The Facts About Intermittent Fasting

Fasting in midlife is popular for shedding pounds. Find out if the hype lives up to the science.

Fasting, Defined

Fasting is simply going without food. While fashionable, fasting is nothing new. It’s been part of religion for centuries, and you could say fasting is in our DNA.  Our ancient ancestors, lacking refrigeration, grocery stores, and 24-hour restaurants, regularly alternated between long periods without eating and feasting on food when it was plentiful.  These days, fasting is often used as an alternative to traditional calorie-restricted plans for weight loss and to reduce the risk for diabetes, heart disease, and inflammation.

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Fasting for weight loss isn’t about what you eat as much as when. Intermittent fasting (IF), also called time-restricted eating, is toggling between periods of no food and eating on a set schedule.  You consume all of your food during a designated “window” every day and you’re allowed calorie-free beverages at all times.  With no meal plans, calorie limits, or certain meals or foods to prepare or buy, IF is far from a prescriptive way of eating. In fact, any food is fair game on IF.

There are many ways to go about IF, but the 16:8 plan - fasting for 16 hours and eating during an eight-hour time frame - is one of the most popular choices. Fasting for 20 hours and eating for four (the 20:4 plan) is another form of IF.  The 5:2 approach is modified fasting for two, preferably non-consecutive days when you consume 25% of your daily calories (about 500 for women and 600 for men); on “feast” days, you eat regularly. Alternative day fasting (ADF), which is going without food every other day of the week, allows only for calorie-free drinks on fast days.  

Intermittent Fasting and Weight Loss

Intermittent fasting lengthens the time between your last meal or snack and the point when your body starts tapping into its fat stores to keep going.  Blood glucose levels go up after you eat, which triggers the pancreas to release insulin to move extra glucose into cells.  Fasting for more than 12 hours suppresses insulin and signals the body to burn stored fat for energy. When this process is repeated daily, it may reduce body fat stores, resulting in a leaner body.

With IF, you watch the clock, not your calories, which may be part of its appeal for women who don’t want to weigh, measure, and log everything they eat and drink. (I hear you!)  Though you’re technically not restricting energy, several studies suggest that practicing IF naturally reduces calorie intake.

In a 2020 Cell Metabolism study, people with obesity who ate only during a four-hour or a six-hour window every day for eight weeks curbed their daily calorie consumption by an average of 550 as compared to the control group, who could eat whenever they wanted. Both of the IF groups (and the controls) could eat whatever they wanted and were not instructed to limit calories but ate less anyway.

A 2023 Annals of Internal Medicine 12-month study found IF helped people lose weight and keep it off, but that the effects of IF were similar to daily calorie restriction. Participants in the IF group ate from 12 PM to 8 PM and lost an average of 10 pounds more than those in the control group who ate whenever they wanted.  The group that restricted (and counted) calories lost an average of 12 pounds more than the controls. Both IF groups reduced calorie intake by about 400 daily, on average.

Still another 12-month study shows that IF does not have the upper hand when it comes to weight loss.  Research published in the New England Journal of Medicine in 2022 found that when people limited their eating window to eight hours and restricted their calories simultaneously, they lost similar amounts of weight as people in the study who simply restricted calories.  

Fasting May Offer Other Health Benefits

Weight loss aside, the other possible health benefits of IF are worth discussing, particularly when it comes to insulin. Fasting reduces insulin levels in the bloodstream, which is a good thing. Consistently elevated insulin concentrations are linked to a greater risk for obesity, type 2 diabetes, cardiovascular disease, and cancer. That doesn’t mean fasting is superior for risk reduction, however.  Weight loss, exercise, and physical activity also help reduce blood insulin levels and normalize blood glucose concentrations.

When it comes to indicators for heart disease, high blood pressure, and inflammation, the answer isn’t as clear.  The 2020 Cell Metabolism study I mentioned earlier found no reduction in blood pressure, blood cholesterol, blood triglycerides or signs of inflammation in the groups that practiced IF as compared to the control group. And a 2023 Aging Biology study of men and women with overweight found that while those on IF for six months lost up to 44 pounds, they did not experience reductions in markers for inflammation, or improved glucose tolerance or insulin sensitivity.

Should You Shut the Eating Window Earlier?

Your body is ruled by circadian rhythms, which are internal 24-hour cycles that follow the day/night routine and regulate energy metabolism and appetite hormones, among other functions.  There is some evidence suggesting that eating earlier in the day is optimal for coordinating with the body’s rhythms that help us process the nutrients from food. We’re not that different from our ancient ancestors, although we like to think so.  Our metabolism has evolved expecting us to eat during the day and sleep at night, but people don’t always do that. Electricity makes it possible for us to be awake and to eat at any time of day, and often when our bodies are least equipped to handle food.

