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Healthy Aging & Lifestyle

Complete Guide to Intermittent Fasting for Weight Loss

By Dr. Martica Heaner, PhD
Intermittent fasting can be an easy-to-follow way to lose weight. You alternate fasting, eating few to no calories, with periods where you eat what you want. So, the practice may feel less restrictive than a conventional diet. Here’s how.

The world is getting bigger in more ways than one. And exploding levels of obesity everywhere mean that promising advancements in longevity can be compromised. Even though people are now living longer, they are often living longer while sick. Despite continuing technological and medical advances, the diseases we develop, such as obesity, mean that our worsening healthspan can erase the potential gains in our lifespan.1 “Overweight and obese people have a considerably shorter life expectancy compared to those with healthy weights and this risk increases exponentially as body mass index increases,” says Jim Mellon, co-author of Juvenescence, and chairman of the biotech longevity company that bears the same name. “Research has shown that men with a BMI of 40 or more are 32 times more likely to die prematurely than those at a healthy weight.”

As the prevalence of obesity started to rise rapidly over the last 20 years, obesity tracking became a thing, with the media regularly reporting the latest stats. Prevalence continues to increase, and the numbers continue to stagger. Current figures from the Centers for Disease Control (CDC) show that nearly 75 percent of the adult population in the US are overweight or obese, and 42 percent, on average, have obesity. The UK is not far behind as 67 percent of men and 60 percent of women are overweight or obese, and around 28 percent have obesity. At last count worldwide, more than 1.9 billion adults were overweight, and over 650 million were obese — a number that has tripled in the last 50 years.2,3,4,5,17 Finding a way to prevent weight gain among those who are at a healthy weight, as well as helping those tipping the scale to lose it — and keep it off — is imperative.

HOW INTERMITTENT FASTING WORKS

Although losing weight is as simple as eating fewer calories than your body burns each day, many people find it tough to stick to diets where they have to constantly be vigilant about what they are (or are not) eating. Intermittent fasting is an approach to dieting that some people may find easier to follow.19 The practice focuses less on what you eat and more on when you eat it.

Eating less, and less often, seems to benefit more than just body weight: Eating at all hours with few breaks in between, as happens more in industrialized countries where food is plentiful and prevalent, may have negative impacts on health and body weight.6

In addition to weight loss, intermittent fasting has been shown to improve glucose regulation, strengthen the body’s resistance to stress, suppress inflammation, improve out-of-whack cholesterol levels and reduce hypertension — as well as increase longevity. Autophagy, the process where cells perform a molecular cleanup of waste products that is linked to longevity, is also up-regulated from intermittent fasting.7,8

As its label suggests, this diet approach involves intermittent periods of fasting: you eat; then you fast. Then you do it all over again. When you eat, or the schedule you follow, can vary, depending upon which type of intermittent fasting you choose to do.

“Fasting” can mean eating absolutely nothing at all. Or it can mean eating just a little bit of food during periods of “fasting,” usually around 40 percent of your normal amount, or what equates to around 500 to 800 calories in a day.8 “The key is to come up with an approach that is sustainable for you, that works with your personality and fits with your lifestyle,” says Dr. Stephen Anton, PhD, associate professor and chief of the clinical research division at the University of Florida’s Department of Aging & Geriatric Research who has conducted studies in intermittent fasting.

INTERMITTENT FASTING DIET OPTIONS

There are many ways to do intermittent fasting (IF.) The most-studied approaches are alternate-day fasting (ADF), 5:2 intermittent fasting and time-restricted feeding (TRF). There is also a fasting mimicking diet (FMD) that induces a fasting state, but while eating a very low calorie, plant-based ketogenic diet.6,9 Here’s how each approach works:

Alternate-day Fasting (ADF)
ADF is a day-off/day-on plan. If you follow a strict ADF plan, you’ll eat nothing every other day, accruing three fast days per week. Or on the Alternate Day Modified Fasting (ADMF) plan, you don’t fast completely, but you do dramatically reduce your intake. On the fast days you eat only 500 calories (women) or 600 calories (men) or fewer. Then on “feed” or “feast” days you eat what you want — and, in theory, as much as you want.

