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Intermittent fasting was the highest ranked diet term searched on Google in 2019, and is one of the diet methods we are personally asked about most. These questions increase in January as new year resolutions begin. This is why we decided to write an article to help answer any questions you may have regarding Intermittent fasting. This article does a deep dive into intermittent fasting, including what it is, types of fasting, how it impacts performance, weight loss, and overall health, the pros and cons, and our overall thoughts on intermittent fasting. It is important to note that we always recommend working with a dietitian to determine if a method like intermittent fasting is right for you.

Definition of Intermittent Fasting and Types of Fasting

It may come as a surprise that the term intermittent fasting doesn’t describe a particular fasting regimen and is used as an umbrella term to describe many fasting methods. Intermittent fasting describes an eating pattern that cycles between periods of eating and fasting (no caloric consumption). This means a person who is intermittent fasting will only restrict food consumption on certain days or during specific time periods. The period a person restricts consumption will be dependent on the fasting method. There are many fasting methods, but the most common are complete alternate day fasting, modified fasting (5:2), and time-restricted feeding.

Complete Alternate Day Fasting: Complete alternate day fasting includes one or two days per week of total fasting (0 caloric intake). This is often referred to as the Eat-Stop-Eat Method. An example of this would be having no food restriction Monday-Saturday and consuming no calories on Sunday.

Modified Fasting: This method alternates between days of no food restriction and days meeting just 25% of daily calorie needs. The most common approach of this method is the 5:2 diet. The 5:2 is no food restriction for five days a week, with 400-600 calorie consumption the other two days a week. These calories are typically consumed in one meal.

Time-Restricted Feeding: Time-restricted feeding is used to describe a pattern of eating during a specific window of time during the day and fasting for the rest of the day. The most common time-restricted feeding method is the 16:8 method, which means fasting for 16 hours and eating within an 8-hour window, for example, between 12-8 pm. Other time-restricted feeding variations include the 18:6 method, 20:4 method, and one meal a day (OMAD).

Unlike other popular diets that restrict certain foods such as carbohydrates or fats, this approach focuses solely on when you can eat and not what you eat. It is important to note that consumption during the feeding time is not meant to compensate for any deficit created by fasting. This was developed as a more straightforward way to simply reduce total calorie consumption, as many believe continuous calorie restriction to be too difficult.

Does The Research Support Intermittent Fasting Claims?

Some of the health claims made about intermittent fasting include weight and fat loss, improved energy levels, increased growth hormone production, reduced inflammation, lowered triglyceride levels, and improved blood glucose levels. Evaluating the research for intermittent fasting can be difficult due to the different fasting methods and the varying results produced. Here we assess the research on how intermittent fasting might impact weight loss and body compositions, overall health, and athletic performance.

Intermittent Fasting for Weight Loss

Weight loss is obtained by creating a caloric deficit. There is evidence to suggest that intermittent fasting may be an effective tool for facilitating weight loss, but is it more effective for weight or fat loss than daily caloric restriction? A review that examined the weight loss outcomes in overweight and obese adults in randomized studies lasting longer than eight weeks  between intermittent fasting interventions and daily caloric restriction interventions determined that both interventions produce similar results. These results indicate that although it may not be superior to typical caloric restriction, intermittent fasting may be another alternative for weight loss.

Intermittent Fasting for Health Biomarkers + Men vs. Women

This review explains that there is some research suggesting that different fasting methods may improve metabolic biomarkers such as blood glucose levels, lipid profiles, and blood pressure. However, there is still more research that needs to be done. It has not been determined whether these improvements result from the weight loss or if they are at all superior to typical caloric restriction.

Additionally, research suggests that intermittent fasting may impact women differently from men, especially as fasting periods get longer. This study demonstrated that a 24-hour fast may result in increased insulin resistance in women. Reproductive hormones that play an essential role in body functions such as metabolism and ovulation may also be negatively impacted during prolonged fasts. For example, estrogen tends to decrease with intermittent fasting; which can lead to increased fat storage and appetite.

Intermittent Fasting for Athletes

A majority of the research examining the impact of fasting on athletic performance looks at athletes during Ramadan. Additionally, there has been a rise in interest in fasted training and if it provides any physical adaptations. If you would like to learn more about the pros and cons of fasted training, check out our previous article on this topic.

Preserving lean muscle mass is a main priority for athletes trying to lose weight. One study in resistance-trained athletes and another in endurance athletes suggests that a time-restricted feeding method may facilitate weight loss while maintaining lean muscle mass. However, both studies also saw a decrease in testosterone and insulin-like growth factor 1 (IGF-1) in both time-restricted feeding groups. These studies indicate that more research is needed to determine their findings and implications to athletic performance.

The current research does not show that intermittent fasting positively impacts performance, but it could negatively impact if an athlete does not have adequate nutrition intake or nutrient timing. If you are an athlete who is interested in attempting intermittent fasting, it is recommended to do a moderate energy restriction to preserve lean muscle mass, and decrease likelihood of hunger and irritability.

Pros and Cons of Intermittent Fasting

Pros of Intermittent Fasting

One of the main benefits to any of the intermittent fasting methods is there are no other restrictions other than the eating window. This makes the plan simple to follow and does not require tracking calories, macros, or avoidance of any foods. Sticking to an eating window (for example 14:10, 12:12) could reduce the incidence of overindulging late at night, which may be beneficial for late night snackers/drinkers looking to lose weight but find it too difficult to reduce or track their calories.

Cons of Intermittent Fasting

While there are no food restrictions with intermittent fasting, it is still a restrictive diet that can be difficult to adapt to for numerous reasons. Symptoms from restricting energy consumption for extended periods include fatigue, headaches, inability to concentrate, hindered performance and recovery, and irritability. Additionally, limiting yourself to an eating window can also cause additional stress in social settings or when schedule changes occur, and can make eating intuitively very difficult.

There are many unanswered questions about intermittent fasting that need additional research. The findings are varied and there is a need for longer studies to confirm safety and ability of weight maintenance or diet adherence.

Special Considerations for Intermittent Fasting

As mentioned above, we always recommend meeting with a sports dietitian to achieve body composition goals. If you are someone who has T1DM, pregnant, breastfeeding, use medications that involve food intake, or have a history of disordered eating or eating disorders, intermittent fasting is not recommended. It is also not recommended for athletes who are highly competitive with busy schedules, especially in season. Restricting yourself to an eating window can result in poor performance and recovery.

Final Thoughts on Intermittent Fasting

Do we typically encourage intermittent fasting? No. Are we completely against intermittent fasting? Also no. While intermittent fasting isn’t the miracle method that it’s rumored to be, there is evidence to support that it can be an effective tool for weight loss. The best way to ensure success with weight loss is to make nutrition habits that you will be able to maintain long-term. If you’re unsure where to start, check out our weight management guide here or reach out to us to schedule a nutrition coaching session.

Article written by Dana Norris MS, RD

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