If you’re like the majority of Americans who’ve tried at least one fad diet, chances are you’ve heard about Intermittent Fasting. In recent years, Intermittent Fasting has gained popularity, with everyone from Hollywood celebrities to high-tech billionaires to neuroscientists touting its ability to help people lose weight, improve health, increase endurance, and sharpen memory.
But does it really work? Books and documentaries make intermittent fasting sound like the cure-all for weight loss and increased quality of life. However, while preliminary research on animals has shown some promising health benefits of fasting, human studies have been more limited. In fact, the benefits of fasting in humans may be from overall calorie reduction rather than any magical effects of fasting itself.
While the jury is still out, intermittent fasting may be an effective tool when done responsibly if you’re committed to making it a long-term eating routine. This article reviews the current research on intermittent fasting and its potential benefits and provides tips for fasting effectively and safely.
The bottom line is that there is no single magic bullet for losing weight or living longer in good health. Unless you’re lucky and have no issues maintaining a healthy weight, it requires hard work and effort, no matter what type of food plan you choose.
The Concept Behind Intermittent Fasting
Intermittent fasting comes from the belief that in the pre-farming era, hunter-gatherers might go days without eating and still stay fit as they covered miles in search of game, fish, nuts, and berries. While that’s one justification for current intermittent fasting plans, there is some debate about how hard it was for our ancestors to find food.
More likely, they migrated to or settled where fish, game, and edible plants were abundant. They may have eaten only one or two meals a day since they had few ways to store fresh food, but they most likely did not adapt to periods of starvation. Those who went too long without food would have suffered from poor health and a lack of energy.
The reality is the human body needs some fat to function properly. Fat plays important metabolic, hormonal, and structural roles. When your calorie intake matches the energy you expend daily, your body primarily burns glucose (sugar) from digested food for fuel. In other words, you burn the calories you consume. Most people can burn through the available glucose in their bloodstream within 8-12 hours after their last meal.
If you consume more calories than your body uses, the excess calories are converted into triglycerides and stored as fat in adipose tissue. If you continue to consume as many or more calories than your body can burn, you maintain or increase your fat stores. Conversely, if you eat fewer calories than your body needs to operate efficiently, your body taps into these fat stores and breaks them down for energy.
With intermittent fasting, extending the time between meals creates periods when the body must tap into fat stores for energy. Evidence suggests the longer cells go without new glucose from food, the more they may adapt to become efficient fat burners over time. So by intermittently fasting for 16 to 48 hours at a time, the body undergoes periods of heightened fat metabolism, which may lead to weight loss.
Of course, overeating during non-fasting periods can reduce the caloric deficit produced during fasting. Intermittent fasting requires some attention to the quality and calorie content of the food you eat and the fasting intervals themselves.
Intermittent Fasting May Expose Cells to Beneficial Stress
Mark Mattson, a neuroscientist at Johns Hopkins who has been studying the benefits of intermittent fasting for many years, has presented the theory that, in addition to weight loss, fasting may provide additional health benefits because it exposes cells to various stresses that make them stronger and more resilient.
In a study published in Cell Metabolism in 2014, Mattson explains that intermittent fasting "triggers adaptive cellular stress responses, which result in an enhanced ability to cope with more severe stress and counteract disease processes."REF#3166
Consider how the stress of intense exercise makes your muscles and cardiovascular system stronger — as long as you give your body time to recover between workouts. Mattson suggests that taxing your body with intermittent fasting and exercise may trigger similar adaptive responses at a cellular level that result not only in weight loss, but also affect oxidative stress and inflammation.
Human research is needed to determine whether this theory fully explains why fasting appears beneficial in animal studies.
4 Different Approaches to Intermittent Fasting
Intermittent fasting does not dictate what you eat, but when you eat. It refers to an eating pattern that involves cycling between periods of fasting and normal eating. Intermittent fasting requires you go for extended periods without eating, typically between 16 to 48 hours at a time.
Intermittent fasting is different from calorie restriction because calorie restriction focuses on reducing overall calories without any prolonged periods of fasting.
There are several ways to implement intermittent fasting. The most common approaches include:
- Alternate day fasting (ADF): Fasting every other day. On fasting days, some people consume zero calories while others aim for around 25 percent of their energy needs or around 500 calories. There are generally no restrictions on the number or quality of calories consumed on the days you eat. However, proponents recommend setting calorie goals and avoiding foods high in saturated fats and preservatives, such as burgers, fries, and processed foods, and satisfying your hunger with plant-based meals with lean protein.
- The 5:2 diet: Restricting calorie intake to 500 to 600 calories for two nonconsecutive days per week and eating normally the other five days.
- Time-restricted eating (TRE): Eating all calories within a set window, usually four to ten hours. For instance, only eating between 9 am and 5 pm. This is the easiest form of Intermittent Fasting for most people to follow.
- One meal a day (OMAD): This is a form of TRE that involves fasting for 23 hours every day and eating one large meal in one hour.
For intermittent fasting to be successful, you should pick a schedule that is sustainable in the long term. Your emphasis is on timing rather than calorie counts. Still, it’s critical to set calorie goals for yourself and not overeat on non-fasting days, which can negate the calorie deficit created by fasting.
