Intermittent Fasting - Does it Really Work? A Registered Dietitian Weighs In

Intermittent Fasting - Does it Really Work? A Registered Dietitian Weighs In



What is Intermittent Fasting?

Intermittent Fasting (IF), also known as time-restricted feeding, has been a traditional practice for many centuries, originating with those who observe Ramadan. In recent years, IF has become a more mainstream practice for non-religious lifestyles for both clinical and athletic performance. 

While IF does not have a formal definition, it generally refers to a period where you restrict food consumption to a small window. Common IF schemes include alternate day fasting, fully fasting one day a week, and the most common being 16:8, where one restricts feeding for 16 hours and eats during the 8-hour window. 

Does Intermittent Fasting Work?

Before diving into the research, it is important to consider the practicality of IF. This lifestyle is not for everyone and requires dedication, especially choosing a more extreme scheme such as fasting for a full day. However, this lifestyle can be convenient for people who work long shifts and may not have time to take normal food breaks (i.e. 3rd shift workers, healthcare providers, travelers, etc). 

Research in animal models, particularly monkeys and rats, show that periods of fasting leads to an increased lifespan. This may be due to a protection from oxidative stress, that normally leads to cell death, damaged proteins and tissues, and ultimately increased risk in mortality.  

Regarding the age-extending benefits, this may be due to the increase in what is known as mitochondrial biogenesis. Mitochondria is best known for generating adenosine triphosphate (ATP), which is the basic energy unit for all metabolic processes. What is fascinating about mitochondria is that they possess their own DNA. Damage to this DNA correlates with age in in vitro models. Thus, caloric restriction, which generates new mitochondria could be the link to improving the lifespan.

However, to date, there is no human evidence validating this proposed mechanism. To further conflict results, animal models do not directly translate to human physiology. Additionally, the scientific community does not know how much of a life extension this would translate to and what the optimal fasting scheme or caloric restriction is necessary to achieve it. 

Not to mention, study design across the literature is riddled with mixed methodologies, such as population characteristics (overweight/obese, trained, untrained, male, female, older, younger, etc), day of fasting (exercise versus rest day), composition of diet, and population training status. 

Despite these inconsistencies, let us dive into the human-based research. 

Focusing on body composition and weight loss literature, research reviews demonstrate that IF works. There tends to be an improvement in body weight, waist circumference, body fat mass, LDL, HDL, total cholesterol, triglycerides and fasting glucose. Interestingly, research has shown that there tends to be a reduced drive to eat. This last point is important because there tends to be an argument that people will overcompensate on a refeeding day, which is not always the case. 

What tends to happen during IF is that carbohydrate consumption goes down, and protein goes up to displace those Calories. While protein and carbohydrates have the same amount of Calories per gram, protein is the most satiating macronutrient (i.e. carbohydrates, fats, protein). Additionally, protein tends to increase the thermic effect of food (the Calories required to digest food) greater than the other macronutrients.  This combined effect of increasing Calories expended + feeling fuller longer, and thus potentially eating less Calories overall, may lead to beneficial weight loss. 

The downside to IF is that it does not necessarily work better than a traditional weight loss diet. It is certainly subject to low adherence due to the intense nature of consuming little to no food for periods. Additionally, there are no long-term studies showing that these benefits can be sustained. 

Regarding athletic performance, research has mixed results regarding efficacy of intermittent fasting. Some studies have shown a decrease in prolonged exhaustive exercise and high-intensity exercise, which may be ascribed to impacts of dehydration. However, other studies have failed to show declines in performance in both aerobic and anaerobic performance. 

What is most interesting regarding the performance studies is that even though the perceived exertion of exercise is higher, the performance may stay the same or even increase. Most likely, these performance studies are impacted by other factors such as sleep, training level, food intake during feeding periods, and gender.  It is possible to maintain performance while fasting, but attention to details during feeding periods are critical to maintain ideal outputs. 

The ultimate question to ask is, for whom is IF the right fit? And the jury is still out. It seems that mood tends to improve on IF (alternate day fast) for those who are obese but declines significantly for those who are lean. 

This diet is certainly not for the faint of heart but can certainly be utilized as a tool in the box to achieve your goals.

Disclaimer: This information is educational only and not providing healthcare recommendations. Please see a healthcare provider.


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