Fasting — that is, abstaining from food for set periods of time — is a longstanding practice that has been part of various religions and cultures around the globe for thousands of years.
Recently, though, fasting has reached trend status among dieters and health-seekers alike in the form of intermittent fasting (IF), with advocates claiming that its benefits include long-term weight loss, improved health and even anti-aging effects.
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What Is Intermittent Fasting?
Intermittent fasting is a structured fasting method that restricts eating (and drinking any calorie-containing beverages) to certain hours of the day. One of the primary goals is to help followers decrease their overall calorie consumption, to promote weight loss or support weight control.
One of the most popular versions of IF is the 16:8 diet, which allows for an eight-hour window to eat, followed by 16 hours of fasting each day, the latter of which includes the time you're asleep.
This dieting method may owe its growing following to the fact that it's so simple. Besides the eating and fasting windows, there are no rules to follow — no restricted foods or calorie counting, and no need to track your macros. Just eat what you like from, say, 8 a.m. to 4 p.m., then don't eat until 8 a.m. the next day.
Benefits of 16:8 Fasting
While the science on intermittent fasting is still in its early stages, promising research has shown a variety of potential health benefits for followers.
1. Weight Loss
Intermittent fasting has been shown to reduce overall calorie consumption throughout the day by restricting the time you're allowed to eat. Indeed, a December 2019 review published in the The New England Journal of Medicine found that in short-term studies involving adults who have overweight or obesity, IF is just as effective as overall calorie restriction for weight loss.
IF might also be a boon for dieters who've hit a weight-loss plateau. A February 2018 study published in the International Journal of Obesity found that alternating periods of energy (calorie) restriction — which can be achieved via fasting — with periods of energy balance allowed subjects to achieve greater weight and fat loss. In other words, IF may help you avoid the metabolic process that causes your body to stop shedding pounds during a weight-loss attempt.
In a nutshell: While more research needs to be done, IF shows promise for weight loss without the need for counting calories.
2. Reduced Inflammation
Budding research also suggests that IF may help decrease inflammation in the body.
Inflammation is your immune system's natural response to injury or infection, according to Harvard Health Publishing. When you sprain your ankle or come down with a cold, your body mounts an attack — led by an army of white blood cells — to help you heal.
While inflammation can be a good thing in small doses, chronic inflammation — triggered by toxins in food or the environment, or sometimes excess fat cells in the body — has been linked to serious health conditions, from diabetes to heart disease and cancer.
But the December 2019 review in the New England Journal of Medicine found that intermittent fasting reduces widespread inflammation in the body, which means IF may help lower your risk of developing certain chronic diseases.
Read more: 14 Inflammation-Fighting Foods to Eat Every Day
3. Improved Blood Sugar Control
Restricting the number of hours you're eating each day has a direct effect on your blood sugar and insulin levels.
Why? Well, when you consume food containing sugar, your body releases insulin, which brings the sugar into your cells to be used as energy. When you fast for 16 hours at a time, though, your body gets to take a break from producing insulin.
The December 2019 review in the New England Journal of Medicine concluded that practicing IF improves blood sugar regulation, which may reduce the risk of developing pre-diabetes or diabetes.
4. Anti-Aging Effects
One of the still-budding areas of research around IF shows that it may help combat the effects of aging — at least in animals.
A study published in the June-July 2011 issue of Mechanisms of Ageing and Development found that a restricted diet using alternate-day fasting increased the lifespan of laboratory rodents. Researchers measured the rats' ERK and PI3K signaling molecules, which are associated with age-related heart failure, and concluded that alternate-day fasting may provide a cardio-protective effect against aging.
The research in this area is new and only being conducted in animals so far, but the results are promising and could lead to a greater understanding of aging in humans.
How to Start a 16:8 Fast Diet
1. Select Your Window
When you begin a 16:8 diet, the first step is to determine your eating and fasting windows. Many people who follow this plan prefer to eat between noon and 8 p.m. For most people, it's easier to skip breakfast, have dinner before 8 p.m. and forego any late-night snacking. Other people may prefer a 9 a.m. to 5 p.m. window, though, or even a 10 a.m. to 6 p.m. option, depending on different lifestyle factors that affect their ideal mealtimes.