What Science Says About Intermittent Fasting
Most human studies on intermittent fasting have focused on individuals who are obese or overweight and have sedentary lifestyles that put them at higher risk of health problems. A limited number of studies have shown intermittent fasting to provide weight loss and other health benefits. More research is needed to confirm these benefits.
One of the perceived benefits of intermittent fasting is that it does not require obsessive calorie counting or a radical change in your diet. It can therefore be easier and more sustainable than calorie-restricted dieting. That being said, you may lose more weight from a food plan by avoiding foods high in saturated fats and sugars and adopting a mostly plant-based, Mediterranean-style diet that has proven health and longevity benefits.
Intermittent Fasting May Promote Weight Loss
In several human studies of the various forms of intermittent fasting, obese and overweight individuals experienced minimum or moderate weight loss, from one percent to eight percent.REF#3167 All forms of intermittent fasting provided weight loss. However, clinically significant weight loss was only shown in the ADF and 5:2 diet plans.
The results from the ADF and 5:2 plans were surprisingly similar to those experienced by a group reducing their calories by 25 percent. The zero-calorie ADF group had a weight loss of three to eight percent. The modified ADF group (those who ate around 500 calories on fasting days) and the 5:2 plan group experienced a four to eight percent weight loss.
In studies on TRE, which placed no calorie restrictions on subjects during the eating window, participants lost an average of three percent of their weight. There was no indication that weight loss was affected by the length of the eating window (from four to ten hours). When TRE was combined with exercise (specifically resistance training), subjects actually gained weight. This may be due to an increased appetite from exercising and higher calorie consumption.
Very little difference has been found between weight loss from intermittent fasting and weight loss from more traditional restrictive calorie diets.
Studies found that most participants on ADF did not binge eat on their non-fasting day, which helped with weight loss. Those on the ADF and 5:2 plans were able to retain their weight loss during the maintenance period following the study period, with a small increase over time, suggesting that to maintain weight, they may need to continue with their fasting routines long-term.
However, longer-term studies are needed to determine the sustainability of Intermittent Fasting. There are no longer-term human studies on weight loss retention in TRE.
Intermittent Fasting May Support Appetite Regulation
Studies on Alternate Day Fasting indicate that this form of intermittent fasting may increase the perceived sense of fullness in the short term but may have no effect over the longer term. No tests have been done on other forms of fasting and appetite regulation.
Intermittent Fasting May Support Brain Health
Animal models show intermittent fasting can produce changes in the brain and gut microbiome that may provide neuroprotection. Intermittent fasting’s effects on human brain health likely depend on the kind of food one eats, with caloric restriction amplifying benefits.
A three-year progressive study published in 2020 investigated intermittent fastings effects on cognition, biomarkers, oxidative stress, and inflammation in older adults with mild cognitive impairment (MCI). The results showed that MCI patients practicing intermittent fasting had improved cognitive scores and reversion to normal/successful aging after three years compared to controls.
The researchers concluded that intermittent fasting offers comparable cognitive benefits as caloric restriction but may be more feasible for long-term compliance in older adults. Though more research is still needed, this study provides evidence that practicing intermittent fasting may help combat age-related cognitive decline in those with MCI.REF#3168
While human studies are limited, there is a growing body of evidence that intermittent fasting is beneficial, especially if you are overweight or sedentary. The effects may not be significantly different from a traditional diet based on restricting calories, but the fasting process could be a lot easier to maintain, especially if you select a plan that works with your schedule.
Is Intermittent Fasting Safe?
Many people have expressed concerns about the safety of intermittent fasting, worrying that it may cause or aggravate gastrointestinal issues, hormonal disruptions, energy loss, or eating disorders.
Human studies have shown intermittent fasting to be a safe intervention for weight loss, and when done responsibly, it can become a long-term lifestyle choice.2
Intermittent fasting has not been shown in studies to affect metabolism adversely, to alter hormone levels, to cause gastrointestinal problems, or to cause eating disorders.
That being said, the few human studies that exist have focused on obese and overweight individuals and may not be appropriate for everyone. You should not consider intermittent fasting without consulting your healthcare provider if you are:
- Pregnant or breastfeeding
- Have a Body Mass Index (BMI) of 18 or less
- Are under the age of 12
- Have a history of eating disorders
- Are a shift worker whose waking hours constantly change
- Take medications with your meals
If you do start intermittent fasting, it is not unusual to have headaches, dizziness, or constipation during the first few weeks of your new eating plan. It’s helpful to increase the amount of water you drink to cope with these issues.
Exercise should be safe even on fasting days, but alcohol consumption should be avoided to reduce the chance of dehydration.
Is Intermittent Fasting Right For You?
The bottom line is that intermittent fasting provides very similar benefits to losing weight by reducing calories. When you are obese or overweight, losing weight can lower your blood pressure and cholesterol, put less pressure on your heart and joints, and reduce your risk of developing chronic illnesses like Type 1 diabetes and cardiovascular disease.
Besides being a relatively safe form of weight loss, intermittent fasting may be the easiest way to lose and/or maintain your weight, since you don’t have to think about what to eat as much as when you’ll eat.
As with any diet, the quality of the food you eat is critical. If you focus on fresh, high-nutrient foods that provide maximum sources of energy and fiber (one nutrient that you often don’t get enough of when you diet), intermittent fasting is a positive step toward meeting your goals for healthy weight, vitality, and longevity.