Home Blog Intermittent Fasting: New Diet Solution or Passing Fad?

Intermittent Fasting: New Diet Solution or Passing Fad?

Written on October 8, 2008 at 8:00 am, by Eric Cressey

This guest blog comes from Brian St. Pierre, a Cressey Performance staff member who specializes in nutrition. Keep an eye out for more great things from Brian; he’s got a lot of excellent ideas and is very well-read.

Intermittent Fasting (IF) seems to be newest dietary fad. IF followers will choose a day or two each week, and on those days, they simply won’t eat for 24 hours. It seems simple, and a lot of people like the idea of giving your body and digestive system a break.

It also stands to reason that eating this way will help you eat less calories and make it easier to maintain a healthy body weight. Seems logical, right? Since IF usually leads to decreased calorie consumption it is often compared to Caloric Restriction (CR).

CR has been shown in a plethora of animal studies to lead to significant improvements in many markers of health and an increase in life span. Extrapolating that data, it would seem that if humans cut their calorie intake by 30-40% we would lead longer, healthier lives. Here arises the problem. In the animal studies with rats, researches observed signs of depression and irritability. With primates, it was even worse (researchers quantified the amount of monkey poo thrown for statistical analyses…okay, not really, but it’s an amusing concept for future reference). If they did not consume enough cholesterol, they became violent.

So, angry monkeys aside, if you are willing to be cold, irritable, and prone to depression and flashes of violence, then caloric restriction is for you!

This is supposedly where IF comes in. More animal studies have shown that when animals were forced to fast every other day, but consume all they wanted on the off days, they were able to maintain their body weight, while also sustaining all the life prolonging benefits of CR. They had better glucose tolerance, insulin sensitivity and blood pressure to name a few. It was the best of both worlds!

After these initial animal studies, human researches jumped on the IF bandwagon. People were expecting big things. Unfortunately, they were very disappointed with the results. Research started showing that people following IF, or even purposefully skipping a few meals per day were developing insulin resistance, decreased glucose tolerance, and increased blood pressure. These problems were not tremendous problems and some might argue that in real world IF, where people don’t fast every other day (only once or twice a week), the studied health problems wouldn’t occur.

Even if this is so, there is another problem with which we have to contend. The largest problem is a decrease in thermogenesis. Essentially saying that these people, even though they purposefully consumed as many total calories on IF as the control group, had drastically suppressed their metabolisms. This is why so many people have found such little real world fat loss from IF. In most real world applications – especially because people were eating diets in a significant caloric deficit – the body downregulated its thermogenesis to such an immense degree as to not allow for almost any weight loss. This to me is the final blow.

So, I guess if fasting is your thing, and it’s not causing you any negative health effects, then have it. As for me, I prefer to stick to my low carb diet, eat whenever I feel like it, and receive all the benefits that IF proposes to give – except mine are real, not imaginary or hypothetical.

Note from EC: You can find a rebuttal to this post HERE.

10 Responses to “Intermittent Fasting: New Diet Solution or Passing Fad?”

  1. Moz Says:

    Any reference studies available?
    I am especially interested in support for metabolic slowdown. This is contrary to the information I have seen thus far…


  2. AT22 Says:

    I second moz’s question – studies have shown that affecting thermogenesis takes much more than our little bits of tweaking.

    Of course, I’ve tried IF and I don’t think it works for me. But I am trying again – today, in fact, which makes the post seem like some kind of omen. All I can say is, right now, 14 hours in, I’m starving.

  3. Ruslan Kedik Says:

    I know that in the past in Easter European block countries many ‘Functional Medicine’ practitioners where very successful with fasting and treating very serious medical conditions.

    Valeriu Popa (1924-1997) was one of the biggest names in Easter Europe who often spoke of fasting…and had a proven track record of ‘healing’ the ill.

  4. Patrick D. Says:

    I’ve been doing Intermittent Fadding for over a year straight now, to great success. If nothing else, you can’t beat the convenience of it for college life.

  5. Gregg Says:

    I fast for 2-3 hours in between meals, so far it has worked for me.

    In all seriousness, some interesting points made in the article and I hope to hear more from Brian in the future regarding his “low carb” dieting among other things.

  6. Martin Berkhan Says:

    Wrong on oh so many many accounts. Nice job on ignoring the shown and proven benefits of IF in humans, though. Show me the studies where IF lead to a decrease in metabolism. You can’t, and in fact the opposite reaction occurs.

  7. Gerkin Says:

    This Cressey guy has a lot of fasting fans, hahaha. Didn’t see that coming.

  8. Omri Says:

    Great post Brian/Eric

    As for the critics: Learn to read studies, not just title. IF studies in rats with positive results (increased life span, etc) were done with a consistent 1 day on, 1 day off diet. However, the total amount of food consumed was still the same.

    That’s easy for rats who will eat anything you put in front of them. Human beings, after fasting for 24 hours, are unlikely to be able to consume 5,000 calories the next day. They would constantly feel sick and nauseated.

    In trained athletes, 12,000 calories a day would be impossible to consume.

    Oh, and in males, IF also caused a downregulation in genes controlling glycolysis and mitochondrial ATP production.

    PS: mouse models for dietetics suck

  9. oyaji poi Says:

    I think the main take home message about IF versus frequent eating is that the metabolism is not grievously affected. All the other “benefits” are just not proven well enough, or supported by human studies to really consider as solid evidence. That goes for both sides.

    Eating more often will not increase your metabolism, per se, nor will eating 1-2 meals cause your metabolism to decrease, it’s just not that simple.

    Omri, do you have the studies that show IF causes a downregulation in genes controlling glycolysis in ATP production in humans? I’d like to see that.

    Regarding consuming massive amounts of calories, you get used to it. Just like when I did 6 meals a day it was tough to get used to, after a few weeks it was fine. Same with taking in large amounts of calories.

    Who, besides Phelps, takes in 12K in calories a day? Let’s be realistic and not go to extremes, I agree that 12K in 1-2 meals would be terribly difficult to pull off.

    You don’t *have* to do IF, but you do *have* to not attribute false and incorrect assumptions about it, just like proponents should not make grand claims about it.

    I don’t consider the “mysterious” benefits of IF and CR when defending IF as an eating habit, the evidence just isn’t solid enough. And when you consider that metabolism is not negatively affected by eating less than 5-6 meals a day, it comes down to convenience. For me, there are no other benefits other than simplicity, and there are no drawbacks to IF.

    I eat the same types of food as I did before, only not at the same time. The results are the exact same, only I have more time during the day. Oh, and I have to put up with guff from people saying I’m killing myself by not eating every 2.5 hours.

  10. Rikard Dahl Says:

    When is the references to the studies coming up? This wasn’t very convincing 🙂

  • Avoid the most common deadlifting mistakes
  • 9 - minute instructional video
  • 3 part follow up series