The What

Intermittent Fasting (IF) has recently been a rising trend in diet culture, where the focus of the diet is to focus on when to eat as opposed to dictating what to eat. The basis of this diet is to trigger the natural “metabolic switching” that occurs between fed and fasted states. Since humans naturally tend to follow the circadian rhythm of fasting- eating during the day and fasting at night, the selling point of this diet appears to be that it is a more approachable and less rigid diet than others. Currently some clinical trials have shown significance in weight loss in the short term (2-12 months) for participants that practiced IF compared to no diet interventions at all. However, the takeaway from most research is that there was no significant weight loss difference between IF and typical caloric restrictive diets, and there are no long-term studies on the sustainability of weight loss beyond the short term. It’s also important to mention that there are risks to consider with this diet as well.

The two most common methods are the 16/8 (daily approach) or 5:2 (weekly approach). 

• The 16/8 method consists of eating for 8 hours and then fasting for 16 hours. An example would be planning your eating hours from 12pm until 8pm, and fasting during sleeping hours + skipping breakfast and possibly a morning snack. 

• The 5:2 method is to monitor eating habits throughout the week, by consuming regular caloric intake for 5 days, and reducing to about 25% of normal calories (~500-600 calories) for the other two days. This was designed for those who are not able to fast completely. It’s important to note that on the two reduced calorie days meals should still be balanced.

Times of fasting would consist of only water and non-caloric beverages, including black coffee/tea, while times of eating should be filled with balanced meals, with many suggesting using the Mediterranean diet as a guideline. The Mediterranean diet consists of leafy greens, healthy fats, lean protein, olives, and lots of complex carbohydrates like whole grains. 

The How

Metabolic switching, as mentioned previously, is the main mechanism this diet proposes to induce weight loss. Similar to other caloric restrictive diets, IF’s rationale mostly rely on the body utilizing alternative energy sources when carbohydrates (sugars) have been depleted to lose weight. Fasting can trigger starvation. Upon starvation or when the body is depleted of readily available sources of energy which is sugar (aka glucose) or sugar stores (aka glycogen), the body will start to break down protein from lean muscles first to convert them into sugar. There is only a certain amount of protein that can come from muscle to be broken down before it becomes too dangerous (hello, many organs are made of muscles). When it does become dangerous to breakdown muscle mass, the body switches to ketosis to burn fat for energy to spare vital muscles mass. Together, the loss of muscle mass, the associated fluid mass in muscle mass, and fat mass contributes to the overall weight loss on intermittent fasting.  

Is it effective?

Weight loss: IF shows similar significance on weight loss as typical caloric restrictive diets. Current research shows it can likely induce short-term weight loss. Long-term effects unknown.

Cardiovascular health: IF shows similar significance on improving cardiovascular risk factors ie. triglycerides, blood pressure, lipid panel as typical caloric restrictive diets in the short-term. It’s unclear whether this is related to the change in weight or this specific dietary pattern. Long-term effects unknown.

Blood-sugar control: IF shows similar significance on improvements of blood sugar control as typical caloric restrictive diets in the short-term. It’s unclear whether this is related to the change in weight or this specific dietary pattern. Long-term effects unknown.

Risks and Side Effects

Cardiovascular risk: Newest research conducted by researchers from Shanghai Jiao Tong University in China have reported long term fasting of 12-16 hours per day is associated with a 91% increased risk of death from heart and circulatory diseases. Although this information is fairly new, and comes only from observational studies which cannot prove cause-and-effect results, this data is alarming and hopefully leads to more future research. For now, researchers have shown the potential link between eating windows and death from cardiovascular disease. 

Hormonal impact for women: As a precaution for women, IF can be beneficial or potentially harmful on women’s hormones. While there is some evidence that fasting for women can address symptoms of PCOS; however, prolonged stress such as fasting may reduce the activity of the hypothalamus and its production of reproductive hormones needed for ovulation and regular menstrual cycles. IF is not recommended for folks with thyroid issues or going through peri-menopause or menopause since hormone levels are dynamic and cycles are unpredictable. 