Intermittent Fasting 5:2
The 5:2 approach to intermittent fasting divides a week into five feast days and two fast days on non-consecutive days. “Dr. Michael Mosley, a British television presenter for the BBC, is credited with developing this particular intermittent fasting diet, which he himself followed, dropping around 10 kg of fat in three months resulting in reversing his type 2 diabetes,” says Mellon. The fast days can be complete fasts or very low-calorie days.

Time-restricted Feeding (TRF)
TRF divides a 24-hour day into an eating window that might last from four to 12 hours, followed by a fasting window, where you eat nothing, for 12 to 20 hours.

Intermittent Fasting 16/8
The most popular way to do TRF is known as the 16/8 plan where you eat for eight hours and fast for 16 hours. “This is one of the intermittent fasting diets that seems to be a better alternative to the misery that simply restricting calories continually would impose on us,” says Mellon. Some people start eating early and finishing early, from 8 AM to 4 PM, for example. Others shift the eating window later and might start eating at noon and finishing by 8 PM.

Fasting Mimicking Diet (FMD)
The FMD diet doesn’t involve complete fasts, but the calorie intake is very low for a five-day period. It’s also a ketogenic diet that is nutritionally replete and plant based. This type of ketogenic plan provides plenty of beneficial fiber and less potentially-harmful saturated fat, instead favoring healthier unsaturated fats, compared to conventional keto diets. The ProLon® plan developed by researcher Dr. Valter Longo, PhD, who is the author of The Longevity Diet, is designed to be followed one week a month for three months. Research has shown that the FMD protocol can elicit a slew of favorable health effects, such as a reduction in fasting glucose and the hormone IGF-1 (which is associated with inflammation) and abdominal fat loss.9,10

Intermittent Fasting Schedules

MON TUE WED THU FRI SAT SUN

ADF

Alternate-day Fasting

Feast Fast Feast Fast Feast Fast Feast

ADMF

Alternate-day Modified Fasting

Feast Fast:
Eat 500 kcals
Feast Fast:
Eat 500 kcals
Feast Fast:
Eat 500 kcals
Feast

5:2 FAST

Intermittent Fasting 5:2

Feast Fast:
Eat 500 kcals
Feast Feast Fast:
Eat 500 kcals
Feast Feast

TRF: 16/8

Time-restricted Feeding

Feast:
8am-4pm

Fast:
4pm-8am
Feast:
8am-4pm

Fast:
4pm-8am
Feast:
8am-4pm

Fast:
4pm-8am
Feast:
8am-4pm

Fast:
4pm-8am
Feast:
8am-4pm

Fast:
4pm-8am
Feast:
8am-4pm

Fast:
4pm-8am
Feast:
8am-4pm

Fast:
4pm-8am

TRF: 18/6

Time-restricted Feeding

Feast:
noon-6pm

Fast:
6pm-noon
Feast:
noon-6pm

Fast:
6pm-noon
Feast:
noon-6pm

Fast:
6pm-noon
Feast:
noon-6pm

Fast:
6pm-noon
Feast:
noon-6pm

Fast:
6pm-noon
Feast:
noon-6pm

Fast:
6pm-noon
Feast:
noon-6pm

Fast:
6pm-noon

FMD

Fasting-Mimicking Diet

Fast:
Eat 1090 kcals
Fast:
Eat 725 kcals
Fast:
Eat 725 kcals
Fast:
Eat 725 kcals
Fast:
Eat 725 kcals
Feast Feast
  • ADF
  • ADF
  • ADMF
  • 5:2 FAST
  • TRF: 16/8
  • TRF: 18/6
  • FMD
Alternate-day Fasting
MON
Feast
TUE
Fast
WED
Feast
THU
Fast
FRI
Feast
SAT
Fast
SUN
Feast
  • ADMF
  • ADF
  • ADMF
  • 5:2 FAST
  • TRF: 16/8
  • TRF: 18/6
  • FMD
Alternate-day Modified Fasting
MON
Feast
TUE
Fast:
Eat 500 kcals
WED
Feast
THU
Fast:
Eat 500 kcals
FRI
Feast
SAT
Fast:
Eat 500 kcals
SUN
Feast
  • 5:2 FAST
  • ADF
  • ADMF
  • 5:2 FAST
  • TRF: 16/8
  • TRF: 18/6
  • FMD
Intermittent Fasting 5:2
MON
Feast
TUE
Fast:
Eat 500 kcals
WED
Feast
THU
Feast
FRI
Fast:
Eat 500 kcals
SAT
Feast
SUN
Feast
  • TRF: 16/8
  • ADF
  • ADMF
  • 5:2 FAST
  • TRF: 16/8
  • TRF: 18/6
  • FMD
Time-restricted Feeding
MON
Feast:
8am-4pm

