March 28, 2016

Running without carbs – a week trialling a ketogenic diet

Almonds nuts ketogenic keto low carb

After spending some a lot of time trying to understand the role food plays in running a marathon, this post is about my attempt train for a week whilst on a ketogenic (super low carbohydrate) diet. For an explanation of what ‘ketogenic’ actually means & why I’d do this to myself it’s worth checking out my previous post “Nutrition for a marathon – what should you eat?”, but here’s a summary of the most relevant part:

  • During moderate to high intensity exercise, carbohydrate is the body’s preferred fuel source.
  • There’s a limit to how many calories worth of carbohydrates you can store, and so for endurance events like the marathon it can be difficult to take on enough in order to avoid running out.
  • The other main energy source – fats – are almost inexhaustible, so making your body more efficient at using them during exercise may help delay the point at which fatigue sets in.

This theory was enough to make me curious about the application and effects of low carbohydrate, high fat (LCHF) diets. Could an approach almost completely opposite to the ‘carbo-loading’ norm actually help you run further, faster?   

Fatty fatty run run!

The assumption behind following a carbohydrate restricted diet is that it helps the body become more effective at utilising fat as a fuel source. In short, eating yourself to metabolic flexibility. Whether this actually translates to improved endurance ability is another debate, but for now let’s just say there isn’t conclusive proof it doesn’t work, so I thought it was worth a closer look.

To be clear from the outset, here’s what the week wasn’t about…

I didn’t take any baseline data.

I didn’t measure any effects (other than how I felt during the week).

7 days isn’t long enough to become properly fat adapted.

I wasn’t expecting it to result in any meaningful physiological changes.

I’m not going to pretend the possibility of even marginal gains wasn’t a factor in deciding to give it a go (as few as 5 days could offer some benefit – again, details in the nutrition post). Testing whether I could use diet to boost fat metabolism & endurance certainly would have been fascinating (no… just me?), but planning and executing it properly would have required time and equipment I didn’t have.

So why bother?

As much I’m a fan of small scale experiments, my interest was in two much more basic questions:

  1. What is it like to follow a ketogenic diet?
  2. Is it possible to train without carbohydrates?

This trial week was an attempt to answer these questions. Here’s how I got on…

The LCHF/ketogenic week

From a diet perspective the aim was simple – keep carbohydrate intake below 25g per day (depleting glycogen and forcing the body to increase fat metabolism to meet energy demands). Exercise wasn’t quite so clear, but the target was to get through my scheduled training unchanged.

Very few carbs - lots of greens.

Very few carbs – lots of greens.


Want to see a full breakdown of what I ate? Here’s the (ugly but simple) spreadsheet I used to plan & record the week. You can also check out my Strava feed for a summary of the matching training runs (7th – 13th March).

Day 1: Super prep

After a LOT of food prep on the Sunday, the week started pretty well. Thanks to some generous portions meals were pretty satisfying, and I didn’t feel any major adverse effects given the sudden reduction of carbs. What became obvious very quickly was that snacks would be the real challenge. Having ploughed through my daily nut quota before lunch, a slightly panicked search for replacements to the standard biscuits/cereal bars/fruit began. Salvation came dried and fried, in the form of beef jerky and pork scratchings. Snack cravings solved, but I did fancy a pint. And a game of darts.

Day 2: Hello exercise

The first 24 hours had been a test of will power, but physically not too difficult. So far though there hadn’t been any meaningful exercise, and after a lethargic start the day I was conscious of the 600m repeats scheduled for the evening. This is a pretty taxing workout at the best of times, a view which was reinforced when my hands started to tingle/go numb near the end of the last repeat (they weren’t cold). Devoured dinner, and would have headed straight to bed were it not for the need to prepare food for the next day.

Day 3: The longest day

Really feeling the effects by this stage. Whether or not I ever actually entered ketosis, it definitely seemed as though glycogen reserves were near empty. The combination of a less than ideal lunch and being on my feet most of the afternoon left me pretty exhausted by the end of the day. Walking home from the station was a genuine challenge so I was concerned about my ability to play football a couple of hours later! Admittedly not my greatest performance (sorry OMFC boys), but by no means a disaster & I actually felt better after the game.

