What Happens In Our Bodies When We Fast
Food provides the cells in our bodies with their fuel: glucose. Our bodies release a certain amount of glucose into the blood and store the rest as glycogen, releasing it as needed. Once that supply is used up after at least 12 hours without food our fat stores are called upon.
Burning fats rather than glucose produces substances called ketones, and high levels of these can suppress hunger . Researchers are looking at the effects of this metabolic state on the body. For instance, research dietician Dr Michelle Harvie at the University of Manchester has found that, by dampening down levels of certain hormones, fasting could help reduce the risk of breast cancer.
Benefits To Metabolic Health
Good metabolic health means having optimum levels of blood sugar , fat , good cholesterol , blood pressure and other markers that indicate your risk of heart disease, stroke and other chronic conditions.
Certainly, from a metabolic point of view we definitely see benefits in intermittent fasting, says Dr Collins. Youre better at clearing, storing and utilising carbohydrate and fat when youre on an intermittent fasting regime, so there are improvements in insulin sensitivity, blood lipids and other cardiovascular risk factors.
Such improvements are evident in lean people as well as those who are overweight, he says. This is possibly because healthy-weight people can still carry harmful deposits of fat in their internal organs, including the liver.
But the effects of weight-loss are difficult to untangle from other processes at play during fasting. Dr Collins is conducting more research to find out whether metabolic health can be improved in people on intermittent fasting programmes without reducing their overall calorie intake.
What Are Fasting Diets
Diets involving daily calorie restriction have often been prescribed to facilitate weight and fat loss, but many individuals have a hard time following the diet long enough to see results . In contrast, fasting diets alternate between periods of substantial calorie restriction and periods where you eat normally rather than restricting calorie intake every day . Intermittent fasting, alternate-day fasting, and time-restricted feeding are among the most researched diets. Intermittent fasting involves 75-100% energy restriction one to three non-consecutive days a week with normal eating the other days. With many interventions, you may consume up to 500-600 calories and unrestricted water and calorie-free beverages on fasting days. On the feeding days, you may eat ad libitum, meaning that you can eat whatever and as much as you want . Extreme over consumption of food, however, will defeat the purpose of this diet. The 5:2 diet is a commonly known intermittent fasting diet where the individual fasts two non-consecutive days each week and eats normally the other five . Intermittent fasting may also be known as intermittent energy restriction or intermittent calorie restriction .
Alternate-day fasting is similar to intermittent fasting, but you fast every other day rather than only once or twice a week .
Intermittent Fasting Promises Better Health And Longer Life But Does It Work
The obesity epidemic has spawned a cottage industry of weight-loss schemes. Currently in vogue is intermittent fasting, which involves alternating intervals of extreme calorie reduction with periods of normal eating. Proponents of an intermittent fasting regimen claim that it helps shed pounds faster than traditional diets, as well as reduce inflammation and other heart disease risks.
Is Water Fasting Safe
Unless you are part of a research study carefully designed and monitored by medical professionals, no.
I would say it’s not a safe choicedefinitely notfor people to do on their own, says Varady.
If you wanted to try just drinking water for one day, which is also known as zero-calorie alternate day fasting, thats a possibility, says Varady. But she doesnt recommend multiple-day water fasts.
I would say it’s not a safe choicedefinitely notfor people to do on their own.
One reason: Although you might lose weight on a water fast, you probably will shed more than fat. Water fasting will likely result in lean muscle wasting, or muscle mass loss that occurs when you dont take in protein, she says.
To compensate, your body starts to break down muscles. You might develop other nutrient deficiencies as well. Again, if you are part of a study where someone checks your blood work regularly for signs of nutrient deficiencies, maybe you could catch these things before they become dangerous. On your own, you could end up in trouble.
Another problem: I’d be worried about eating disorder propensity, says Varady. That seems a little worrisome for me, if you’re not eating, it might change your psychology around food if you haven’t eaten anything for 30 days.
And its not just the fast thats potentially dangerousits the period of time when the fast is over and youre ready to eat everything in sight.
Mayo Clinic Minute: Is Intermittent Fasting Healthful Or Harmful
Intermittent fasting is cutting yourself off from any food or beverages, other than water, for a certain amount of time. Some fasting is for religious reasons, while others fast for weight loss. But is it a healthy way to lose weight?
Jason Howland has more in this Mayo Clinic Minute.
Journalists: Broadcast-quality video pkg is in the downloads. Read the script.
Want to lose weight? Just don’t eat, right?
“We’re hearing a lot more about intermittent fasting, and it essentially means the voluntary abstinence of food or beverage for a prolonged period of time,” says Mikel Bryant, a Mayo Clinic dietitian.