Fast:
4pm-8am
TUE
Feast:
8am-4pm

Fast:
4pm-8am
WED
Feast:
8am-4pm

Fast:
4pm-8am
THU
Feast:
8am-4pm

Fast:
4pm-8am
FRI
Feast:
8am-4pm

Fast:
4pm-8am
SAT
Feast:
8am-4pm

Fast:
4pm-8am
SUN
Feast:
8am-4pm

Fast:
4pm-8am
  • TRF: 18/6
  • ADF
  • ADMF
  • 5:2 FAST
  • TRF: 16/8
  • TRF: 18/6
  • FMD
Time-restricted Feeding
MON
Feast:
noon-6pm

Fast:
6pm-noon
TUE
Feast:
noon-6pm

Fast:
6pm-noon
WED
Feast:
noon-6pm

Fast:
6pm-noon
THU
Feast:
noon-6pm

Fast:
6pm-noon
FRI
Feast:
noon-6pm

Fast:
6pm-noon
SAT
Feast:
noon-6pm

Fast:
6pm-noon
SUN
Feast:
noon-6pm

Fast:
6pm-noon
  • FMD
  • ADF
  • ADMF
  • 5:2 FAST
  • TRF: 16/8
  • TRF: 18/6
  • FMD
Fasting-Mimicking Diet
MON
Fast:
Eat 1090 kcals
TUE
Fast:
Eat 725 kcals
WED
Fast:
Eat 725 kcals
THU
Fast:
Eat 725 kcals
FRI
Fast:
Eat 725 kcals
SAT
Feast
SUN
Feast
INTERMITTENT FASTING RESULTS

Ultimately, IF helps you consume fewer calories. But some other mechanisms are in play that might make it easier. During fasting periods, if there is a reduction of 500 to 700 calories per day or less, or very low levels of carbohydrate intake, a metabolic switch can occur, where the body switches to ketosis, accessing fat to produce ketones for fuel. Ketosis has an appetite-suppressing effect. So, this can make it easier to make it through the fasting portions of the regimen.7,11

The ability to alternate between periods of feasting and fasting may provide a psychological edge over conventional diets where you continually restrict what and/or how much you eat. “Conventional caloric restriction can reduce body weight and increase longevity, but it can be an unpleasant and impractical way of going about it,” says Mellon. “You might end up living a little longer. But if you’re living in constant hunger, as the old joke goes, it might feel much, much longer.”

HOW MUCH WEIGHT CAN YOU EXPECT TO LOSE

One study found that participants lost 2.5 percent of their starting body weight, and 4 percent of their fat mass, after 22 days of alternate-day fasting. Another study in the International Journal of Obesity found that overweight or obese premenopausal women following the 5:2 plan lost an average of 6.4 kg (14 lbs.) over six months. A systematic review that analyzed the results of 400 overweight or obese participants who were enrolled in six studies found that an average of 5 to 10 kg (11 to 22 pounds) was lost during interventions that lasted an average of 5.6 months. The fasting-mimicking ProLon® diet has resulted in an average weight loss of 5.7 lbs. when done for three five-day cycles over three months.9,12,13,14

In studies that compared intermittent fasting to conventional caloric restriction, generally, weight loss results appear to be similar. However, intermittent fasting appeared to produce significantly better losses in waist circumference and visceral fat in the belly. And one study published in the British Journal of Nutrition found that two different intermittent fasting regimens result in significantly greater weight loss and fat loss among adult women. The majority of participants (over 65 percent) lost at least 5 percent of their starting weight after three months on the regimens.6,12,13,14,15

HOW INTERMITTENT FASTING WORKS FOR WEIGHT LOSS

No matter which type of intermittent fasting you do, “feasting,” where you eat all that you want during the periods that you are not “fasting,” is part of the allure of this eating regimen. And while some people practice IF for its health benefits, many do it to lose weight. But that begs the question. How can you “feast” and still lose weight?

