Quick Hits: Consuming carbs won’t cause you to bonk

Posted by Dave on July 7, 2011 | 8 Comments

Two separate individuals have pointed me to a new site, The Natural Running Center, which claims to be “a comprehensive web resource of information and education for all runners.” The site was founded by minimalist running gurus (advocates of running barefoot or with very reduced footwear), some of whom also happen to operate stores where you can buy minimalist running gear.

I’m always a little leery of accepting advice from folks who are trying to sell you something other than advice, but I do suspect that the people running the site probably believe in what they are doing. There’s also little doubt that there is some truth in some of the general principles they advocate. That said, so far the site unfortunately doesn’t live up to its promise.

Let’s just consider one example, a post on nutrition and injuries, which claims that diet is “perhaps the most important factor” causing injuries in minimalist runners. The article, written by a chiropractor, makes literally dozens of unsupported claims. I won’t try to debunk every one of them, but instead focus in on this one:

A high-carbohydrate diet, especially refined sugars like high fructose corn syrup, white sugar and flour, and yes, even agave, will contribute to the inflammation and even increase insulin levels over time. Eventually your tissues will become resistant to the insulin and blood-glucose handling problems will result. You may bonk or under-perform in a race because of this, or have mood swings and general body aches due to the carbohydrate sensitivity.

Don’t get me wrong, high-carbohydrate diets have contributed dramatically to the obesity epidemic in the U.S. and elsewhere in the world. But if you’re a runner who trains regularly and is at a healthy weight, does a high-carb diet in and of itself cause you to “bonk or under-perform in a race”? The evidence suggests otherwise. Carbo-loading, while difficult to achieve, does elevate muscle glycogen levels and may lead to improved performance in long races.

As Matt Fitzgerald points out in his book Racing Weight, a high-carbohydrate diet — over 65 percent of total calories — has been shown to improve racing performance in extreme circumstances. While more realistic tests have come up with mixed results, never has a study found that a high-carb diet diminishes performance or leads to bonking compared to other diets. And as Fitzgerald observes, the most-accomplished Kenyan runners as a group tend to consume extremely high proportions of carbohydrates: Over 75 percent of total calories, according to one study. If the best distance runners in the world consume extremely large volumes of carbs, there is little doubt that carbs don’t cause them to bonk.

But most nutritionists agree that highly-processed carbs like corn syrup and white bread are not great diet choices; they would suggest consuming most of your carbs in the form of fruits and vegetables and whole grains.

What does this mean for you? As long as you’re not gaining unwanted weight, there is no evidence that a diet rich in high-quality carbs like fruits and vegetables will impair your performance in races. Indeed, even some “low-quality” carbs like energy gels and drinks are probably fine as long as you’re burning those calories off in workouts.

And, most importantly, when reading nutrition advice, make sure your sources cite scientific data to back up their claims.

Fitzgerald, Matt (2009). Racing Weight. Boulder, CO: Velo Press.


8 Responses to “Quick Hits: Consuming carbs won’t cause you to bonk”

  1. Jeff
    July 7th, 2011 @ 4:51 pm

    I’m glad you’ve pointed out this site. I stumbled upon it the other day and and immediately skeptical of anything that uses natural as a value judgement. It’s such a loaded term here and just propagates the “natural always equals better” fallacy. Not to say there isn’t some good content here, that content was framed differently and put under a more skeptical lens.

  2. Dan O'Neil
    July 7th, 2011 @ 5:59 pm

    I’m pretty sure that your UK readers will find this hilarious… Presumably bonk means something different to what it means over here… Sounds like it’s nothing like as fun either!

    On a serious note, I totally agree with you that you need to check out the science and evidence behind all nutrition advice… There’s so much garbage out there.

  3. Dave
    July 7th, 2011 @ 6:07 pm

    Dan: It means that here too — though apparently the usage is less common! Hopefully after you had your laugh you figured out what I was talking about…

    Jeff: I agree. “Natural” doesn’t necessarily equal good, even though it can be a pretty good proxy in a lot of instances: For example if I had to choose between a “natural” and an “artificial” diet (based on the common understanding of those terms), I’d definitely go “natural.”

    But on the other hand sometimes “natural” is thought of as “avoid modern medicine,” and then I’d say no thank you.

    Too often, “natural” versus “artificial” is simply not a useful distinction to make.

  4. Jeff
    July 8th, 2011 @ 2:46 pm

    I’d have to disagree on diet, because I don’t believe there is a common understanding of those terms, at least among the general population. An example would be GM food, which a large portion of people who consider “artificial” and not eat it (even though they probably do anyway without knowing it). They believe this despite substantial evidence that GM food is completely safe and that it is an essential scientific development for our growing world population.

  5. Chris
    July 8th, 2011 @ 7:19 pm

    In the US, it is boink as opposed to bonk as a euphemism for shagging.

  6. BikeMonkey
    July 11th, 2011 @ 6:26 pm

    The bonk, you perve, the bonk. Hunger knock. The great flattener.

  7. Dan
    July 13th, 2011 @ 10:56 am

    Just started reading this blog and I love it. However this post seemed a bit off the mark strange. While I agree that the quote you’ve debunked is a confused mishmash of information and fabrication I’m not very clear why a lot of your rebuttal is relevant to most runners.

    If might be the case that a high carb diet improves performance in extreme circumstances. But what are extreme circumstances? I’d guess that most people’s running (training) doesn’t qualify as extreme so it’s not clear to me why this stat would be relevant.

    And it may well be the case that the most-accomplished Kenyan runners consume a diet of 75% carbs. But again it strikes me that most runners are not in that vanishingly small elite group.

    I do think both facts are interesting but I’m left wondering how relevant they are to the majority of runners.

  8. Dave
    July 13th, 2011 @ 2:19 pm


    The main point I’m making here is that according to Fitzgerald, no study has every shown a high-carb diet to impair performance.

    The extreme evidence, in fact, shows the opposite: high-carb diets are associated with improved performance. So it certainly seems implausible that a (less extreme) high-carb diet can harm your performance as long as it doesn’t cause you to gain weight.

    And arguably, in that case, the problem isn’t so much the ratio of carbs but the overall caloric content of your diet.