Intermittent fasting has become a big hit in the nutrition world over the last several years. And for good reason!
It’s been suggested that it may help people lose weight, improve their metabolism, and maybe even slow the process of aging. What magic is this? It’s no wonder so many people have started adding fasting to their health routines. I’ve been curious myself, so I dove into the research to figure out exactly what this practice can do for us.
What is Intermittent Fasting?
Fasting is a broad term that describes the act of voluntarily withholding from eating for a set period of time. It has long been practiced for religious or cultural purposes, as well as for health benefits, which is what this article will focus on.
Intermittent fasting (also known as IF) is a style of short-term fasting followed by a period of eating, and then repeating the process. This differs from many typical diets because emphasis is placed on the timing of meals versus the kinds of foods being included at meals or portion control.
There are many forms of IF, but the two most popular and well-researched are Alternate Day Fasting, and Time-Restricted Eating.
Alternate Day Fasting involves switching between fasting days and feeding days during the week. On fasting days, little to no food is consumed, and beverages are usually calorie-free, like black coffee or water. Food is eaten normally on feeding days. There are many variations of alternate day fasting. One requires fasting every other day of the week, while another popular version is the 5:2 method, with two days of fasting and five days of feeding.
Time-Restricted Eating is a form of IF where someone fasts each day for a certain period of time. Common fasting periods are 12-18 hours each day, and most of the fasting is done during sleeping hours. This allows a much shorter period of time for eating but does not require a full day of abstaining from food.
What does Science Say About IF?
Unfortunately, data is limited at this time. A lot of early research that showed really positive results was gathered using animals as subjects. And since we’re not the same as rats or monkeys, we’re not sure how useful this is. The good news is that human studies are developing, so we may know more about the long-term effects in just a few years. For now, what we do know about IF is really interesting. Here is a list of some of the potential pros and cons, based on the most current research.
Possible Intermittent Fasting Pros:
It can lead to weight loss. Even without restricting calorie intake during eating periods, both versions of IF listed above have been shown to lead to loss of fat. And with continued practice weight loss can be maintained.
It may help with blood sugar control. Both types of fasting can reduce fasting blood glucose levels. A select amount of studies have shown that people can have better insulin sensitivity and lower post-meal insulin levels. This may help to reduce risk of diabetes and related conditions. Not enough evidence for long-term risks versus benefit exists for this, though.
Better gut health. Digestion seems to improve and participants report less belly bloat with short-term fasting, especially when the eating period ends earlier in the day (say, around 6:00 pm.) It’s unclear if this is related to the fasting alone or if weight loss is a factor.
It could improve heart health. Short-term studies have shown some improvements in LDL and HDL cholesterol levels, as well as reducing or normalizing blood pressure levels.
Inflammation and oxidate stress may improve. Some studies show that inflammatory markers, such as C-Reactive Protein (CRP), show improvement the longer fasting between meals goes on. This could potentially help to lower risk of some chronic diseases.
Possible Intermittent Fasting Cons:
Unpleasant side-effects like extreme hunger, nausea, dizziness and headaches may occur, especially with the Alternate Day method of fasting. These could make IF challenging to maintain in the long run and may also reduce quality of life.
Lean muscle loss. The evidence is not strong here, but there is potential that this may occur with the time-restricted version of IF.
Increased hunger hormones. Studies are a bit mixed here, but there is evidence that our “hunger hormones” leptin and ghrelin increase with fasting. When they are elevated, these hormones can cause us to overeat, and this could interfere with weight loss. It appears these hormones go down the longer fasting continues.
Dehydration and electrolyte imbalance can occur if calorie restriction is also present or if intense exercise is combined with longer periods of fasting.
Should Anyone Avoid Intermittent Fasting?
Certain people should not practice IF at all, or at least until they get approval and supervision from a healthcare provider. This includes:
• anyone with a chronic condition, such as diabetes, especially when treating the condition with medications
• anyone under the age of 18
• pregnant and breastfeeding women
• anyone with a history of disordered eating or with a poor relationship with food (i.e. obsessive thoughts about weight loss and food)
My Thoughts on Intermittent Fasting
While IF may provide some useful health benefits, it is not for everyone, and we need a lot more research to understand it. Weight loss seems to be the biggest benefit of IF. This makes sense – limiting your eating window will likely reduce your overall calorie intake. The longer you fast, the less opportunity you have to eat, or overeat. If your goal is simply to lose weight and you would like a way to modify your behavior, restricting your window for eating might be a great technique. And I think the time-restricted version of IF seems like a good fit for many people‘s lifestyles. Abstaining from food for entire days several days per week might take a lot more planning and adjusting.
If you are considering IF or have already begun, I strongly encourage a conversation with a dietitian first. It seems important that since eating periods are limited, they be balanced with mostly nutrient-dense foods. This can help prevent nutrient deficiencies and lessen the risk of unnecessary side effects. It’s also useful to consider schedule requirements, health history and relationships with eating when determining which version may work best. A dietitian can work with you to help identify a best plan for you.
Finally, remember that the best way of eating is whatever is sustainable for you. Be sure to share any thoughts or experiences you have on this topic!
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R. Patterson, D. Sears. Metabolic Effects of Intermittent Fasting. Annual Review of Nutrition 2017; 37:371–93.
S. Anton, et. al. Flipping the Metabolic Switch: Understanding and Applying Health Benefits of Fasting. Obesity (Silver Spring) 2018 Feb; 26(2): 254–268.
D. Lowe, N. Wu, L. Rohdin-Bibby. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity. JAMA Internal Medicine 2020; 180(11):1491-1499.
U. Espelund, et. al. Fasting Unmasks a Strong Inverse Association between Ghrelin and Cortisol in Serum: Studies in Obese and Normal-Weight Subjects. Journal of Clin. Endocrinology & Metabolism 2005; 90(2): 741–746,