Do You Need Carbs To Run Fast? Concluding The Experiment, Sort Of

Common wisdom says you need copious amounts of carbohydrate to run your best, but who doesn’t love challenging common wisdom once in a while? After reading the Art & Science  of Low Carbohydrate Performance by Volek and Phinney last year, and following the work of Dr. Timothy Noakes, who was kind enough to give me some advice during the process, and other low carb athletics advocates, as well as no-sugar-no-grain advocate, trainer, author and gifted podcast raconteur Vinnie Tortorich, I put it to the test. I started eating high-fat-low-carb Nov. 2, 2013 and stayed either in ketosis or just on the edge of it for the next nine months. (*Reasons, expanded on?)


On no running, or lower weekly mileage (say under 30 miles), I am really happy eating extremely low carb – staying in ketosis even. My brain feels sharp and my sleep is rock solid, a real boon given a lifetime of nagging insomnia. I don’t feel hungry all the time the way I did on a high-carb-low-fat diet, and I can keep weight off  even if I’m not running or exercising at all. It’s a fantastic lifestyle to combat the many negative effects of the ridiculously high-sugar, processed food culture we live in (and which is at the heart of the obesity issues Americans are facing).

But – and this is the big but – I simply couldn’t hit the performance levels I’d been achieving prior to going low carb. This played itself out first in my prep for Boston this spring. I’d been rehabbing an injury, so didn’t run from Nov. 2013 through Jan. 2014, and when I finally started again, I really had a short cycle to back into shape for Boston.

I blamed the too-short training cycle and the two months with no running for my inability to get back into three-hour marathon shape, but likely  it was those two factors combined with the ultra-low-carb diet. Starting in March, I experimented with adding strategic carbs before and after especially tough workouts, and carb reload days, but I was still dragging, and felt like I was stressing my system as well.

Further, most disturbing, I really wasn’t enjoying running, especially long runs, which had been my escape, my mental health time, and often, deeply spiritual. Now I was just grinding through them.

I ran Boston more to participate in the event than to race as hard as I could, and ran a very enjoyable 3:12, which didn’t leave me too beat up. Even so, I took time off after, contemplated what to run next, and decided, nothing. I’d put nothing on the calendar and just run short stuff. I ran one 10K in June, and bailed on the idea of racing it after only a few miles, dropping back to run it with my wife instead. I just didn’t have the juice in my legs to do what I wanted to do – what I’d been able to do just one year before. It was as if, after only two fast miles, the glycogen stores were completely empty. A bonk. Yet not a bonk, since at the slower pace, I could have run all day. It was a top-end-speed bonk only. Frustrating.

Fat Adapted? Yes, But Not Enough For Tempo Running, Racing

They say adapting to low-carb is a long process involving significant physiological changes. I think I made quite a bit of progress – I managed to get back into the 40-mile-a-week range, felt pretty good on short speed work staying very low carb, but still couldn’t manage the long tempo runs. Even just running marathon pace (6:49) felt like a struggle after a few miles, even though I could feel other markers improving, including capability in longer interval workouts.

Despite having said I wasn’t going to run a long race in the fall, I started training in earnest in July and signed up for Manchester a month later. I missed the discipline of the training calendar. And I was seeing improvements, yes, but still, where was that tempo endurance? And why were the long runs still so dang plodding?

Like Flipping A Switch

So a few weeks ago I finally acknowledged that if this was to really be an experiment, I needed to test the other side of the coin and try training higher carb again.

Could I have have gotten even more fat adapted if I’d given it a year, or two years? Perhaps so. Likely so. But I’m in my mid-40s, and I don’t have that many years left to try to improve my marathon times. Or just to enjoy the cruising sensation of a long run, fully glycogen loaded.

Still, I didn’t want to give up all of the benefits I’d been getting  in other areas of my life from low-carb eating. I figured maybe increasing carbs but sticking in general to lower glycemic foods like beans, squash, etc., might keep my blood sugar more steady and maintain some of the happy aspects of low carb living, while allowing me to repack muscles with glycogen consistently throughout the week, not just in bursts.

It was like flipping a switch. My pace on longer runs dropped (9-18 miles) between 15-30 seconds a minute per mile or more, on any given day, with no additional perceived effort. I allowed myself one or two Hammer gels (carbs but very little sugar) on runs over sixteen miles. I increased my mileage back into the mid 50s and felt as strong at the end of the week as I did at the start. And finally, near the end of a 60 mile week this week, I ran a treadmill 9 miler with 5 tempo miles in the middle. Now I mentioned I’d been having to work very hard with a 6-mile marathon pace (6:49) run, even on the treadmill. After a three-mile warmup, the next five tempo miles flew by, with the first four miles at 6:35 pace and the last at 6:31. I could have gone farther or faster. Maybe both. I felt like a different runner. Or more accurately, like the runner I had been.

Now I get it, there are so many factors in play here, mileage, place in training cycle, sleep, it’s hard to control for any one thing perfectly, but the change was too abrupt and notable to be coincidence, and not just on speed days, but on about every run of the week for three weeks in row.

So what now?

So now I stick to the good, clean, whole foods mostly no-sugar-no-grain eating (excerpt for the grain in a good IPA now and again), but with many more carbs in the mix. I’ll make sure I’m still getting plenty of healthy fat. And instead of counting carbs or calories, I’ll keep an eyeball on the glycemic load of the foods I’m eating, though not too close of an eye. As long as the performance continues to improve, and the gnawing hunger or poor sleep doesn’t return, I’ll figure I’ve found a happy medium. Between race cycles, or when my knee finally fails or some other injury sidelines me for good, and I’m no longer burning 7,600 calories a week (this week’s output), I’ll switch back to very strict low carb living and enjoy that.

I continue to both advocate a low-carb diet for the many modern humans struggling with weight and metabolic disorders, as well as for any athlete who’s having luck with it and will benefit more from weight loss and appetite control they might by accessing their top end speed, whatever that might be. And if you don’t go completely low-carb, at least play around with no sugar no grains and see how you feel. I think Vinnie’s onto something easy to follow with that simple, straightforward formula.

In the meantime, someone pass the sweet potatoes.  Today was long run day.


* Note my full reasoning for going low-carb isn’t necessarily clear from the intro above. Those who follow the blog know there are two primary drivers – a lifelong struggle with weight, hunger and adverse reaction to sugar when not running a high-mileage training schedule, as well as a desire to see if increased fat adaptation (burning fat in a larger proportion during exercise) would improve my abilities in marathon-distance running.  

Follow me!