Can two days of changed diet significantly affect my running capabilities?

May 16, 2012 7:14 AM

Can two days of changed diet significantly affect my running capabilities?

I'm a regular runner, 3x per week around 4 miles. My usual pace these days is 8:45 - 9:00 min/mile. Prior to Tuesday of this week, my diet was decent but nothing special: bagel with light cream cheese for breakfast, sandwich for lunch, various stuff for dinner. Some peanuts and Rice Krispies treats (damn the free snacks at work!) in between.

I've run faster in the past, but I've always attributed that primarily to age and weight, not diet (though when I've lost weight in the past, I'd been eating the diet below, so those two go hand-in-hand).

On Monday of this week, I decided to go back on my informal low-carb diet. I've started eating primarily eggs, lean meats, fresh fruits, peanuts, with a couple of sweets in between (I stayed out of the packaged snacks, but damn my coworkers who bring in homemade sweets!). Other than those small amounts of sweets (once per day), I've eaten virtually no other carbs in the form of breads or sweets.

Last night, I went for a run as usual. I use a GPS app on my phone that announces my time, distance and pace every 1/2 mile. When the first announcement came, I was shocked to discover I was running a 7:53 pace! By the end of the run, that had settled down and averaged 8:15/mile pace.

Could two days on this new diet really have improved my running by 30 to 45 seconds/mile?

posted by tippiedog to health & fitness (14 answers total)

I would suspect it's just a lucky coincidence. You aren't really low carb enough to be getting your energy from fat (and you'll probably want to add more fat to your diet, lean meat isn't great on a low carb diet as you'll be most of your energy from the fat though everyone's bodies are different so if low fat is working for you ignore what I said). If I had to pick anything I'd tie it up to all the sugar from the snacks and fruits. IANAD. IANYD.
posted by wwax at 7:33 AM on May 16, 2012 [2 favorites]

Another possibility: Phone GPS isn't the most accurate thing in the world. I use an app (Runkeeper) that does something similar to what you describe to measure my pace when walking, and when walking the exact same route that I do every single night, I've found that the distance it measures can vary by as much as 5% - a distance error that could possibly explain your time difference if it's measuring on the low side of that variance.
posted by deadmessenger at 8:03 AM on May 16, 2012

Virtually every study I've seen says that low carbohydrate diets are problematic for running because they decrease your glycogen stores. On a shorter run I wouldn't expect it to have a lot of effect, since you are not going to deplete your glycogen stores in a 30 minute run. On longer runs, I would expect you to tire more easily. I've also seen articles that suggest that low carb diets may be associated with higher injury risk. Apparently, there is not much known about the effect of diet on shorter periods of exercise. (The Role of Carbohydrates in Exercise and Physical Performance)

I also doubt that just two days would be long enough to form any kind of conclusion about the effect of diet on your performance. I do find that my pace can vary by a minute a mile or so depending on how good I feel on any particular day. Get back with us when you have a couple of weeks data. For instance, every time I buy new shoes I feel wildly enthusiastic that they make me much faster, but it stops working after a few days.
posted by Lame_username at 8:05 AM on May 16, 2012

@deadmessenger - I use RunKeeper.
posted by tippiedog at 8:09 AM on May 16, 2012

Info I should add in case it's useful:

I'm:
- male
- 48 years old
- 6' 1"
- 225 lbs

Here's my RunKeeper log: http://runkeeper.com/user/tippiedog/
posted by tippiedog at 8:16 AM on May 16, 2012

When I get to cycling medium to longer distances (60-100 miles) at a whack, what I ate in the last hour has a tremendous difference on my speed. When I forget this, it hurts, hard. So, yeah, I could totally see two days making a bit of a difference.

As others have mentioned, there's a whole bunch of stuff that can impact speed, so I'd wait to claim victory until you've got a larger set of times, but I could totally see this.
posted by straw at 8:30 AM on May 16, 2012

Thanks for the feedback. I guess I'll have to continue the diet and run a few more times to see whether this was just a coincidence.
posted by tippiedog at 8:59 AM on May 16, 2012

I would agree that the likely impact of carb restriction per se is to run slower, or have to run harder to achieve the same speed. This is offset if you lose weight, because all else being equal being lighter makes you faster. But it is unlikely that your diet changed things that significantly in such a short time. I'd suggest that you just had a good day.
posted by OmieWise at 9:15 AM on May 16, 2012

You might be interested in Peter Attia's reflections on his athletic performance over time on a low-carb (though more hard-core low carb) diet. I think basically it takes time to transition to a low-carb diet and you might in what he calls the "zone of misery" where you're not getting enough energy from the carbs you're consuming, nor are you low enough in carbs to shift completely to fat-burning.
posted by peacheater at 9:20 AM on May 16, 2012

Very low-carb can absolutely impair exercise performance in the short term. Your body is used to burning glucose for fuel, and now it is shifting to burning more fat (and using ketones, which are produced from fat, in place of glucose in some places). The adaptation period can be a week to a month, after which you should be pretty much back to your original condition for endurance exercise. You may even be more resilient to "the wall", as you are more capable of running your body on fat.

Intense exercise like sprinting and lifting requires more muscle glycogen, which is never fully replete when eating very low carb, and so will always be harder on very low carb.

Personally, I do best when eating 100-150g carbs - enough to keep me out of the keto zone and keep my muscle glycogen at a decent level, but low enough to avoid the carb crashes and hunger that I used to get when eating a lot of pasta and grains.
posted by Earl the Polliwog at 9:40 AM on May 16, 2012

I am not a runner but I do exercise and often follow a low-carb diet. On low-carb days I generally feel lighter overall (less stomach bloat, more clear-headed, etc.) and when I feel lighter overall, I find my workouts to be more effective and enjoyable.

Considering you did have some homemade sweets - and you only followed this way of eating for two days - you likely did not enter into ketosis, which for some people can cause sluggishness at first but for others (including myself) it seems to give a boost of energy. Regardless, you body may have simply welcomed the protein and break from heavy carbs and gave you a reward of a great workout.
posted by click at 10:24 AM on May 16, 2012 [1 favorite]

@click: I'm inclined to agree with your hypothesis in this instance. I know that when I eat this diet, my mood is very steady throughout the day, I don't get sleepy in the afternoon (thus better clear-headedness, as you mention) and consequently, I feel better in general. Since I ran at the end of the day, it's quite likely I benefited from two days of feeling better in general and not overeating (food in general, not just carbs) like I was often doing before.
posted by tippiedog at 11:36 AM on May 16, 2012

My cycling is always more comfortable and faster when I have just eaten fruit directly before. I'm guessing it's the sugar and liquid that does it.

Since you say you were eating fruit on this "low carb" diet, it is actually possible you got as much or even more sugar/carbs than you usually do, especially if your usual diet isn't hugely heavy with bread and pasta, but doesn't involve much fruit.
posted by lollusc at 9:44 PM on May 16, 2012 [1 favorite]

@lollusc - I believe that neither was my diet was low enough in carbs nor was I exercising long enough to have encountered the issues some other commenters have mentioned regarding extreme low carb diets and/or high performance athletes, such as ketosis, etc.

My eating focus is not primarily low-carb; rather, it's avoiding processed foods, eating lots of fresh fruits and veggies, lower-fat meats, etc. (Probably) lower carb is more a result than a goal. Also (because of the way the body processes them), sweets and breads/pastas are the things I tend to overeat, so avoiding them helps me to lower my overall caloric intake and lose some weight.
posted by tippiedog at 8:19 AM on May 17, 2012