Day 4: The missed workout

Due to plans to meet up with friends after work, I’d intended to shift this day’s workout to first thing in the morning. As soon as my alarm went off however, I knew it wasn’t happening. Whether it was down to a lack of carbs, two days worth of exercise, or not enough sleep, I needed more time in bed. This turned out to be a pretty good decision as I felt better as the day went on. Made sure I took on plenty of food before heading to the pub to meet friends after work. This didn’t stop me looking enviously at the burgers & chips they ordered, but discovering I’m partial to a (zero carb) whisky & soda was a bonus.

Day 5: More fats please

Ended the working week tired but feeling better & with more energy than days 3/4. Went for a run in the evening to compensate for the previous days missed session; hard work, but pretty close to my normal pace. Having expected to be getting sick of rich food by now, a cheese & cream laden dinner was still very welcome. Some more fruit wouldn’t have gone a miss though.

Day 6: Avoiding the ‘douche’ trap

A light strength & mobility workout in the morning then off to the pub to watch the rugby! Aside from a nervous moment scanning a Thai menu for low carb options, managed a day out participating in the real world whilst sticking to the diet. This hasn’t stopped me becoming boring though – a concerted effort was required to avoid bringing running or food into every conversation. Oh dear.

Day 7: The long run

Here was the real test. Under 25g per day of carbs for 6 days, culminating in an attempt to cover some decent mileage. Due to the previous night being unnecessarily late and boozy, I ended up sleeping in longer than planned. Felt surprisingly fresh when I did get up however, so fuelled with fat and set off for Richmond park. At around 11 miles it was my longest training run so far & with a decent amount of gradient. Aside from on the steepest hills the pace was pretty comfortable (under target marathon time), but by the last couple of miles my legs were starting to feel heavy. No doubt this was because I was lacking both strength and energy, but difficult to tell which was a bigger factor. Either way I was pleased & relieved to have got through it. Celebrated with a double lunch; scrambled eggs & smoked salmon followed shortly by fish & chips when I realised carbs were back on the menu!

“Was it really bad? It was horrific wasn’t it…”

The first thing to note is that overall my energy levels were actually more even than usual… just evenly low! On day 3 in particular, I was mentally and physically just about functioning. The feeling is hard to explain – similar to the ‘low blood sugar’ lull you get if you haven’t eaten in a while, although without ever feeling faint or particularly hungry. This brings up an important point…

This was in NO way a calorie restricted diet.

Ketogenic frittata keto rocket eggs

One of my lighter meals – is there any food you can’t improve with cheese?

The only thing I monitored closely was carbohydrate intake, but suspect the daily calorie intake was equivalent to – if not higher than – what I typically average. Gains in fitness happen during the recovery in between exercise, so if you’re training hard on low calorie diet you’re not doing yourself (or your body) any favours.

So no lack of food overall, but it was certainly restrictive in other ways and as expected left me feeling pretty crappy at times. Towards the end of the week however (and when I wasn’t jeopardising things with late nights and alcohol), it did start to become more manageable. This seems to match with reports from people who stick with the diet for longer, that energy levels eventually return to normal. There’s no doubt the process/transition is difficult, but if you can achieve more stable energy levels without the overall fatigue I can understand why you might want to persevere with it.

Thoughts & observations

Looking back at the week here are the other things that stand out:

  • Carbohydrates are awesome, but I eat a lot of simple carbs out of habit & ease, rather than really needing them.
  • Snacks are the killer. Most convenient snacks are dense in simple carbohydrates and you have to think/plan very carefully if looking for alternatives.
  • I really craved fruit. Probably more so than biscuits, cakes, pasta etc. which was not expected! Whilst it’s probably sensible to go easy on some fruits due to their high sugar content, I can’t help but think that the variety and density of other nutrients you get from having them in your diet is likely to be a net positive. This is my biggest issue with maintaining an extremely low carb diet long term.
  • Low carb meals are just as easy to cook as any other food, and can be just as tasty. Whilst I definitely craved starchy or sugary snacks, I didn’t miss/want for much when it came to actual meal times.
  • I need to get more sleep.
  • I should probably cut back on alcohol!
A handful of fruit - about your lot for the day on a ketogenic diet.

A handful of fruit – about your lot for the day on a ketogenic diet.

Time for answers

So where does that leave things? Here’s the answers to my two main questions:

What’s it like to follow a ketogenic diet?

