Intermittent fasting (IF) has emerged as one of the many trendy diet options these days. Basically, “intermittent fasting” is the practice of periodically alternating between fasting – drinking just water and perhaps low calorie drinks like coffee – and non-fasting, i.e. eating normally.
IF comes in a variety of plans and structures. The most popular of these are:
Periodic Fasting – eat normally for 5 days of the week. For 2 non-consecutive days, reduce calorie intake, usually to 500-600 calories. You can spread out the calories into smaller snacks or eat one meal after 24 hours of fasting (so, say you started at 7 pm the night before, you could eat 500-600 calories at 7 pm the next day).
Restricted Eating Period – eat normally, but only for a set window during the day. Most people using this plan eat during an 8 hour window starting around 10 am – 12 pm and lasting until 6 – 8 pm. This essentially equates to skipping breakfast and making lunch your first meal.
Benefits of Fasting
Supporters of intermittent fasting have claimed a wide range of benefits from the practice, including
- Reduced inflammation
- Weight loss
- Faster metabolism
- Lower LDL and total cholesterol levels
- Improved blood glucose and insulin levels
- Protection against cardiovascular disease
- Sugar cravings. When you first start out fasting, the body will need to adjust between using carbs for fuel and using fat for fuel, during which time you may experience some cravings. However, this is common when starting any lower carbohydrate or reduced sugar diet.
- Blanket Prescriptions. Most alternate day fasts prescribe 500 calories for women and 600 for men on fasting day. But what about differences in energy needs among different people? If I’m a small woman doing little exercise, 500 calories might be about 1/3 of my usual daily needs. But if I’m a larger man doing high intensity interval training (like CrossFit) 5 times a week, 600 calories might be less than ¼ of my usual daily needs.
- Does It Make Sense? IF first reached the mainstream around 2003, when The Warrior Diet was published. The Warrior Diet basically prescribed fasting all day and eating one large meal at night because this is what Paleolithic man and Roman soldiers did. But just because Caesar’s army or Paleolithic man did something doesn’t mean we should be doing it. They ate the way they ate because it was all they had. Now we have the knowledge and ability to really optimize our diet. Let’s not waste it blindly copying our ancestors.
- Is There a Ramadan Bias? Many studies I found citing the benefits of fasting on health markers were based on studies conducted among observers of Ramadan, a religious fast during the ninth month of the Muslim calendar. During Ramadan, observers do not eat or drink anything after sunrise and may eat again when the sun sets. However, many IF protocols look nothing like Ramadan. So, are the benefits the same?
After a brief review of PubMed (an extensive online research database), I came to the following conclusions:
A lot of the studies with drastic positive findings were conducted in animals – mostly mice, rats, and fur seals. While the physiological changes observed in these studies support the benefit claims, the findings have not been replicated on a large scale in humans.
A good portion of the human studies on intermittent fasting – especially in athletes – were conducted among people observing Ramadan. Research conducted as of 2012 indicates that the effects of Ramadan fasting are small if the athlete maintains energy and fluid intake during eating times, maintains a normal training schedule, and gets 8 hours of sleep. Studies have also shown that Ramadan fasting lead to decreased power and increased muscle fatigue in football players, decreased performance among middle distance runners, loss of sleep, and increase in subjective fatigue. In addition, most studies on Ramadan have been conducted when it took place during winter months – with shorter days and milder weather – and more research is needed for times when Ramadan falls in the summer.
As for the research on humans not observing Ramadan, the pickings were slim. One study found that both IF and continuous diet with energy restriction resulted in weight loss and improvements in insulin sensitivity, leptin, cholesterol and other health markers among obese women (Arqin et al 2012). Similar results were seen in obese, young women and men. (Harvie et al 2011). Another study found that calorie restricted IF, both with food and with liquid meals may help reduce CHD risk factors, although the IF with liquid meals resulted in greater results. It’s important to note, though, that the IF diet in this study was not compared to a similar continuous diet.
My Takeaway? Cutting out extra calories is a well-recognized strategy for losing weight and improving health overall. IF is just as good a strategy as a “normal” lower calorie diet for cutting back on calories.
So, Should You Try Fasting?
I don’t think you SHOULD fast, but I think you COULD fast. I always tend to recommend a stable diet consisting of quality foods, with intake based on hunger cues. However, the what, why, when, and how of eating is different for everyone. So if you wanted to try fasting as a way to lower calorie intake, then I think it is a good strategy worth trying.
There is one situation in which I would recommend IF – if you have lost your hunger. It’s easy in our American culture of food availability and glorification of “busy” to stop eating for hunger and start eating for a bunch of other reasons. You eat breakfast because it’s 8 am and you need to leave for work. You eat lunch because your coworkers are all going to that new Mexican place at noon. You eat a snack at 4 because you’re bored. And so on. In a case like that, a week or two of intermittent fasting can help reset your awareness of hunger and recognize the difference between actual hunger and other types of hunger. A great read on this is a recent blog post by Robb Wolf’s RD Amy Kubal, “What Kind of Hungry Are You?”
A few tips if you choose to fast:
- Make sure your “normal” diet is on track. Limited eating periods and non-fast days are NOT an excuse to eat whatever whenever. IF only works as a method for calorie restriction if you maintain healthy, moderate eating habits during non-fasting periods.
- Plan ahead. If you decide to do your long run or 2½ hour Olympic lifting class on a fast day, you might pay for it in the form of poor performance and fatigue. Especially when you’re starting out, plan fast days to coincide with lighter training days.
- Listen to your body. Don’t stick to IF because it’s supposed to have all these benefits if it doesn’t feel right for you. If you’re tired all the time, losing sleep, and not seeing results, it’s time to try a new strategy.