What Is Intermittent Fasting (IF)?

Intermittent fasting (IF) is an eating pattern that alternates between periods of eating and fasting, specifically focusing on the timing of meal intake. Fasting itself is not a new concept: There are various fasting practices done for different religious, cultural, and personal purposes.[i]^, [ii] IF has gained significant popularity recently, mainly for its simplicity and its potential benefits in weight management and overall health.[iii]^, [iv] People who partake in IF also report gaining psychological and spiritual health benefits.[v]

IF does not require individuals to dramatically overhaul their current eating patterns nor substitute all their foods for other options.[vi] IF is an umbrella term for three different diets: alternate-day fasting (ADF), the 5:2 diet, and time-restricted eating (TRE). ADF involves a 24-hour fast on alternate days. The 5:2 diet is a modified version of ADF wherein there are 5 feast days and 2 nonconsecutive fast days per week. TRE is when fasting occurs every day with variable hours. Usually, there is a 4- to 10-hour window of eating, for example with breakfast in the morning and an early dinner before 3 p.m., which allows for a 14- to 18-hour fasting duration.[vii]^, [viii] More modern variations of intermittent fasting involve deviating from the usual daily eating patterns that align with the 24-hour day/night cycle, instead restricting caloric intake to specific timeframes.[ix]

How Does IF Work?

During fasting, caloric consumption can range from zero to 25% of daily caloric needs. On nonfasting days, caloric consumption may be liberal, restricted to a dietary composition (e.g. low-carbohydrate, high-protein), or aimed to reach a specific caloric intake (e.g. up to 125% of regular caloric needs).[x]

The exact mechanisms of how IF can improve health markers are unclear. Proposed theories are mostly derived from animal studies and suggest that IF is associated with increased ketosis (wherein your body burns fat for energy instead of glucose), reduction in production of reactive oxidative species (also known as oxidants), and optimizing circadian physiological processes.[xi]

What Are the Benefits of IF?

In general, IF principles target weight concerns and other metabolic-disease risk factors (e.g., cardiovascular disease, type 2 diabetes).[xii] IF can impact various mechanisms that help maintain metabolic balance, including those related to our circadian rhythm. The innate circadian rhythm is like our master clock that coordinates all our biological clocks and multiple functions within our body, including digesting and making energy. This can be significantly affected by our daily feed-fast cyclical rhythm (eating or not-eating time).[xiii] Negative impacts on metabolism (e.g., increased oxidative stress, insulin resistance, and hormonal secretion impairments) can be attributed to disruptions in circadian timing and daily food-intake patterns.[xiv]^, [xv] Studies suggest that intermittent fasting (IF) may reduce systemic inflammation by decreasing energy (caloric) intake and lowering the production of reactive oxidative species by the mitochondria. This, in turn, leads to a reduction in cellular inflammatory processes.[xvi]^{, [xvii], [xviii]} IF also seems to improve resistance to stress, regulation of blood-sugar levels, and—more recently discovered—promote autophagy, which helps remove damaged molecules and recycle their material all while conserving energy.[xix]^, [xx] IF, regardless of the type implemented—ADF, 5:2 diet, or TRE—produces mild to moderate weight loss (3–8% loss from baseline) over short durations (8–12 weeks).[xxi]^{, [xxii], [xxiii]}

How Long Should IF Be Done?

Research studies on IF that look at its impact on biomarkers like weight, blood sugar levels, immune function, lipid profiles, and cardiovascular disease risk typically average from 8 weeks to 1 year.[xxiv]^, [xxv] Alongside other nutritional strategies, IF is also implemented for a short period of time as a way to optimize performance and maximize training benefits in athletes.[xxvi]

Who Should Consider IF?

IF can be considered as part of a comprehensive health-management plan by those with obesity and weight-management issues; those who have cardiometabolic disorders such as insulin resistance, type 2 diabetes, high blood pressure, and heart disease; and those who may be looking to enhance their exercise performance.[xxvii]^{, [xxviii], [xxix], [xxx], [xxxi], [xxxii], [xxxiii]}

Who Shouldn’t Be Doing IF?

IF is not for everyone. Because there is some degree of planned caloric restriction involved, IF is generally not recommended for people who are underweight, have a history of eating disorders, or have dementia, to proceed without the careful supervision of a health-care practitioner.[xxxiv] Glucose metabolism worsened following an ADF in nonobese women, and ADF in general seems to have a minimal impact on insulin sensitivity in nonobese people.[xxxv] Additionally, using IF in supporting patients with diabetes poses a risk of hypoglycemia, particularly for those who are taking insulin or insulin secretagogues (e.g., sulfonylureas).[xxxvi]

What Are Other Safety Concerns for IF?

Some of the commonly reported adverse side effects with implementing IF include hypoglycemia, dizziness, weakness, polyuria, mood swings, and lethargy.[xxxvii]^{, [xxxviii], [xxxix]} There were also people participating in IF trials who reported binge-eating, bad breath, feeling cold, sleep disturbances, lack of concentration, nausea, and constipation on occasion during the study.[xl] It has also been observed that fasting, practiced without adequate protein intake or replacement, can cause muscle wasting. Fasting is also generally not recommended for people with hormonal-dysregulation issues, women who are pregnant and/or lactating, young children, adults of advanced age, and those who are immunocompromised.[xli]

How Do I Know if This Is Right for Me and Get Started?

IF is undoubtedly a promising approach to improving one’s metabolic health and wellbeing, backed by a growing body of scientific research. However, implementing IF requires planning and commitment and should be approached with careful consideration of your unique circumstances and health status. Seeking professional guidance is invaluable in tailoring a health-management plan for you that safely includes intermittent fasting.

Dr. Kim Abog, ND

Dr. Kim Abog is a registered naturopathic doctor and doula based in Toronto, Ontario. She has a special interest in fertility and reproductive health. She helps people by facilitating health-management plans with them, connecting them to evidence-informed integrative health solutions, and helping them thrive.

kimabog.com