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Pick up a fitness magazine at the grocery store, or listen the pre-spin-class gossip, and the chances are that someone is talking about the alleged merits of intermittent fasting. But is this anything more than another fleeting fad?
The act of fasting has been around for millennia, with countless cultures and religions forgoing food for specific occasions or holidays. Cultural practices aside, these days intermittent fasting, or time-restricted eating, is seen as an easy and super-trendy way to drop a few pounds.
Here we look at the facts and science around intermittent fasting, including the benefits and downsides.
Unlike other diets that restrict specific foods (like carbs or dairy or meat), with IF, it's not so much about what and how much you eat, but rather when you eat. The timing of your fasting periods will depend on the specific regime you choose, and like the world of "detoxes," there is something in the IF world for everyone. I'm sure if you Googled long enough, you would find every possible permutation of the fast. Some variations restrict your eating to a specific number of hours each day. For example, you might have a daily 16-hour fast, so you could eat from 9 a.m. until 5 p.m., but nothing before or after. Other variations, like the popular alternate-day fasting, allow you to eat as much as you want on some days of the week, while severely restricting calories on the other days.
Not surprisingly, weight loss is a common (and often desired) outcome, largely because when we reduce the opportunities to eat, we just end up eating less. Taking in fewer calories means creating a bigger calorie deficit and thus greater potential for weight loss. But is there anything more to it?
Here are some of the potential benefits of an intermittent-fasting diet.
In one study, the IF regime was compared to a heart-healthy diet, and researchers found that while both groups lost comparable amounts of weight, the IF-regime group regained less of the weight lost. Since many dieters who lose weight gain back most of it, regaining fewer pounds is a benefit to long-term weight loss. Another study found that, compared with daily calorie restriction, participants who used a IF weight-loss regime were able to maintain more lean muscle mass while losing weight.
Although research in this area is in its infancy, some early research suggests that fasting may slightly increase your metabolic rate, meaning you may burn slightly more calories, even at rest.
Emerging research suggests that fasting may have some benefits for your brain. One early study on rats found that IF helped improve memory, while another animal study found it helped protect the brain from damage due to aging. While we can't necessarily generalize these results to humans quite yet, it's worth keeping an ear out as evidence evolves.
One preliminary study found that a fasting regime helped improved insulin secretion while producing new pancreatic cells in mice with type 1 and type 2 diabetes. In human trials, fasting helped improve participants' insulin sensitivity, while a new review of the literature found that IF was just as effective as daily calorie restriction at reducing insulin resistance.
A short preliminary human study found that fasting reduced cholesterol and blood pressure levels, while another study found it to increase good cholesterol and decrease the bad cholesterol levels in men. Unfortunately, a recent expert report on IF suggested that due to some inconsistencies in the data and preliminary study methodology, we still need a lot more research to confirm IF's heart health potential.
In an early animal study, IF seemed to reduce the risk of lymphoma in rat subjects, while another study found that fasting increased the longevity of rats who were inoculated with cancer cells. Unfortunately, while these studies may be an interesting jumping-off point for future research, it's far too early to suggest that there's any real measurable benefit of fasting on cancer in human patients.
So even if a lot of the research is young, and animal-based, it seems like there may be some serious perks from IF. But what about the potential risks involved? Let's take a look.
Severe dietary restriction is generally not recommended for women trying to conceive, so it makes sense that an early animal study found that fasting impaired the fertility capacity of rats. Again, while we shouldn't put too much weight on this early research, this type of calorie restriction (via IF or another restrictive diet) isn't recommended for pregnant or nursing women or those trying to conceive.
Diets with the most restriction usually have the highest dropout rates, and IF is no walk in the park. One study found that the dropout rate was significantly higher among IF dieters compared to dieters restricting daily calories, so if long-term change is your goal, it might not be the best diet for the job.
While there are many iterations of IF, the underlying concept remains constant—dieters have a fasting phase and a feasting phase, the latter of which can lead to some dangerous tendencies. When you've been restricting most of the day, just trying to manage the hangry beast inside each time you get a whiff of someone's lunch, it's quite natural to go overboard when the clock strikes "feast." For some, these feasting episodes have the potential to overshadow the caloric deficit you make during the fast, thwarting any weight-loss efforts and risking dangerous disordered eating behaviors.
Like most diets, IF is not a one-size-fits-all diet. IF may seem like it gives you the flexibility you crave in a diet, because you're not necessarily cutting out cheese or subsisting only on green juice, but restricting your eating at times when you're legitimately hungry is defying your body's unique intuition. That's just not healthy or sustainable.
Furthermore, while IF may work well for some people or some health concerns, most of the research to date is on animals and is very young, so we need a lot more long-term studies to evaluate its benefits and risks. If you think that IF may be right for you and want to give it a try, work with a registered dietitian to help you reach your own individual goals in the safest possible way.