Bryant says, while fasting can be a tool that helps some people with weight loss …
“… It’s not necessarily the entire answer to everybody’s problem. Our body needs a given amount of nutrients every day.”
Denying your body of calories for an extended period of time isn’t recommended for anyone who’s physically active, pregnant, under 18 or has certain medical conditions, such as diabetes.
She says, if you’re considering fasting as a weight-loss tool, make sure that, when you do eat, you’re taking in quality, healthy ingredients, such as the proper amount of fruits and vegetables, whole grains and lean protein.
“So, bottom line, fasting may not be for everyone,” Bryant says. “But it is worth taking a look at your individual diet, your lifestyle, your behaviors and seeing if there can be any improvements that can be made to reach your weight goals.”
You’re Looking For Sustainable Weight Loss
While some people find intermittent fasting helps them succeed in their weight loss goals, some studies show that it is not much better as restricting calories in general. For many people, going for long periods without food is really difficult, and can cause them to overeat once the fast period is over. So if you end up eating the same or more calories than if you had just followed a calorie restriction plan with normal meal times, it may not be worth it if weight loss is your goal.
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May Support Weight Loss
Many dieters turn to fasting as a manageable approach to weight loss. Studies show that controlling the times we eat or undertaking short-term fasts can aid weight reduction, fat loss and improve blood lipids. Thats not all: other studies have shown fasting to increase the ability to switch metabolism to fat burning, preserve muscle mass and improve body composition in overweight people.
Are Fasting Diets Effective For Improving Health And Losing Weight
While more research is needed, preliminary studies have found that fasting diets, namely alternate-day and intermittent-fasting diets, effectively reduce calorie intake and cause weight loss . Two studies found that despite intense energy restriction every other day in ADF, participants ate only slightly more than normal on feeding days, resulting in calorie reduction of about 25-35% over 48 hours . There is no evidence, however, that fasting diets are more effective in weight loss than daily calorie restriction diets .
While studies show that people lose similar amounts of weight short-term on both intermittent fasting diets and daily calorie restriction diets, many people on both diets regain the weight later on . Those following a fasting diet, however, have regained less weight from fat than individuals on a daily restriction diet in some studies . Maintaining weight loss after a diet is only possible with long-term changes in health and nutrition habits. Emphasizing behavior change during dieting and completing a weight maintenance program may minimize weight regain , but fasting diets alone do not reduce the risk of weight regain.
Ask a doctor before beginning a diet, especially if you:
- Are pregnant or breastfeeding
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To Fast Or Not To Fast
Does When You Eat Matter?
What you eat matters. Many studies have shown that the types of food you eat affect your health. But what about the timing? Scientists are just beginning to understand that when you eat may also make a difference.
Throughout history, people have experienced periods when food was either scarce or completely lacking, says Dr. Valter Longo, an NIH-funded longevity researcher at the University of Southern California. So, they were forced to fast, he says.
But current technologylike refrigeration, transportation, and electric lightinghave made food more readily available.
This has shifted our eating patterns, explains Dr. Vicki Catenacci, a nutrition researcher at the University of Colorado. People now eat, on average, throughout a 14-hour period each day.
Studies suggest that this constant food intake may lead to health problems. Researchers have started looking at whether fasting can have potential benefits for some people.
What Is Fasting And How Does It Work
A good rule of thumb for those wanting to fast to improve their metabolic and overall health is to leave at least 12 hours between meals ideally more on a regular basis, according to Dr Adam Collins, Principal Teaching Fellow in Nutrition at the University of Surrey.
Time-restricted eating is often used for weight-loss, and some people increase the daily fasting period to 14 hours or 16 hours. There are other forms of intermittent fasting, including the 5:2 approach, which involves very restricted eating on two days of the week, with normal eating on the other five. The 4:3, or every other day, approach involves similar restrictions on alternate days. Whichever one you choose, make sure youre doing it consistently, Dr Collins advises.
Read Also: What To Drink While Fasting On Keto
It Typically Focuses On When To Eat Not Necessarily What To Eat
Theres no one-size-fits-all fasting diet plans can be highly individualized. Some folks allow themselves to drink black coffee and green juice during the no-food period, while others may give themselves a cap of 500 calories on fasting days.
For instance, Kimmel told Mens Journal that on fasting days his meals might consist of peanut butter and an apple, the whites of hard-boiled eggs, or possibly a bowl of oatmeal. The rest of the week Im a gluttonpizza and pasta and steak, he told the mag.
But here, Kimmel reveals one of the issues with fasting diets, says Libby Mills, RD, a spokesperson for the Academy of Nutrition and Dietetics. The focus isnt always on nutrition, she says. A lot of the time its just about calories.
