The advice sounds counterproductive, but I have always told meal-skipping clients “add a meal, lose a pound” and it usually works… but is there more to the story? There is a multitude of evidence, anecdotal and research-based, that suggests skipping meals adds up to big problems on the weight maintenance front. You skip breakfast and lunch, overeat at dinner and into the night, and there go any calorie savings from foregoing meals earlier in the day.
But what if you DIDN’T overeat at night… and didn’t each much the whole day? Is intermittent fasting really an effective weight loss solution? It may take willpower of steel, but could missing breakfast, lunch and dinner 1 – 2 times per week give superior weight loss results to the traditional “eat less, move more” philosophy? More importantly, would any weight loss as a result intermittent fasting actually be sustainable?
Note: we are just focusing on the evidence for intermittent fasting in overweight and obese individuals here, not athletes. That’s a whole different story!
The Canadian Medical Association Journal has been ALL OVER intermittent fasting in recent months (1-3) – could have something to do with the publication of several intermittent fasting books tearing up the bestsellers lists, including one by Canadian Brad Pilon called Eat Stop Eat. In fact, Pilon tells the CMAJ that he thinks intermittent fasting is going to be the next big thing in weight loss fads (3).
Get ready for a lot of grumpy and hungry co-workers, friends and family!
What is Intermittent Fasting?
Intermittent Fasting (IF) involves alternating “normal” eating days with days where caloric intake is severely restricted… and we mean severe at a maximum of 500 Calories per day on most plans...
This can be as strict as fasting during the sunlight hours of EVERY day (as is done in the month of Ramadan, the inspiration for IF as we know it today), to an every-other-day plan, or fasting on one or two days per week (4).
How is IF supposed to work?
Much like any calorie deficit does. Caloric restriction leading to weight loss is a pretty simple concept – less energy in as calories from food, more energy out as basal metabolism and exercise = weight loss.
As you draw on body reserves to compensate for this net loss of calories, you lose weight – predominantly fat stores, but some lean body mass particularly in severe, strict diets. This is the philosophy behind intermittent fasting: a severe restriction of calories for only a short time period to reduce the risk of losing muscle mass while still losing fat.
As for whether or not intermittent fasting gives better results than daily restriction of calories, a lot of the evidence is anecdotal and unsubstantiated in the literature. Mechanisms for how and why intermittent fasting *could possibly* result in improved weight loss with preservation of muscle are largely unknown.
But won’t you be hungry ALL THE TIME? Depending on how long your fasting period is, you may deplete your carbohydrate stores and begin burning ketones for fuel – the notorious bad breath-causer in low carb diets – and ketones mildly suppress appetite, at least after you adjust to the restriction (1-4). Note: we are getting hungry just thinking about not eating!
Proponents of IF also claim that IF is a desirable weight loss solution because it removes food-based decisional stress on fasting days and reduces food preoccupation (1-3). If you are not supposed to eat anything, or only allowed a specific beverage, then you don’t have to worry about WHAT to eat! We’ll see about that…
What’s the evidence?
Like most emerging “fad” diets, most of the evidence for IF is limited to animal models, but the evidence from animals *is* positive. As for the notion that intermittent fasting is superior to regular, ol’ daily caloric restriction… the evidence just is not there in humans, certainly not for any sustainable improvements.
Klempel et al. (5) looked at an 8-week low-calorie liquid-based diet (similar to classic breakfast and lunch SlimFast plan) vs. food-based diet (both provided ~ 800 – 1000 kcal per day), with both plans having one day of 120 kcal fasting each week. Both groups lost weight (no duh!), but compliance was letter with liquid shake group and they lost more weight.
Hmmm… a low calorie plan with one day of fasting is effective for weight loss BUT how much did they regain after they went back to eating normally? There was no follow-up data, but we would be very curious! Also, they did not compare the IF interventions to a non-fasting, calorie-reduced control, and these results would be expected from a diet that low in calories!
Harvie et al. (7-8) have compared IF protocols to a daily restricted calorie intervention… and weight loss wasn’t better in the intermittent fasting groups, particularly not after a weight maintenance follow-up phase (7-8). In two IF protocols – one 3-month program (6) and another 6-month program (7) – Harvie et al. randomized overweight and obese women to daily restricted calories (~ 1400 kcal) or regular eating 5 days per week with 2-fasting days at 600 kcal. Both studies showed major improvements in weight, glycemic control and cholesterol levels.
The intermittent fasting group DID have significantly more fat loss in the 3-month program (6) and only a modestly better improvement insulin resistance in the 6-month study (7). Otherwise, the groups had IDENTICAL results. Aside from preoccupation with food, that is, which was reported more often in the fasting group! So much for worrying less about food when you are fasting.
No Baloney’s advice? The evidence that intermittent fasting is successful at achieving weight loss is growing, but there is no real evidence that IF is any better than standard calorie restriction. And this kind of diet is not for everyone. For people who live to eat like us, going without food for six hours is cruel and unusual, let alone 1 – 2 days! Brad Pilon, one of Canada’s biggest proponents of IF is a former bodybuilder who got interested in IF to lean out without losing muscle mass. We are not exactly talking about couch potato-turned-to-fasting success story!
We believe that it would take a very specific type of personality to be successful with an intermittent fasting plan and not lead to bingeing or back-sliding as you transition back to “normal” eating. Maybe that is you, maybe it isn’t. Keep in mind, all of the studies above had dietitian involvement to ensure participants were meeting their nutrient needs. This is not a willy-nilly kind of plan – guidance is a must for success AND safety.
Who SHOULD NOT try intermittent fasting? Anyone with a history of low blood sugar, those using insulin injections or oral diabetes medications that stimulate insulin production (like glyburide and other sulfonylureas), and people taking certain high blood pressure medications. Severely restricting calories is not like simply choosing you have ANY chronic disease, you should talk to your doctor first.
Now for an intermittent plan we can really get on board with. Arguin et al. (8) randomized older overweight and obese women to a 20-week program of daily calorie restriction with or without alternating periods of weight maintenance focus (the opposite of fasting!). Both groups lost a similar amount of weight AND there was no difference in the follow-up after 12 months. NO FASTING – just intermittent focus on good habits and weight maintenance rather than deprivation. We think this is most promising method for intermittent dieting we’ve come across!
- Collier R. Intermittent fasting: five quick questions with fasting expert Brad Pilon. CMAJ 2013; April 15, 2013 [epub ahead of print].
- Collier R. Intermittent fasting: the science of going without. CMAJ 2013; April 8, 2013 [epub ahead of print].
- Collier R. Intermittent fasting: The next big weight loss fad. CMAJ 2013; 185(8):E321-2 [epub ahead of print].
- Brown JE, Mosley M, Aldred S. Intermittent fasting: a dietary intervention for prevention of diabetes and cardiovascular disease? Br J Diabetes Vasc Dis 2013; 13:68-72.
- Klempel MC, et al. Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutr J 2012 11:98.
- Harvie M, et al. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. Br J Nutr 2013 [epub ahead of print].
- Harvie MN, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes 2011; 35:714-27.
- Arguin H, et al. Short- and long-term effects of continuous versus intermittent restrictive diet approaches on body composition and the metabolic profile in overweight and obese postmenopausal women: a pilot study. Menopause 2012; 19:870-6.