Intermittent fasting is one of the most popular diet trends right now. The hype around preliminary research has encouraged many to start experimenting with fasting in various ways. While fasting has been an element of spiritual practices since ancient times, it’s still relatively new to the mainstream market as a diet trend. Research on human subjects remains very limited.
To get started, I pulled some information from the most readily available resources. Here are a few of the methods people have used for intermittent fasting so far:
Alternate Day Fasting: periodically fasting for an entire day at a time. Typically your week alternates between fasting days and “feasting” days, leaving you with 3 fasting and 4 feasting days. A variation of alternate day fasting called the 5:2 plan, has you fasting for 2 days of the week and eating normally the rest. Another version yet has you fasting for only one day out of the week.
Alternate Day Modified Fasting: you follow one of the alternate day fasting patterns, but are allowed a small amount of calories (maybe 500kcal) on fasting days.
Daily Intermittent Fasting: designating the same restricted eating window every single day. Essentially, you restrict the amount of time you spend each day “allowed” to eat. Most are allowed to eat for 8 hours (or so), leaving 16 hours (or so) of fasting every day. There are no apparent firm guidelines on length of eating window or timing of that window.
So . . . what’s the research saying about fasting?
Does intermittent fasting cause weight loss?
Research has shown that intermittent fasting can lead to varying amounts of weight loss. That’s because most study participants ended up eating less overall. The science here is clear: eating fewer calories than you use equals weight loss, and vice versa. This means if one were to compensate for skipped meals by extreme binging during the “feast” phase, then they aren’t going to lose any weight.
Does intermittent fasting improve metabolic health?
There are correlations between intermittent fasting and improved cholesterol, blood sugar, and blood pressure. However, research does not say whether or not the fasting itself has meaningful benefits beyond what a general caloric deficit or weight reduction provides. Intermittent fasting may have favorable effects for those with diabetes or impaired glucose regulation. But there are also studies that say just the opposite, suggesting fasting damages the pancreatic cells that secrete insulin. From where I’m standing, these aspects are certainly inconclusive. Not to mention, there are already effective ways to improve those markers that are backed up by extensive research.
Will intermittent fasting make you live longer?
Long story short - we don't know. Fasting may initiate certain regenerative processes, but much more research has yet to be done to determine whether or not these processes will have any effect on prolonging life in humans.
What if I just skip breakfast?
Once again, reducing calories means weight loss. If you typically eat breakfast and then suddenly decide to start skipping breakfast, you will experience a caloric deficit and likely lose weight. That is, unless you are so ravenous by lunch time that you eat the whole refrigerator. Some people may worry that skipping breakfast lowers their metabolism. In reality, studies have not shown significant reduced resting energy expenditure with skipping breakfast. This being said, higher blood sugar spikes at lunch and dinner have been observed in those skipping breakfast, even if they didn’t overeat at either of those meals. Better results have been seen when eating more calories in the beginning of the day. If you think about it, it’s better to use the energy from food to move your muscles for daily activities than lying down to sleep. Maybe breakfast really is the best meal of the day!
Here’s my opinion in a nutshell:
As a nutrition professional who uses evidence based diet approaches, I cannot yet recommend intermittent fasting as a credible health intervention. It’s natural for people get excited about making new drastic changes to their diets, but setting strict boundaries can really damage your relationship with food. The weight-yo-yo-ing tendency of very restrictive diets is very real. If you aren’t ready to commit to the fasting lifestyle for good, you’re at high risk for rebound weight gain once you fall off the program.
If periodic fasting causes you to binge on poor choices during your eating window, affects your normal social life, or causes you physical or mental distress, I definitely can’t support it. However, I do support any lifestyle change that ends in a healthier body and mind. If you need to lose weight and find it easier to limit your caloric intake by fasting at times, maybe it’s worth exploring on your own. Who knows? Maybe as research continues this process will become a viable option for professionals too.
Nutrition Technique: Initial research on intermittent fasting has shown positive effects on weight loss, but no more so than for traditional lower-calorie diets. There may be something to be said for positive metabolic responses in the fasted state, but much more research is needed in that area. For now, what matters most is managing calories in vs. calories out. Those who find it easier to control when they eat as opposed to what or how much they eat, may benefit from giving it a try. Just keep in mind: when the ultimate goal is to maintain a healthy weight for life, it’s a good idea to ask yourself what kind of diet pattern you’re willing to follow for the duration.
Note: fasting is strongly discouraged for pregnant women, children, those with impaired glucose regulation, or a history of disordered eating until more research has been done.
Amy Jones, MS, RDN, LD