There has been an explosion of diet books (The Fast Diet, The 5:2 Diet, The 8-Hour Diet, etc.) suggesting that manipulating the time frames you (allow yourself to) eat within can create metabolic-magic leading to easier and/or greater success with weight loss.
There are multiple possible time frame variances among these protocols, including:
a) limiting eating to the same 8-hours daily,
b) an alternating pattern of under- and over-eating days,
c) having two fasting days per week and eating normally the other 5 days,
d) having regular splurge days (in the middle of stretches of calorie restriction), and on and on.
Some authors even suggest you can splurge and eat as much as you want of whatever you want, on the non-fasting days and still lose weight. That’s the kind of promise that causes diet books to fly off shelves!
Traditionally, fasting has been considered primarily the act of willingly abstaining from all food, drink, or both, for a period of time. Alternately—and what is recommended by most these diets—is eating only sparingly (just 500 to 600 calories) on “fasting” days, and sometimes eating only certain kinds of foods on “fasting” days.
History - Fasting of course has been around for centuries, and is commonly used at different times of the year in religious observances. Various kinds of fasting regimes (i.e. "juice fasts," etc.) are sometimes advocated in alternative medicine. Within the field of weight management very-low-calorie “liquid diets” are often referred to as “fasting” as well.
The roots of this new genre of "fasting" diet seem to spring from longevity research showing that long-term calorie restriction (eating only about 75% of normal) improves health and lengthens lifespan. While it’s true that long-term calorie restriction does wonders for worms, flies, yeast and mice, in fact there’s almost no research on humans in this area.
Many of these diet book authors tout a magical metabolic shift to “burning fat” as a result of these plans. Such a shift however is the natural result of calorie restriction in general, and not the method of under-eating used to create a calorie deficit.
Your body is an energy burning machine. Carbohydrate, fat and protein (and alcohol) are fuel sources to your body, which is always burning BOTH carbohydrate and fat (and even a small amount of protein). The ratio of carb:fat oxidation varies depending on multiple factors, including: a) when your last meal was, b) what you ate (balance of carbs/fat/protein), c) have you been sitting on the couch, or did you just finish a 5K, d) whether you’re currently in energy “deficit” or “overage,” and myriad other biological-based nuances.
How does your body decide what fuel to burn? Upon ingestion it preferentially increases the oxidation of fuel(s) it has the least ability to save/store for later use. The first two of those fuels are alcohol and protein. Next is carbohydrate; although you are able to store some carbohydrate (as glycogen) in your muscles and liver, your body still increases oxidation of carbohydrate as it becomes available.
So in the end, upon ingestion/digestion, your body preferentially burns fuel in this order: Alcohol>Protein>Carbohydrate>Fat
Note that we are also burning a small smount of protein (relative to CHO and FAT) all the time. The protein which is being oxidized is believed to be the quantity available--over and above--that currently needed for other physiological requirements. The greater the percentage of energy from protein (or total protein) we ingest, the more protein gets oxidized this way.[viii]
Remember however, that you’re also always burning a ratio of carb:fat. Fat oxidation simply reduces when the oxidation of other fuels increases. So by default your highest level of fat oxidation occurs on an empty stomach (when you’re relaxed on the couch or sleeping) after the other fuels have been cleared.
Ultimately the only way to reduce your fat stores (excess body fat) is to sustain a long-term energy deficit which forces your body to continue to draw on stored fat for energy. It doesn’t matter what type of calorie-restriction scheme is employed to get there (at least not from an energy metabolism perspective; overall health is another important conversation). [For a more thorough discussion of human energy metabolism see: When is a Calorie Not a Calorie?]
Another claim the authors of these regimes make is that their methods prevent your metabolic rate from having the usual (normal) drop associated with sustained energy restriction. The notion they present is that by alternating calories on different days you trick/confuse your body so that it doesn’t notice the ongoing/overall calorie-restriction. It’s a great claim, but there’s NO data supporting it. In fact the only studies in the medical literature that have actually looked at rotating calories between various levels found neither an enhanced weight loss effect, or any beneficial effect on metabolic rate.[i],[ii],[iii]
The suppression in metabolic rate related to calorie restriction is strongly tied to the magnitude of (sustained) calorie restriction. Inexplicably, suppression of metabolic rate doesn’t happen to everybody.[iii] So these studies [i, iii] finding no difference (in RMR changes) between groups eating the same total weekly calories, but one group alternating between high and low days, suggests that it’s harder than we think to outsmart Mother Nature!