Some people may also interpret the normal eating time as free rein to go calorie crazy, Mills adds, which can backfire.
Improves Insulin Resistance Which Can Help Diabetes Prevention And Management
Intermittent fasting and weight loss may help reduce fasting blood glucose and improve insulin sensitivity through lowering concentrations of leptin, the hormone produce by fat cells to regulate hunger, and increasing adiponectin, a hormone that plays a role in glucose and lipid metabolism.
In some studies where fasting was used as a weight-loss intervention and strategy for maintaining a healthy weight, people who intermittently fast had lower levels of blood glucose which happens to be a central goal in the prevention and treatment of diabetes .
These possible benefits may be driven more by reductions in body weight and body fat percent stemming from caloric restriction influenced by intermittent fasting, but research still shows an overall positive effect on blood sugar levels when intermittent fasting .
Intermittent fasting may help improve glycemic control and insulin resistance with a reduction in weight in those with diabetes and those without.
Recommended Reading: What To Expect When You Start Intermittent Fasting
Changes In Body Composition
composition are among the most studied effects of intermittent fasting.
Several studies have shown that weight loss of between 37% body weight in an average of 8 weeks was achievable through intermittent fasting. Research also noted that this method could result in fat loss.
Fasting in a 14:10 pattern an eating window of 10 hours and a daily fast of 14 hours can act on the risk factors of metabolic syndrome, including by reducing waist circumference, body fat percentage, and visceral fat.
You Have Period Pain Pms Or You’re Trying To Get Pregnant
Intermittent fasting requires you to restrict food for periods of time, which overtime, can make your body more sensitive to insulin. Insulin sensitivity is a good thing, it means your cells can use the energy from the food you eat more efficiently, which ultimately helps lower your blood sugar. But for some, low blood sugar and hunger means more stress on the body. Going through long periods of fasting, resulting in low blood sugar and stress on the body, can mean bad news for your hormones and fertility.
“I don’t recommend if have a lot of stress, aren’t sleeping enough, are exercising a lot, are trying to get pregnant, have a history of disordered eating or have hormone imbalances or thyroid or adrenal issues…which pretty much excludes most women,” says Melissa Groves Azarro, an integrative and functional medicine dietitian specializing in women’s health and hormones.
Studies performed on young rats showed that fasting negatively affected reproductive health for male and female rats. “Women’s hormones are super susceptible to perceived stress and scarcity, so intermittent fasting can worsen existing hormone imbalances. I do think it may be a useful tool in menopause, if sleep, stress and exercise are balanced,” Azarro says.
Read Also: Fasting For Beginners To Lose Weight
Benefits To Gut Health
Short periods of fasting, such as skipping breakfast , may benefit your gut microbes, explains Professor Tim Spector of Kings College London in his book Spoon-Fed. Although research in the field is in its early stages, accumulating evidence suggests that during fasting, certain species of microbes replicate and feed off carbohydrates in the gut lining. This makes the gut barrier healthier and more efficient.
One explanation for this could be that gut microbes follow a circadian rhythm like you do. Microbe communities, like us, may also need to rest and recuperate as part of a daily circadian rhythm which could be important for our gut health, writes Spector.
Everyones body responds differently to skipping meals, so Spector suggests experimenting by giving breakfast or another meal a miss to see how you feel. Leaving a minimum of 12 hours between eating, even if its only occasionally, could improve your mood and energy levels in the short term , he argues.
What Are The Benefits Of 16
16-hour intermittent fasting is also called a 16:8 diet plan where you consume just water, black coffee, green tea, and other unsweetened beverages. Then, you can eat all of your meals and snacks for the remaining eight hours of the day.
The 16-hour period may appear to belong, but if you’re receiving adequate sleep, you should be asleep for around half of that time.
The benefits of a 16:8 intermittent fasting diet include:
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This article was reviewed on 16th November 2021 by Kerry Torrens.
Kerry Torrens is a qualified Nutritionist with a post graduate diploma in Personalised Nutrition & Nutritional Therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine and a member of the Guild of Food Writers. Over the last 15 years she has been a contributing author to a number of nutritional and cookery publications including BBC Good Food.
All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.
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What The Studies Show
Intermittent fasting has been shown, in many short-term research studies, that it can help you lose weight, improve your health and potentially protect you from developing certain diseases. And whats most appealing, is that it doesnt restrict you daily, like conversional dieting.
Studies have also shown that intermittent fasting may help prevent or even reverse diabetes. This is because it helps to control and reduce insulin and glucose concentrations in your blood.
However, theres still not enough studies to show how safe intermittent fasting is if you do it for a long time. But in general, most research highlights that its safe in otherwise healthy adults.
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