In the first week or so, it may be necessary to experiment with a few different timeframes to determine which works best with your hunger and lifestyle. For example, if you are active and exercise regularly, it's a good idea to time your eating window to begin within a half hour after your workout, so you can properly refuel. If your goals include increasing your muscle mass, you'll want to consume protein immediately following strength training.
2. Ease Into It
You don't need to jump feet first into a 16-hour fasting window. If you've never fasted before, begin with a 10-hour window and gradually work your way up to 16 by increasing your fasting time by an hour each day for a week. This will allow your body time to adjust and should help you avoid potential side effects like low blood sugar, fatigue and headaches.
While it doesn't matter when you eat during your selected window, spacing meals and snacks throughout the day will help to keep hunger at bay as well as stabilize your blood sugar. Eating just one large meal per day, on the other hand, could lead to dizziness, headaches and fatigue, and is not recommended.
3. Fill Up on Nutrient-Dense Foods
With intermittent fasting, there's no restriction on the foods you can or can't eat. But if you want to reap the biggest benefits, you'll want to opt for nutrient-rich foods. Fruits, veggies, whole grains, healthy fats and lean sources of protein will keep you feeling full longer and maximize the amount of vitamins and minerals you're taking in.
It's also important to keep meals balanced and include a mix of lean protein, healthy fats and carbohydrates at each meal. This will ensure your body is getting the nutrition it needs to sustain your energy, muscle mass and metabolism during the fasting period.
4. Don't Forget to Drink
Staying hydrated is also important while fasting. Drinking water and unsweetened coffee or tea is recommended even during the fasting period. Bonus: They may help reduce hunger, too.
It's important to note that the amount of water you drink is influenced by your local climate, activity level, and personal needs. If you are more active or live in a climate that is dry, hot or humid, you will need more water than someone who's not physically active or lives in a cooler climate.
According to a publication from The National Academies of Sciences, Engineering and Medicine — which is still considered the standard — the majority of healthy people meet their daily H2O needs by letting thirst guide them. The publication's general daily recommendation is 2.7 liters (11 1/2 cups) for women and 3.7 liters (15 1/2 cups) for men, which includes total water from all foods and beverages.
Intermittent Fasting Risks and Drawbacks
Restricting your food intake to eight hours per day isn't right for everyone. Pregnant or breastfeeding women and those who struggle with hypoglycemia (low blood sugar), have a history of an eating disorder, are taking certain medications or have a chronic condition like heart disease, diabetes, or kidney disease should consult with a doctor before starting 16:8 fasting or any type of fasting diet.
Also, some studies have shown that limiting food throughout the day can cause increased eating outside of the fasting window. For some people, this can lead to stalled weight loss or even weight gain in the long-term due to the increased number of calories consumed during a session of overeating.
Symptoms such as hunger, weakness, fatigue, dizziness and headaches have all been reported by those just starting out with intermittent fasting. While these symptoms resolve within a week or two for most people, some may have continued side effects, and should stop IF or consult with an expert to make sure their approach is healthy and sustainable.
- Nutrition Research: "Intermittent fasting during Ramadan attenuates pro inflammatory cytokines and immune cells in healthy subjects"
- International Journal of Obesity: "Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study"
- Ageing Research Reviews: "Impact of intermittent fasting on health and disease processes"
- USDA ChooseMyPlate: "10 tips: Make better beverage choices"
- Harvard Health Publishing: "New insights about inflammation"
- The National Academies of Science, Engineering and Medicine: "Dietary Reference Intakes: Water, Potassium, Sodium, Chloride, and Sulfate"
- Mechanisms of Ageing and Development: "Alternate-day fasting reverses the age-associated hypertrophy phenotype in rat heart by influencing the ERK and PI3K signaling pathways"
- The New England Journal of Medicine: "Effects of Intermittent Fasting on Health, Aging, and Disease"