You probably know someone who swears by intermittent fasting. Maybe you’re even interested in trying it yourself. It’s one of the most Googled diets in the world (right up there with keto!); clearly, people are hungry for the facts.
So what is it, and what’s the science behind it?
The basics
Fasting is thought to boost metabolism and reduce the risk of chronic disease. In animal studies, fasting is associated with lower cholesterol, lower triglycerides and fasting glucose levels, less body fat, and better overall health—even a longer lifespan.
Intermittent fasting—an eating pattern in which you eat little or no food during set, cyclical blocks of time—is emerging as a beneficial tool in managing health and reducing the risk of metabolic diseases. In one popular method of intermittent fasting, eating happens within eight hours (daytime), and the rest of the time is spent fasting (unsweetened drinks, however, are allowed). During the times when there’s no available energy from food, the body uses calories stored as fat.
Unlike other popular eating patterns, no particular foods or nutrients are prioritized or banned; instead, the clock is king. The latest research reveals that it’s not just swearing off eating for a few hours that counts, but also the timing of it.
The circadian rhythm connection
Insulin release follows your circadian rhythm, rising by day and dropping at night, which explains why we seem to deal better with richer meals earlier in the day. Basically, our circadian clocks are designed to help us stay healthy as we go between feeding and fasting.
With the arrival of artificial lighting, the boundary between day and night became blurred, and not without consequences. Shift work, social engagements, and unhealthy habits have us eating calorie-rich meals and snacks late at night. Obesity and diabetes (due to insulin resistance and high blood glucose levels) are on the rise.
Constant snacking keeps the body in a perpetual “fed state,” while fasting helps the body switch to breaking down stored resources and performing repairs.
Recent revelations about the microbiome also need to be considered. Our inner team of bugs, normally in sync with the circadian clock, are now believed to adapt to our late-night shenanigans by making it even harder to resist after-hours temptations.
Exercise to the rescue? Before you go for that late-night treat while vowing to run it off tomorrow, consider this: Exercise alone is not thought to prevent timing-related diet-induced obesity. Instead, aim to avoid nighttime eating when possible so you work with your circadian clock, not against it.
Effects of intermittent fasting
“There are studies showing that intermittent fasting decreases inflammation and reduces oxidation, and it reduces the risk of heart disease,” says Orsha Magyar, MSc, a registered holistic nutritionist.
This has many asking: Could intermittent fasting increase our lifespan? Multiple studies on many organisms, from yeast to worms to mammals, have concluded that fasting promoted longer, healthier lives. Fasting mice even developed fewer and slower-growing tumors in one study. Fasting has been shown to help rodent brains too. “Intermittent fasting increases the brain production of a special kind of protein, which helps neurons grow and stay healthy, and it may even protect against neurodegenerative diseases,” says Magyar.
Fasting is not for everyone
“Some people may experience improved health outcomes on intermittent fasting; for others, it may not lead to any changes or may cause negative health outcomes,” caution registered dietitians Alexandra Inman and Stephanie Dang in a written interview. No single diet plan works well for everyone, they add.
Children and teenagers should not fast, as their developing bodies need a constant supply of nutrients.
“We recommend that people with chronic diseases such as diabetes, cardiovascular disease, or cancer don’t fast, as they’re at risk of becoming malnourished,” say Inman and Dang. Avoid fasting if you’re pregnant or planning to become pregnant or are breastfeeding, they add. People with a history of eating disorders should also steer clear of intermittent fasting.
Don’t fast if you take blood pressure or heart medication, as it can affect the electrolyte balance in your body. And don’t fast if you have any blood sugar issues, cautions Magyar, as you’ll risk hypoglycemia.
Make sure you speak to your health care practitioner before adopting the diet.
Fasting or not, eat healthy
Yes, fasting may prove helpful in preventing or even possibly reversing metabolic disease symptoms and reducing cancer risk, but it is by no means a standalone silver bullet. “Focus on incorporating more vegetables, fruit, whole grains, and plant-based proteins such as legumes, tofu, and nuts/seeds and their butters into your diet to make sure you’re getting the nutrition you need,” advise Inman and Dang.
Fasting also cannot counteract the negative health effects of a diet high in unhealthy fats, sugars, and refined carbohydrates. In other words, shop the perimeter of the grocery store, where the wholesome foods are.
Cook for the love and health of it and make meals a social affair. “The reasons that people eat are for more than just nutrition. Our food choices are impacted by social, economic, environmental, and cultural factors,” say Inman and Dang.
If there’s a takeaway anyone can apply to their eating habits, it’s to enjoy every bite, choose healthy options whenever possible, and remember your circadian clock.
Find your fasting style
There are a few intermittent fasting methods, including these: 5:2 method: Eat normally for five days a week, then switch to 600 calories a day for the remaining two.
Alternate-day method: Alternate between regular feeding days and fasting days, consuming just 500 calories or so on the fasting days.
16:8 method: Eat for a period of eight hours and fast for the rest of the time, consuming only water and unsweetened drinks.
Monitor the impact on your lifestyle, whichever fasting method you choose. “You need between three and six weeks to adapt to intermittent fasting,” advises registered holistic nutritionist Orsha Magyar, so be patient but aware of how you feel.
Is one method better than another? More research is needed on the effectiveness of all of them. And the intermittent fasting method that’s right for you depends on your goals and what you find sustainable. The 5:2 and alternate-day methods are the most widely studied in human trials so far. Both methods appear to lead to weight loss, plus improvements in blood pressure and insulin resistance. A recent 12-week pilot study of obese adults found that the 16:8 method also led to mild weight loss and reductions in blood pressure. Notably, no one dropped out of the study due to difficulties with the diet, which suggests the 16:8 method may be easier to stick with than other forms of intermittent fasting.
Another study—this one of men with prediabetes—found that limiting eating to earlier in the day offered benefits even without weight loss (the study was designed to maintain each participant’s weight). A six-hour eating window that ended by 3 pm led to improved insulin sensitivity, lower blood pressure, and reduced appetite.