Losing weight ultimately boils down to burning more calories than your body takes in. So even though intermittent fasting allows for days where you can “feast” or eat ad libitum, you probably won’t overeat by enough to stifle weight loss. Studies have shown that people who do intermittent fasting overall eat fewer calories than normal, and not enough to make up for the caloric deficit they accrue from fasting periods.13,16, 18,19

Here’s how it works: When you consume the same amount of calories as your body burns each day, you are in energy balance. So, if you are weight stable, you might burn around 2,000 calories per day and eat around that same amount. In a typical week, you might eat around 14,000 calories.

Alternate-day Fasting (ADF)
If you follow a strict ADF plan, you’ll eat nothing on three fast days per week. Or you might just reduce your intake and eat only 500 calories on three fast days. Then on “feast” days you eat what you want. If you eat more than normal, say, 2,500 calories, you’d accrue a deficit of 10,000 calories on the stricter plan. If you eat small amounts on fast days, you would still accrue a deficit, consuming 11,500 calories instead of the usual 14,000 in a week. Those calorie reductions add up over time to weight loss and fat loss.

5:2 Intermittent Fasting
The 5:2 approach divides a week into five feast days and two fast days. Let’s say you fast by dropping down to eating 600 calories on two days a week. That leaves five days to indulge. You might eat 3,000 on day one, 2,500 on day two and 2,000 on day three. Depending upon your activity level and other factors, you might even eat less than 2,000 on days four and five, perhaps 1,800 calories. Overall, you’d eat around 12,300 calories, lower than the usual 14,000.

Periodic Fasting
This approach entails fasting completely for two or more consecutive days. Since extended periods of zero-calorie intake are a form of starvation and can result in nutritional deficiencies over a period of time, this approach should only be done under the supervision of a physician.6

Time-restricted Feeding
TRF divides a day into eating and fasting windows. Fitting a day’s three meals into shorter windows can be tough, and usually results in fewer calories consumed because when your meals are spaced close together, you’re not that hungry by the time you get to the next one. A typical eating pattern may look like this: breakfast, or at least coffee — often with sugar or cream, upon waking, let’s say at 7 AM. Lunch might take place at noon or 1 PM. There may be other snacks during the day. Dinner might start at 6 PM and end at 7 PM. (If you eat like this normally, you might already be following a 12/12 time-restricted eating plan.)

But not everyone stops eating after dinner. There might also be a bedtime snack at 9 PM. And not everyone eats dinner early. Some may eat as late at 8 or 9 PM, finishing even later. So, you might normally eat that 2,000 calories over 12 or 15 hours, or more.

Trying to eat 2,000 calories within a six- or eight-hour window can be tough, especially if you are eating healthier, whole foods. Typically, the more you eat, the fuller you feel. So, if you ate breakfast at 7 AM, you’d need to eat lunch and dinner by 1 PM if you are on the six-hour eating window, or by 3 PM if you are on the eight-hour plan. So, you finish breakfast at 7:30 AM, eat lunch at 10:30 AM and dinner at 12:30 PM. Chances are, by dinnertime, just past noon, you aren’t going to be hungry enough to meet your 2,000-calorie quota for the day. So, if you reduce your calorie intake by 500 calories because you are fitting all your meals within a shorter eating window, you might consume only 10,500 calories in a week, instead of 14,000, thereby creating a caloric deficit that will lead to weight loss over time.

IS INTERMITTENT FASTING SAFE FOR YOU?

People with certain health conditions such as diabetes should consult their physician before following a regimen such as this. People with a history of eating disorders or who are pregnant or breastfeeding should not follow this type of plan. Be aware that many types of intermittent fasting that involve total fast days where no food is consumed can risk nutritional deficiencies if done for extended periods of time. When carrying out total fasts, it is important to eat nutrient-rich foods during eating periods to meet essential nutrient needs.8

You don’t have to dive right in and be a total perfectionist, You can start by simply delaying your breakfast, having it later in the day, or by eating dinner earlier.”

Dr. Stephen Anton, PhD
HOW TO START INTERMITTENT FASTING

Different protocols may be better suited to different personalities. So, if one approach isn’t the right fit to your lifestyle, try another. “And keep in mind that you don’t have to dive right in and be a total perfectionist,” says Dr. Anton. “You can start by simply delaying your breakfast, having it later in the day, or by eating dinner earlier. Then you can gradually work up to incorporating the extended periods or days of non-eating or very-low-calorie eating.”

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