Side-effects of the diet commonly include but are not limited to constipation, dizziness, headache, lethargy, irritability, loss of concentration, sleep issues etc. 

Limits of Intermittent Fasting Research

As there have been no long-term studies conducted on IF, it is unclear whether the weight loss is subsequently regained on average as does most caloric diets have shown in the past.

The most important thing to keep in mind is evaluating how reliable information is. In today’s day and age with social media and everything available at the click of a button, it can be easy to fall into traps of misinformation. Intermittent Fasting has been rising in popularity on platforms such as Tik Tok, where people who claim to be highly credential promote this diet as a fast and easy way to lose weight. 

Same thing applies to researching articles online. Although it may not be obvious, the majority of clinical trials and subsequent data available on intermittent fasting were conducted on animals. This poses a large issue on the efficacy and safety of the data because humans and animals are vastly different both in our physiology as well as psychology as it relates to impact from food. Search engine bias towards more engaging content also results in studies that align with diet industries to show up more quickly and more often. These tend to be animal studies and very short-term studies. 

It takes time to dig deeply into academic sites and journal articles to find more reliable sources that show human trials. And those studies tend to conclude more long-term research is needed before wide-spread promotion of IF to the general public. 

Specifically for IF, there is a major gap in long term effects of adhering to the diet. Recent research supports some benefits in the short term for weight loss; however at this point in time, it is unclear whether long term side effects or risks may arise. Not only due to the lack of research, but also every person is different and may react to fasting differently. As a result, it’s important to proceed this diet with caution and if you do persist it over a long time, be attentive and listen to any signs from your body indicating any issues. 

Our Thoughts

Intermittent fasting remains controversial in the evidence-based practice community for nutrition and it is most definitely not for everyone. Prior to adapting this diet, it’s essential to check in with your primary health care practitioner to make sure abiding by this diet won’t cause any problems in the long term.

This diet is especially not recommended for people who are still developing such as child and youth, women who are pregnant / breastfeeding / peri-menopausal / menopausal, any folks who have thyroid concerns, have type 1 diabetes and take insulin, and/or those who have a history of eating disorders.

Although there is promising research on numerous health benefits shown on rat models, human studies have not shown significant effectiveness beyond what’s shown in other fad diets. In addition, fasting is difficult and dangerous for some people. Fasting is associated with increased cortisol levels, the stress hormone, which can lead to metabolic changes that often increases weight in the long-term and can trigger more food cravings. As a result, many people undertaking this diet report overeating and binge eating as two common side effects. Since there are some unnatural restrictions associated with IF, disordered eating behaviors may arise similar to other caloric restrictive diets. It is important to keep your mental health a priority and stop fasting immediately if disordered eating symptoms (ie. recurrent binge eating, significant emotional eating, intensifying food restrictions, strong food cravings, food preoccupation, increased guilt and shame with eating etc) are present.

In sum, we would recommend proceeding IF with caution and best if it’s under guidance of relevant health care professionals such as dietitians. As fad diets can trigger disordered eating symptoms which do put people at risk for eating disorders, it is important to examine the reason behind wanting to engaging in dieting in the first place. As fad diets come and go every decade promising a solution to weight problems (where is Paleo and Atkins now?), we still have yet to find one that sticks around long enough to make a meaningful impact. Perhaps because the problem is not with weight at all, but with our relationship with food and bodies, our systems and culture.

Disclaimer: the information provided is not intended as medical advice or to diagnose or treat a medical disease. It is strictly for informational purposes. Consult with your medical provider such as a dietitian before implementing any dietary changes, the information provided does not replace medical advice provided by your healthcare provider.

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Research assisted by Elisa Sahraie, UBC student

Reviewed and edited by Abby Hsiao, RD

References-

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