What should be obvious is that the real challenge people have with diets is that they’re hard to follow for very long! Any (reduced calorie) diet will lead to weight loss IF you stick to it. The question we should be asking is, “are these intermittent/alternate day fasting regimes easier to follow than other diets?” I don’t find any evidence, or hypothetical reason, to suggest that that might be the case.
Of six published human trials on intermittent fasting and alternating calorie diets in the medical literature only one tracked hunger[ii], three tracked resting metabolic rate (RMR)[i, ii, iii], and two tracked fuel oxidation[i, ii]. A couple studies only looked at markers of cardiovascular health (in addition to weight loss).[iv],[v],[vi]
In the trial that tracked hunger [ii], the authors wrote: “Hunger increased on the first day of fasting and remained elevated” (for the 3 week study). This study had two “fasting” days a week, and 5 normal eating days (no overeating). That arrangement set participants up for a weekly weight loss of a bit more than a pound, which overeating days would quickly have eroded. I think it’s fair to say that if you’re hungry ALL the time, the diet won’t be easy to stick to.
In the same study, “RQ did not change significantly from baseline to day 21.” RQ (respiratory quotient) measures the balance of carb:fat oxidation. Fat oxidation in the study did finally increase on day 22 by 15g/d. As I said earlier however, that shift is the natural result of calorie restriction in general, and not the method of under-eating used to create a calorie deficit. In the study by Hill (which included a comparison group), there was no difference in weight or body fat losses between the constant or intermittent calorie-restriction groups.
Ignoring the elephant in the room?
Despite lots of misinformation and mythology to the contrary weight loss comes down to “calories-in/calories-out” as noted in the first sentence of the recent position paper from the American Society for Nutrition, “Human physiology complies with the first law of thermodynamics…”[vii]
Nevertheless, it's also true that there is always somebody that does well on a particular diet. That’s what leads to the well known “results not typical” advertising disclaimer required by the FTC on the inspiring testimonials that are the backbone of weight loss advertising.
Things worth thinking about:
One of the concerns professionals voice about some of these regimes is that they may actually encourage binging and/or disordered eating by over selling the notion of overeating days.
Another concern is that some of these regimes seem to encourage an unhealthy (anything goes) diet on non-fasting days. It should be obvious that the quality of what you eat becomes MORE (not less) important if/when you’re consuming less total calories. Furthermore, such eating would probably obliterate any calorie deficit created on fasting days. That might explain why I saw more than a few (book) reviewers complaining of no weight loss after several weeks.
Having worked with patients in medically-based fasting programs I know that many people initially do very well. Compliance to any diet however is generally short-lived (because our environment and busy lives don't naturally support restricted eating plans). Non-compliance (cheating) usually starts early and continues to escalate until people throw in the towel. Unfortunately they usually blame themselves instead of the silly diet they were trying to follow.
The Bottom Line: If you want success with your weight and health the tried and true approach is to learn how to eat in a healthy way and in healthy amounts. My best advise is to start where you are at, and look for "easy changes" to improve the quality of your diet. Forget "going-on-a-diet," instead approach it as a lifestyle change. It would also help a lot to understand more about how your body works. Here are some links that can help you get started:
All the Best!
-Dorene
PS--if you like this blog you may also like my book: The NEW Healthy Eating & Weight Management Guide.
[i] de Groot LC, et al. Adaptation of energy metabolism of overweight women to alternating and continuous low-energy intake. Am J Clin Nutr 1989;50(6):1314-23.
[ii] Heilbronn LK, et al. Alternate-day fasting in nonobese subjects: effects on body weight, body composition and energy metabolism. Am J Clin Nutr 2005:81:69-73.
[iii] Hill JO, et al. Evaluation of an alternating-calorie diet with and without exercise in the treatment of obesity. Am J Clin Nutr 1989;50:248-254.
[iv] Varady KA, et al. Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. Am J Clin Nutr 2009;90:1138-1143.
[v] Klempel MC, et al. Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutriton Journal 2012;11:98.
[vi] 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(5):714-727.
[vii] Hall KD, et al. Energy balance and its components: implications for body weight regulation. Am J Clin Nutr 2012;95:989-94.
[viii] Stubbs JR, et al. Carbohydrates and Energy Balance. Annals New York Academy of Sciences. 1997;819:44-69.
Related blog added 11-APR-13 - Diet Book Review: The Fast Diet