Most diets are focused on what you can’t eat. But what if you only had to worry about when? Following the lead of Silicon Valley, fasting diets are gaining mainstream popularity. The science is still fairly new, but there’s compelling evidence that fasting causes positive changes in the body.
What is a fasting diet?
People have been fasting for thousands of years. Food scarcity and hunter-gatherer lifestyles have imposed fasts throughout history. And fasting as an act of religious worship or self-control exists in almost every culture.
Today’s fasting diets are best described as eating programs. They’re mostly concerned with when you eat, and don’t restrict specific foods. Some people may incorporate healthier food choices into a fasting diet, but this isn’t usually required.
Types of fasting diets
Alternate day – Fasting happens every second day, where participants consume one small meal of around 500 calories.
5:2 – A 5:2 program allows normal eating 5 days a week, while the remaining 2 are spent fasting. Most versions of this diet feature a small meal on fasting days equal to about one quarter of a typical day’s calories (500 for women, 600 for men).
Periodic – Involves fasting for longer periods of time. One of the most popular versions is called a fasting-mimicking diet, where participants eat a low number of calories for 5 days straight. This initiates a fasting response in the body that’s both easier and safer than a zero-calorie diet. Periodic fasting can happen anywhere from monthly to yearly.
Time-restricted feeding – Could you eat all of your meals between noon and 6 p.m.? By eating within a smaller window of time, this plan has your body enter a fasted state once in every 24 hour period.
Are fasting diets all hype, or the key to living a longer, healthier life? Some of the potential benefits include:
Resetting circadian rhythms
- Before electric lights and abundant food sources, people ate when it was light out, slept when it was dark, and went longer between meals.
- The body’s circadian rhythms are responsible for our internal 24 hour clock. Circadian rhythms control more than just the sleep/wake cycle, and are also closely related to energy intake and blood glucose levels.
- Round-the-clock eating appears to disturb circadian rhythms, while time-restricted feeding creates a feed/fast cycle that can help reset the body’s natural rhythms.
- Fasting diets tend to help people consume fewer calories. Even in fasting diets where overall calorie intake is unchanged, there’s a trend towards weight loss.
- Mice on restricted feeding schedules tend to do better on fitness tests and are less likely to become obese or develop metabolic disease.
- Alternate-day fasting and calorie-restricted diets lead to similar weight loss in obese people.
Lowering blood sugar
- Fasting-mimicking diets appear to promote pancreatic cell repair in diabetic mice.
- In a human study, cycling through 5 day fasts every 30 days saw participants’ blood sugar lowered after three fast cycles with no harmful effects detected.
- But diabetics, especially those on medication, shouldn’t begin a fasting diet without their doctor’s consent. The long-term safety of fasting while diabetic hasn’t been established, and these diets could cause rapid changes in blood glucose.
- Scientists describe fasting as a challenge to the brain. By causing a small amount of stress, it helps strengthen neural connections the way exercise strengthens muscles.
- Fasting appears to increase a protein (BDNF) that strengthens neural connections and can act as an antidepressant.
- This increased neural activity may prevent plaques from forming in the brain, one of the characteristic features of Alzheimer’s disease.
- Older mice on fasting-mimicking diets show more brain activity in the hippocampus (associated with memory) and improved cognitive performance.
- Fasting along with chemotherapy has been shown to delay tumor progression in mice models of breast cancer. Mice that fast also have lower instances of some types of cancer.
- Adults who finished three cycles of a fasting-mimicking diet showed reduced risk factors for cancer and cardiovascular disease, including blood pressure, cholesterol, signs of inflammation, body fat and IGF-1 (a hormone associated with cancer risk).
Intermittent Fasting Diets: Potential Downsides
Hormones and fertility
- Fasting causes some women to experience hormonal imbalances and symptoms like mood swings, difficulty sleeping, and missed periods.
- To date, there haven’t been any studies specifically looking at how fasting impacts human fertility. Animal studies have found that calorie restriction and fasting can decrease ovarian size and stop ovulation.
- Since we don’t know exactly how these findings translate to humans, it’s a good idea for women to start slowly on a fasting diet. Instead of jumping right into alternate day fasting, begin gradually with something like time restricted feeding. Pay close attention to any changes you notice in your body or mood.
Losing muscle mass
- This is mostly a concern with long-term fasting that completely cuts food out of the diet for more than 48 hours.
- Studies of alternate day fasting and time-restricted feeding in people generally show weight loss and reduced body fat, while lean muscle is unchanged.
Difficult to stick to
- The best diet is one you can stick with. But for most people, that probably isn’t a fasting diet.
- Fasting diets range in their levels of difficulty. This is especially true if you have an active social life or a job that includes lunch and dinner meetings.
- Some people find fasting is easier when they have something to focus on, and prefer to schedule fasts on work days.
Changes your relationship with food
- This can be both positive and negative. Fasting diets may help to reduce emotional eating or eating out of habit.
- If you enjoy social eating or family mealtime, a fasting diet might change your positive associations with food.
- Some people are tempted to stretch fasting to unhealthy lengths, especially if they have a history of disordered eating.
Who shouldn’t fast?
Certain conditions make fasting more difficult or even dangerous. Unless approved by your doctor, don’t being a fasting diet if you fall into one of these categories:
- Pregnant, breastfeeding, or actively trying to conceive
- History of an eating disorder, body dysmorphia, or obsessive compulsive disorder
- Low blood pressure
- Chronic stress
- Taking multiple medications
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