Breaking Down Intermittent Fasting


By: Samantha Poccia, MS RDN LDN


There seems to be a constant cycle of new fads in the nutrition world, with intermittent fasting being one of the latest to cause a craze. What is intermittent fasting, and how does it impact nutritional habits?

Intermittent fasting (IF) is a dietary pattern with alternating periods of fasting and non-fasting. The overarching idea is to not necessarily focus on what to eat, but rather, when to eat. There are several approaches to IF, with one of the most popular being time-restricted eating. In this method, the fasting period is typically either 16:8 or 14:10, meaning one would fast 16-hours with an 8-hour eating window per day or fast 14-hours with a 10-hour eating winder per day, respectively. Typical eating windows are between the hours of 10/11am-6/7pm for 16:8 or 9am-7pm for 14:10. Other types of IF include alternate day fasting, the 5:2 method, and the 24-hour fast.1

The Good: Pros

Circadian Rhythm: Following an eating pattern that aligns with the circadian rhythm, such as time restricted IF, allows the body to better maintain metabolic balance.1 This can lead to decreased body weight, body fat, inflammatory markers, and blood pressure, as well as improved gut health, glucose tolerance, and cortisol levels. 


Weight Loss: IF has been regarded as a dietary pattern that can aid in sustainable weight loss. It must be noted that although the focus is on food timing, it is still important to consume a healthy, well-balanced diet. The main mechanism at work when it comes to IF and weight loss is referred to as “metabolic switching”.1 Our body naturally uses glucose as its main fuel source. When we go into a fasting state, usually of at least 12 hours or more, our body will switch to using ketones for energy. These ketones are derived from stored fatty acids and become our fuel source. As a result, our bodies will burn fat to create energy due to a lack of available glucose. This metabolic switch protects muscle mass and function and leads to potential improvements in body composition, including a reduction in fatty tissue and improved appetite control.1 It also aids in enhanced metabolism, increased longevity, and improved health span.1

Reduction in Metabolic Syndromes

Most IF clinical research trials reveal the positive impact this diet has on healthy living by slowing down the progression of metabolic syndrome, which includes reducing the prevalence of T2DM, cardiovascular disease (CVD), and hypertension.1

T2DM – The development of insulin resistance is hypothesized to be caused by several factors. One theory correlates increased adiposity (fatty tissue) and chronic inflammation, which could lead to the development of insulin resistance in tissues. It’s known that IF can decrease adiposity by way of reducing caloric intake, which might lead to a decrease in insulin resistance.1 Another theory predicts that by following an IF diet, which subsequently cuts back energy intake, it will cause an extended decrease in insulin production.1 This decrease could lead to improvements in insulin sensitivity and glucose balance. 

CVD – An increase in inflammation can cause a host of negative consequences, some of which include elevated C-reactive protein, insulin resistance, the development of atherosclerosis, and coronary artery disease. Therefore, following an IF diet can reduce major cardiovascular incidence.         

Hypertension – Lowering blood pressure through an IF diet may be due to a link to the activation of the parasympathetic system.1 Incorporating extended periods of fasting is a stimulus to increase parasympathetic activity, which is the state of “rest and digest” and regulates digestion.    


The Bad: Cons

Although there are numerous health benefits to following an IF diet, it is not recommended for everyone. Side effects can include hypoglycemia (low blood sugar), dizziness, weakness, hunger, fatigue, decreased concentration, nausea, and headaches.1 Those with certain medical conditions, such as hypotension, T1DM, etc., as well as elderly and pregnant women, should consult with their physician before starting an IF diet.

Those who have a history or tendency toward eating disorders should also stay away from an IF diet.1 It has been linked to bulimia nervosa and can instigate feelings of perfectionism, impulsivity, and mood instability, which are all risk factors in developing disordered eating patterns.

Most clinical research also notes the difficulty of adhering to an IF diet, citing high dropout rates in their studies.1 Fasting can pose a challenge and is not always practical in everyday life. Especially, alternate day fasting, compared to time-restricted eating, is harder to maintain with no significant difference in benefits.1 Therefore, it makes sense to choose a 16:8, 14:10, or even 12:12 time-restricted eating window.

The Ugly: Long-term impacts 

When evaluating IF diets, there have not been enough long-term comparative studies to validate if any of the diet approaches have long-term effectiveness.1 

Many studies have limitations. Some of these limitations include a small sample size, poor study design, lack of including people with various health backgrounds, testing using animals, high drop-out rates, and shorter-term study parameters.1 Further research is needed to truly understand the long-term potential benefits and sustainability of following an IF diet.

Bottom Line

Intermittent fasting has numerous health benefits. It can aid with weight loss, reduce the prevalence of T2DM, CVD, and lower hypertension. However, this diet is not for everyone and can sometimes be difficult to adhere to.

1 Vasim, I., Majeed, C. N., & DeBoer, M. D. (2022, January 31). Intermittent fasting and Metabolic Health. MDPI. Retrieved February 17, 2023, from