Fasting can be a tool for weight loss but is not necessarily any better than simply reducing caloric intake. A 2015 meta-analysis of 12 clinical trials focused on IF found weight-loss results comparable between fasting and caloric restriction groups. When researchers compared the fasting group with the continuous calorie restriction group, they concluded, “There was no significant difference in weight loss amounts or body composition changes” (Seimon et al., 2015).
Similarly, a 2018 meta-analysis of six studies found that “intermittent energy restriction was comparable to continuous energy restriction for short-term weight loss in overweight and obese adults,” again showing that fasting isn’t necessarily superior to caloric restriction in terms of weight loss (Harris et al., 2018).
So why do people seem to lose weight with IF?
When it comes to any diet or plan, the reason people lose weight is more about the caloric intake and awareness of food, portions, etc., rather than the specific dietary protocol they are following. This appears to be true for fasting as well. IF may work as a tool but isn’t superior for weight loss. Further, fasting is a high-level strategy for managing weight.
Much like moving from body-weight exercises in the gym to performing higher-level body-building movements, there are certain nutritional foundations that should be mastered before manipulating caloric intake with fasting.
A 2019 study found that people who eat a diet with more highly processed food end up eating approximately 500 more calories per day. The study suggested, “Limiting consumption of ultra-processed foods may be an effective strategy for obesity prevention and treatment” (Hall et al, 2019). Clients wanting to improve weight or health could benefit by eating fewer highly processed foods in addition to more fruits and vegetables, before moving to higher-level strategies like fasting.
Intermittent Fasting and Managing Cholesterol and Glucose
Researchers have also been interested in the use of IF to help manage both cholesterol levels and glucose. The results, however, have been less than consistent.
One randomized controlled trial that followed 100 obese participants for 12 months found “no significant differences in blood pressure, heart rate, fasting glucose, and fasting insulin. At 12 months, although there were no differences in total cholesterol and triglycerides, the alternate-day fasting group showed significantly increased low-density lipoprotein (LDL) cholesterol levels.” The authors did not comment on a possible cause (Trepanowski et al., 2017).
While Trepanowski et al. (2017) noted an increase in LDL cholesterol among subjects who fasted, a 2018 study conducted by Ganesen and colleagues noted a decrease in LDL cholesterol levels in a meta-analysis of four studies on IF. More research is necessary to investigate the long-term impacts of fasting on metabolic markers.
Suttun et al. (2018) found that early time-restricted feeding (i.e., dinner is eaten by 3pm) improved both insulin sensitivity and blood pressure. Unfortunately, the study was only conducted on men. Further, the early feeding window could be particularly challenging to implement and maintain over the long-term.
While the research results on IF are mixed, previous studies have shown that the strategies that have the biggest impact on managing cholesterol and glucose/A1c are increasing fiber intake with fruits, vegetables and whole grains in addition to increasing exercise (Mcrae, 2017).
Practical Concerns of Following an Intermittent Fasting Diet
Supporters of IF assert that fasting is simple because tracking calories or eating specific food isn’t necessary—simply eat only during your specific days or eating windows and you’ll lose weight. However, as research suggests that implementation isn’t quite so simple.
In the 12-month randomized control trial mentioned above, the dropout rate was higher in the alternate-day fasting group, which suggests that fasting may be more challenging to stick to. The prospect of going without food for a whole day or sticking to a very low-calorie intake a few days a week may not be feasible in the long-term (Trepanowski et al., 2017).
Putting fasting into practice in daily life becomes more complicated due to social events, cravings or high stress levels. One recent study tracked individuals who intended to fast to determine whether or not they followed through. The study found “emotional, stress eating, and food craving are disinhibiting traits that seem to increase intention-behaviour gaps” (Reichenberger et al, 2019). In other words, despite the best of intentions, common aspects of daily life often get in the way of being able to maintain an IF diet.
Is Intermittent Fasting Appropriate for Everyone?
While there is no specific research on IF and female hormones, there are some studies that suggest fasting could adversely impact ovulation and fertility in women. For example, Fujiwara and Nakata (2010) found that skipping breakfast was related to menstruation disorders in college women. In subsequent research, Fujiwara and colleagues (2018) found evidence that fasting negatively impacted ovulation in female rats. While animal data cannot directly be applied to humans, it does suggest that further research is necessary, especially considering the impact fasting can have on hormonal health.
There are a few groups of people who should NOT fast. These include:
- Pregnant or breastfeeding women
- Anyone with an unhealthy relationship with food, or a history of eating disorders or disordered eating habits
- Anyone with diabetes, or who experiences low blood sugar
Additionally, women with a history of irregular periods or who are trying to conceive are urged to proceed with caution, as IF may adversely impact hormone levels.
Conclusion: More Research is Needed
A lack of high-quality, large-scale controlled studies and longitudinal data, as well as multiple definitions and methods for fasting, make it challenging to extrapolate recommendations from the current research. Without long-term research studies, including studies that specifically investigate the impact of IF on men and women, it’s too early to know the long-term benefits or drawbacks of fasting.
In their comprehensive research review of animal and human studies on fasting, Harvie and Howell (2017) sum up what we can safely conclude about IR: “We do not know conclusively whether long-term IER [intermittent energy restriction] is a safe and effective method of weight control for subjects who are overweight or obese or whether IER may confer health benefits to people of any weight independent of weight loss. High-quality research comparing long-term outcomes of IER and CER [continuous energy restriction] are required to ascertain any true benefits or detrimental effects which IER may have for controlling weight and improving metabolic health in the population.”
So, if your clients ask for your opinion on the merits of IF or if they should try it, your best bet is to let them know that there just isn’t enough quality research yet to make a determination. You can, however, remind them of the basic tenets of a healthy diet, such as consuming a variety of whole, fresh foods, practicing portion control and reducing saturated fats and processed foods. In doing so, you can stay well within your scope of practice while also providing quality information that can help guide their food choices and improve their overall health and well-being over the long-term—not just the duration of a short-term diet.