Not nearly as difficult as I thought. It required a lot of preparation and some significant willpower, but in retrospect I feel like this was mainly because the change, rather than the diet itself is difficult. If you’ve been eating a certain way for the majority of your life, any significant switch is going to be a shock – at least mentally if not physically. It supposedly takes between 10-12 weeks to become fully fat adapted. Right now I can only guess whether I’d find the diet sustainable over that length of time, but based on a brief experience it seems more viable than I’d first suspected.

Is it possible to train without carbs?

Absolutely. Is it advisable? Probably not… well, not until your body has had time to fully adapt to the change in diet. The interesting part is that whilst it made exercise harder work, I don’t think I lost a huge amount of capability (even at higher intensities). My experience suggests that you can train and run on low carb intake, but probably not very effectively unless you’re already used to it.

5 takeaways, tactics and tips

  • The really valuable part of the long run on day 7 was proving to myself that it was possible to keep going in a glycogen depleted state. The universal advice to marathon runners seems to be “don’t do anything on the day you haven’t tried before”. As I won’t be going close to race distance in training, it seems this was a pretty good way to give me an indication of what it might be like.
  • It’s worth challenging your view of what makes up a ‘complete’ meal. Try some different things. Pick up ingredients you don’t usually buy (especially if they don’t come pre-packaged). Experiment with different ratios of fat, protein and carbohydrate to see how it makes you feel.
  • Fat, in isolation, does not make you fat. Low or zero fat are not bywords for healthy (whatever that means) and foods labelled that way aren’t by default the better choice. Where fat can have a negative impact is if eaten in high quantities alongside or shortly before/after simple carbohydrates. Think of the fatty foods that you’d consider the most unhealthy. I bet you a tub of ice cream they’re also high in sugar/carbohydrates (see what I did there).
  • If you want to successfully make a change to your diet, preparing (or even just planning) food in advance is a big force multiplier. I’m all for spontaneity, but leaving the choice of what to eat until you’re hungry seems to correlate pretty closely with picking up foods that fall under the ‘guilty’ category. Sure, it’s satisfying at the time, but deep down you know it’s filth & will likely leave you with a nagging sense of regret.
  • One small tweak that I found made a big difference was to make sure I always had alternatives to sugar heavy snacks. Will it stop me eating biscuits etc. altogether in the future? Don’t be silly. What it does provide however, is the ability to choose between when I actually want something like that, and when I just need something to satisfy general hunger. As a result, the overall quality of food I eat of the course of a day is likely to go up, which I’d consider a win.

Apologies for the fat rant by the way. A bit off topic from marathon running & I try to avoid sweeping statements, but this one annoys me so I’m going for it. Feel free to call me out on it if you think I’m wrong (and can explain why).

Theory into practice – what will my diet look like going forward?

Nutrition is clearly an important component of being able to run a marathon, so any improvements in diet are likely to make a difference in training, recovery, and ultimate performance.

Despite the short time frame, the lessons taken from trying a ketogenic diet will certainly inform how and what I eat going forward. I believe that LCHF diets offer some interesting (and under-researched) benefits for both athletic performance and general health, so whilst it’s unlikely I’ll ever adopt on a full time basis, plan on setting aside time in the future to continue experimenting.

With less than a month to go however (!), I can’t be confident that drastically changing what I eat would have a meaningful effect on performance (or that if it did it would be a positive one). As a result, I’m going to follow some of my own advice; focus on quality over quantity. In this case that means not worrying so much about macro-nutrients ratios. A low carb diet may still play a role in my preparation, but probably only at the start of my taper week, before re-introducing complex carbohydrates in the last few days before the race.

Talking of complex carbohydrates, some UCAN Superstarch arrived recently… more on this another time!



This post was in danger of being a sensible length, so thought I’d throw out a prediction related to my trial week on a low carbohydrate diet:

Synthetic/exogenous ketones will be the next big product in the sports supplement market.

Only recently launched as commercial products (early versions were reportedly tricky to keep down) they’re still pretty pricey. Despite this, the range of potential benefits means I can certainly see the value in using them. Why am I so convinced? Have a listen to two Tim Ferris interviews with Dom D’Agostino & Patrick Arnold and make your own mind up.

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