Humans have fasted for millennia, either because of food scarcity or religious reasons. The first physician to introduce fasting as a medical treatment (to improve well-being) was Hippocrates (460-375bce), a Greek physician considered the father of medicine. Although there was medical research on fasting after Hippocrates, Jason Fung, a Canadian nephrologist, popularized intermittent fasting about six years ago.
Fasting implies eating less (or not at all), either by reducing calories (caloric restriction), eating fewer hours per day (time-restricted eating), eating less on certain days of the week (alternate day fasting), or mimicking a water fast (fasting-mimicking diet).
What Are the Different Forms of Fasting?
With caloric restriction, one can restrict calories a little by skipping dessert, eating smaller portions, and replacing high-calorie meals with less caloric options. (An example of this would be swapping a double-cheeseburger for a salad or a soup.)
One can also restrict calories a lot with a very low-calorie diet.
A very low-calorie diet (VLCD) limits calories up to 800 calories per day, usually through meal replacements and with the help of a physician or a health coach. Intermittent fasting involves alternating cycles of fasting and eating—an eating pattern that includes hours or days of no or minimal food consumption without deprivation of essential nutrients.
Alternate-day fasting and time-restricted eating are two other forms of intermittent fasting.
Alternate-day fasting consists of eating less on certain days of the week, such as fasting every other day or a 5:2 regimen (fasting two days each week), where fasting usually involves eating about 25% of what one eats on non-fasting days. Finally, time-restricted eating consists of eating fewer hours per day, such as eating only during a four or six-hour window and up to a 10-hour window.
And the fasting-mimicking diet, created by Valter Longo, Ph.D., involves eating a vegan diet that is low in calories, protein, and sugars but high in unsaturated fats over five consecutive days once a month for three months (or seven consecutive days twice a year). This technique mimics a water fast, which involves consuming no food or drink except for water between 24 and 72 hours.
There are other styles of fasting, most of which are combinations of the above forms. One of the most popular is the combination of caloric restriction with intermittent fasting. This fasting technique restricts our calories on some days and when we eat them on other days.
How Does Intermittent Fasting Work?
A six-to-eight-hour fast triggers a metabolic switch from glucose-based to ketone-based energy, which we call the ketogenic diet-like effect. In this instance, the body uses ketones to obtain energy instead of glucose (sugar).
Fasting increases fat tissue metabolism, which helps reduce weight and cholesterol, increasing longevity and decreasing the risk of chronic diseases, including cancer and obesity. Fasting lowers inflammation associated with chronic conditions like diabetes, obesity, heart disease, and osteoporosis. Time-restricted eating restores our circadian rhythm (sleep-wake cycle), which is associated with obesity and cancer when disrupted.
What Are the Benefits of Fasting?
The benefits of calorie restriction include weight loss of up to eight months, better blood pressure control, and better blood sugar regulation, which are beneficial for heart health.
In fact, following a very low-calorie diet (VLCD) resolved diabetes and induced weight loss in one study.
The drawback of the VLCD was that over time (after one year), many participants regained the weight they had lost.
Most studies, although not all, show an improvement in sugar control.
Weight goes down, especially in obese people, ranging from eight to 20 pounds after about eight to 12 weeks. Blood pressure, blood glucose, and cholesterol also decrease, which along with lower weight, improve cardiovascular health. Intermittent fasting might be better than a ketogenic diet for heart disease.
In studies of time-restricted eating, eating during 10 hours, compared to 14 hours or more in a day, improves weight, sleep, and energy. Compared to 12 hours, eating within six hours improves weight, insulin (a hormone involved in sugar metabolism), blood pressure, and appetite. In one study, eating earlier in the day led to more weight loss, better sugar control, and lower inflammation.
However, time-restricted eating that involved eating after 4 p.m. was associated with worse sugar control, worse blood pressure, and worse lipid parameters.
Some benefits are similar for caloric restriction and intermittent fasting: increased energy, increased fat mobilization, improved brain connectivity, blood pressure, cholesterol, and sugar regulation. Intermittent fasting might have an advantage for improved fasting glucose and adherence (it is easier to maintain over time). According to a 2014 study, the caloric restriction might be superior in weight loss, but it does not affect ketone formation and restore circadian rhythm. Intermittent fasting that is centered around time-restricted eating restores circadian rhythm, in addition to promoting some ketone formation.
Longo's fasting-mimicking diet improves sugar metabolism and gut microbiome, lowers inflammation, and improves mitochondrial function (mitochondria are the energy centers of our cells). Following a fasting-mimicking diet for three months lowers body weight, blood pressure, and blood sugar in healthy people, who have type 2 diabetes and have prediabetes. In addition, metabolic parameters, triglycerides, cholesterol, and a marker of inflammation also improve. Adhering to a fasting-mimicking diet for seven consecutive days followed by a Mediterranean diet for six months was as beneficial in patients with multiple sclerosis as following a ketogenic diet, one study showed.
What Are the Risk Factors Associated with Fasting?
A very low-calorie program carries a risk for gallstones. Long-term intermittent fasting carries a concern for worse sugar regulation and the release of damaging chemicals from tissues (based on data from animal studies).
The excessive loss of body fat causes a decline in sex steroids, which can lead to menstrual irregularities and lack of menstruation, bone thinning, the development of osteoporosis in women, and low libido in men.
For people taking blood pressure or sugar regulation medications, the doses might need to be decreased, or the medications stopped.
Speak with a physician if you consider any of these fasting protocols. Do not take fasting lightly.
What Fasting Model is Best for You?
For weight loss, any of these methods will help in the short term. However, although caloric restriction might be superior for weight loss, it is difficult to maintain, and over time, weight regain is common due to hormonal regulation.
Early time-restricted eating (eating up to 3 p.m.) is beneficial for weight loss and might be easier than caloric restriction.
For sugar control, early-time restricted eating, a supervised very low-calorie diet for two months, and Longo's fasting-mimicking diet are beneficial.
Intermittent fasting and the fasting-mimicking diet have improved risk factors associated with heart disease.
For neurological conditions, intermittent fasting seems promising, as well as Longo's fasting-mimicking diet.
- When we eat is as important as what we eat. Eating in alignment with our circadian rhythm has health benefits because our metabolism changes throughout the day. For example, a slice of bread consumed at breakfast leads to a lower glucose response and is less fattening than an identical slice of bread consumed in the evening. And eating during a shorter period (10 hours or fewer) allows our bodies to restore their circadian rhythm, which is beneficial for our health.
- It is important to ensure we are not falling into malnutrition when fasting. It is the alternating between fasting and non-fasting, not the chronic restriction, which seems to be beneficial for our health.
- Adhering to a plant-based diet (increasing the number of vegetables, nuts, and fruits) will lead to a lower-calorie diet.
- Working with a health coach and creating accountability is beneficial.