies related to the long-term effects of intermittent fasting. Research is ongoing to observe long-term health outcomes, including weight loss maintenance and change in metabolic parameters associated with diabetes, cardiovascular disease, and cancer. Additionally, interest remains in the long-term effects of intermittent fasting on gut health, circadian biology, and sleep.

Potential Pros and Current Claims:

Weight Loss: Intermittent fasting Could be an effective weight loss strategy as an alternative to traditional calorie restriction. Because there is no caloric tracking involved (you are either eating or not), it might be a more manageable method for some.

Metabolic Benefits: Some very small studies show reduced insulin resistance and improved glucose control in overweight adults after an intermittent fasting diet. Rodents following an intermittent fasting diet showed reduced and reversed aspects of metabolic syndrome, marked by abdominal adiposity, insulin resistance, elevated triglycerides and/or hypertension.

Lifespan: It’s not new science (studies date back to the 1930’s) that restricted calories in some fashion may extend lifespan, however, research has been mostly conducted on rodents, fruit flies and fish. In animal models, long term caloric restriction has been shown to increase lifespan and improve tolerance of metabolic stressors.

Brain benefits: In rodents, alternate-day fasting enhances brain function indicated by improvements on tests related to sensory and motor function as well as learning and memory. Additionally, intermittent fasting protects against cognitive deterioration in rats.

Potential Cons & Risks:

Lack of human studies: Unfortunately, the majority of our science is currently done on rats and mice. The few studies that have been conducted in humans are very small (100 participants at most) and short term. As we know, rodents are not humans – results in rodents cannot be directly translated into expected outcomes in humans. Studies in rodents provide a starting point and lend support to further investing in human studies. Until there are more studies conducted on humans, particularly on the long-term effects of a fasting regimen, recommending intermittent fasting to clients is questionable.

Lack of information means unanswered questions: With a lack of long-term studies, we also don’t know what method of intermittent fasting, if any, is most effective. Is there an optimal window for the fasting? How few calories should one eat on their fasting days? What is the safety (mentally and physically) of ongoing intermittent fasting?

Quality of diet and Nutrient deficiencies: When restricting food consumption to only a few days per week, you miss opportunities to consume nutrient-rich foods that are essential for optimal health. On the days that dieters can eat, many tend to binge on all foods regardless of nutrient quality, limiting the opportunity for nutrient-rich foods, and therefore increasing risk for nutrient deficiencies.

Workouts may suffer: On fasting days it is not recommended that intermittent fasting dieters exercise due to the lack of food consumption. Intermittent fasting can lead to dizziness, lightheadedness, and hunger, all of which can be dangerous during exercise. Additionally, if your goal is muscle gain, fasting can make building muscle extremely difficult due to calorie deficiencies.

Social Aspect: It’s already hard enough going out to eat and choosing healthy food options. But going out to eat, and not having the option to eat at all? Personally, I’d rather stay home. Intermittent fasting does not leave room for a social life, unless your scheduling your dates around your non-fasting days.

Stress & Fatigue: I don’t know about you, but when I don’t eat- I’m hangry! Participants in studies have reported fatigue, lack of concentration, mood swings, headaches and poor performance.

Lack of long-term sustainability: I get it, you want to lose the weight, and you don’t care how. Losing weight quickly sounds great, which may happen via intermittent fasting. However, what happens when intermittent fasting dieters start eating their regular foods in their regular routine again? The weight will likely come back, due to the drastic switch from fasting to regular calorie consumption.

Additionally, studies show that Dropout rates are higher in intermittent fasting dieters compared to dieters following a regular calorie restriction, indicating that participants were unable to sustain the diet for an extended period of time.


Is your goal weight loss?

Proponents would argue that fasting works for weight loss because the dieter eats less in one week than they normally would. However, this is true for almost any individual sticking to a health and disciplined diet regardless of whether such individual is fasting.

People like intermittent fasting because it provides renewed hope for weight loss goals. People are told exactly what to do, and in the short term, it provides a model needed to reach their goals.

Working with a registered dietitian to meet your unique goals can help you establish long-term, sustainable habits that will get you where you want to be without removing your favorite foods. Recent studies have shown that intermittent fasting and regular long term calorie restriction are equally effective for weight loss and cardiovascular health.


Bottom line:

Personally, I believe we need more research, particularly on the long-term effects of intermittent fasting, before recommending to clients. The research in rodents appears promising, but studies are needed in humans.

Any diet that requires a person to disregard innate hunger and satiety cues is not sustainable, and likely not very enjoyable. Other methods of weight loss and improved health are proven effective in the science. The evidence is not clear that intermittent fasting is superior to other methods.

Work with a Registered Dietitian to create a program based on your unique health and nutrition needs and goals. I am following closely with the research on the other potential benefits of intermittent fasting, such as longevity, and brain and gut health.

I do not recommend intermittent fasting for people with diabetes, during pregnancy or breast-feeding, or individuals with a history of eating disorders or disordered eating.

Consult a doctor, especially if you take medication, before starting any sort of intermittent fasting routine.




bottom of page