Perhaps IF would work better if the eating window was earlier in the day to stay in sync with our internal clock.  A 2023 Nature Communications study compared the effects of an eight-hour eating window starting between 6 AM and 3 PM to an eight-hour window that started between 11 AM and 8 PM. What’s interesting about this study is that the there was no significant difference in calorie intake between the two IF groups, yet the researchers found that those who started and ended eating earlier in the day had a greater improvement in insulin sensitivity than those who ate later in the day. The “early” eaters also had lower fasting glucose and less inflammation than the “later” group.

Eating earlier may be a weapon in the battle against prediabetes and type 2 diabetes, which increase with menopause and age.  In a 2018 Cell Metabolism study in people with prediabetes, limiting food intake to six hours and finishing by 3 PM every day for five weeks decreased insulin levels, reduced insulin resistance, and improved blood pressure even without weight loss, suggesting that when you stop eating enhances fasting’s effects.  A 2022 Obesity Reviews study that examined the outcomes from nine clinical trials with nearly 500 adults found that people who ate most of their calories earlier in the day lost more weight than people who did the opposite. The “earlier eaters” also had greater improvements in their blood levels of glucose, cholesterol, and for insulin sensitivity, which is a marker of diabetes risk. It may be that eating earlier enhanced the synchronization of circadian rhythms to improve metabolic indicators.

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What You Eat Matters, Too

Setting time limits on eating can naturally curb calorie intake, but it may also compromise the quality of your diet. It can be challenging for busy midlife women to include all the protein, calcium, fiber, and other nutrients they need in a shorter eating window.  As a perimenopausal or post-menopausal woman, your eating plan should be packed with the nutrients your body requires and low in saturated fat, sodium, and added sugar. In other words, IF isn’t an eating free-for-all for women interested in protecting their health during the menopause transition and afterwards.

Nutrition matters, and so does the amount of exercise and other physical activity you get every day.  One of the benefits of IF is that it’s possible to shed pounds without counting calories.  Yet, it’s possible to gain weight, too, if you go all out during your eating window.  Think of IF as one more tool in your weight control toolbox, not as a magic bullet.

What to Consider Before Trying Intermittent Fasting

Fasting isn’t for everyone. Certain people require a steady flow of nutrients to support health. While going without food for is safe for many people, don’t fast if you’re over 75, you take blood pressure medication or need to take medication with food at certain times of the day, you’re an endurance athlete or you exercise intensely, or you have a chronic medical condition, such as kidney disease, type 1 diabetes, and possible type 2 diabetes (check with your health care provider first before trying IF).

Women who have a disordered relationship with food are also risk for harm from IF. Preoccupation with the timing of eating can exacerbate existing anxieties or negative behaviors around food. If you’re a parent or guardian, you may want to think twice about any form of fasting that has you forgo eating in front of kids, which could set confusing expectations, potentially jeopardizing a child’s relationship to food.

Given all that menopausal women deal with, consider that going long stretches without eating can leave you feeling cranky and may result in obsessive thoughts about food. Depending on the length and timing of the eating window, IF could put a crimp in family and social activities that involve food, like family dinners and celebrations.

How to Try Intermittent Fasting

During intermittent fasting, you focus more on the clock than on calories, and every type of food is allowed. It’s possible that designated eating times can make it easier for you to control food intake, especially in the evening.  As long as you consume adequate calories and satisfy your nutrient needs, and IF doesn’t interfere with your mental health, or your home, work, or social life, and provided that you don’t go any longer than 16 hours without eating, IF is probably safe and sustainable. (Tell your health care provider about your eating plan.)

If the idea of long stretches without eating lacks appeal, a shorter daily eating window may provide benefits similar to more drastic IF plans. For example, eating satisfying meals and snacks during the day and not snacking after dinner could cut hundreds of extra calories daily.  In fact, that’s one of the principles Hillary Wright, MEd, RDN and I advocate in our book, The Menopause Diet Plan, A Natural Guide to Managing Hormones, Health, and Happiness. In our decades of counseling women, we’ve found that eating too little during the day sets people up to overdo it at night. If you don’t want to do IF, choose a satisfying eating plan that’s right for you from the book and stop snacking at night.

If you are ready to try IF, make sure you have a balanced eating plan in place first.  Ease into fasting by extending your nighttime fast to 10 hours and then to 12 hours.  Try to match mealtimes to your body’s rhythms by starting to eat earlier in the day and stopping earlier. For example, on a 16:8 plan, this could mean eating from 9 AM to 5 PM, rather than from 12 to 8 PM. To get the most out of IF, choose a regular fasting schedule and stick to it every day.

The Bottom Line on Intermittent Fasting

There’s no proof that fasting provides a weight loss advantages compared to restricting calories on a daily basis, but IF can make it easier to eat better, as long as you maximize meal timing and food choices. Above all, listen to your body and eat if you’re hungry. You’ve got enough on your plate!

What do you think about intermittent fasting? Have you tried it? Would you like to try it and why?