Carb loading folklore

Three myths debunked

I broke an athlete’s heart yesterday. He’s a new runner, less than a year in, and is at that wonderful phase of PB'ing every race. He has a 10k off-road race this weekend, for fun, but nevertheless he wants to do well.

He is also a huge foodie. Rich in pasta and sauces, from the Med so he knows how to cook properly - when we run together 90% of our conversation is about what are we having for dinner. Even if we've just eaten.

So, when I told him that he didn’t need to carb load before a 10k he was crestfallen. When I explained that eating a big breakfast was likely to give him more pain than pleasure, particularly with the off-road lumps and bumps, he was deeply fed up. He had been promised big bowls of pasta days before, seduced by large breakfasts full of delicious carbs – white ones too, with sugar. You know the type.

Search online for “race day nutrition” and you can see why. Actually, no need – I’ve done it for you:

All tips backed up by references and in line with the other "information" around carb loading. The problem is that nowhere in the search or answers did it define the distance of the race. Neither did it talk about the runner. Nor does it suggest alternatives.

Myth #1: The race

Sorry team, but carb loading is only needed for races more than two hours long. [1] So not for 5ks, 10ks, or half marathons which you will finish in 2 hours or less. For half marathons over 2 hours, marathons and ultras then yes, you'll be in carb loading territory (More on that below).

Here’s why.

Carb loading (or carbo-loading, glycogen loading, or glycogen supercompensation) is used by endurance athletes to reduce fatigue and enhance performance. (It’s also a perk of running for us carb-loving foodies). Our muscles and liver hold carbs as glycogen, i.e. our fuel.   If we don’t top up midrace – which is a whole different blog post – then even with a full fuel tank, at moderate pace a less conditioned athlete’s tank will be depleted within 1 hr 45 mins of continuous moderate exercise. That’s when fatigue starts to kick in and performance drops. You are, quite literally, running on empty.

So while we need to focus on topping up the tank mid-run, equally we need to ensure our fuel tank is full to the brim to start with. For those with delicate stomachs, who struggle to ingest enough during the race, that’s even more important. It’s far nicer to have a big bowl of pasta with your feet up in front of Netflix, than to be worrying about how many gels you can get in before being sick mid-run.

But not all athletes’ tanks are created equally. A conditioned athlete, consuming a normal diet of 45% carbohydrate can store 330g glycogen in their liver and muscles. Bumping that up to 75% can increase those reserves to 880g, i.e. over 2.5 times more. However, an untrained individual consuming a high-carb diet can only store around 490g.

So, if you’re not running far enough – or you haven’t (yet) trained your tank to be big enough – then topping up your tank won’t lead to the gains you expect. That excess fuel will simply overfill the tank.

Myth #2: The runner

The link between carb loading and performance improvement is so well regarded as fact, that I even started this blog post with it. But the truth is that this mainly applies to men.

In a time trial following 4 days of carb loading, men exhibited a 45% improvement in performance, while similarly trained women showed no significant increase . [2] Instead of drawing readily on carbs as a source of energy, women seem to utilise more fat. [3]

The menstrual cycle impacts this further. Women in their follicular phase (the phase after bleeding before ovulation, when you feel normal) have a greater reliance on carbs than those in their luteal phase (the phase just before bleeding, when you feel decidedly wonky). [4] The ability to store carbs also fluctuates, with a greater ability to store glycogen in the luteal/wonky phase. [5] Taken together, this suggests that women would benefit from carb loading in their follicular phase BUT, and it’s a big but, the same study also found:

Despite achieving an improvement in glycogen storage in the midfollicular phase, carbohydrate loading did not enhance 16-km time-trial cycling performance after 75 min of low- to moderate-intensity cycling in this group of moderately trained eumenorrheic female athletes when they consumed breakfast beforehand.

For post-menopausal women, the body's ability to process carbs reduces, increasing your glycaemic responses (i.e. blood sugar levels) for high carb meals. [6]

Of course, it could be that the women did not exercise long enough in the above study. Yet it seems for most women, doing most levels of exercise, a good breakfast and a normal diet is all that’s needed. And if you’re in your luteal phase or post-menopause, there’s less need to top up your tank because it’s already pretty full.

Myth #3: The method

Carb loading has been around for a while. The classic approach devised in the late 1960s involved high-intensity exercise 7 days before the race followed by 3 days of no exercise, and a high-carb diet. This carb depletion approach however raised the risk of injury and unbalanced athletes’ regimes. In 1981, a new approach was developed. Eating normally for 7 days but beginning an exercise taper, with the final three days increasing carbs to 70% of the diet. Subsequent research found that while glycogen stored in the liver can deplete quite quickly, muscle glycogen stores can stay full for 3-5 days post-loading. [7]

So if you are topping up your fuel tank, you don’t need to drain it on the forecourt to start with. Just top it up. But by how much? General guidance tend to be:

“Optimal carbohydrate loading can be achieved if approximately 600g of carbohydrate is consumed daily for two to three days.” [1]

“You should aim to consume 8-10kg per kg of body weight per day. As an example, an athlete weighing in at 70kg should aim to consume 700g of carbohydrate per day.” [8]

For reference – 600g of carbs equates to 24 slices of whole wheat bread. 700g is 2.5 pizzas. Even for a foodie, this is a lot to consume in the run-up to a race.

It’s not just the food itself you’ll be storing. For every gram of glycogen stored, an extra 2.7g of water will be stored too. So a well-conditioned athlete with a full fuel tank can be about 2kg heavier at the start of the race. [1]

If such a volume of carbs fills you with dread, there is an emerging alternative. Prof. Tim Noakes – author of a running bible Lore of Running – has changed his mind on carb loading. Once he considered glycogen depletion the biggest factor impacting performance, now he goes so far as to say rip that chapter out of his book. [9]

Instead, the emerging view is that protein and fat are the fuel we need, not carbs. Instead of worrying about carb loading, we should be primarily concerned with fat adaptation (certainly something relevant for women, as per #2).

Subsequent studies have indicated that a low carb high-fat (LCHF) diet might not translate into performance increases, although this depends on the runner. [10] For those with a sensitivity to carbohydrates, or just can’t stomach the idea of 24 pieces of bread, a LCHF approach might work.

So while we’ve been looking at the type of tanks, refuelling rate, and quantity of fuel – we might be looking at the wrong fuel source. Instead of petrol, some of us might be electric cars.

The takeaway

So do you need to carb load before a run? Not necessarily. For races under two hours, for women, and for those sensitive to carbs, carb loading may not give you performance gains and risk leaving you sluggish and bloated on race day. Experiment in your training runs to find out what works for you. As ever, it is a question of balance.

For carb-loving foodies though, who run because running = food, go for it. If food gives you joy, have permission to stock up. Because outside of the science, the best performance enhancer I know is happiness.

[1] MarathonTraining.com - The Science of Carbohydrate Loading (archive.org)

[2] Carbohydrate loading and metabolism during exercise in men and women - PubMed (nih.gov)

[3] Dietary energy intake across the menstrual cycle: a narrative review | Nutrition Reviews | Oxford Academic (oup.com)

[4] Menstrual cycle phase and sex influence muscle glycogen utilization and glucose turnover during moderate-intensity endurance exercise | American Journal of Physiology-Regulatory, Integrative and Comparative Physiology

[5] Carbohydrate loading and female endurance athletes: effect of menstrual-cycle phase - PubMed (nih.gov)

[6] Menopause Is Associated With Postprandial Metabolism, Metabolic Health and Lifestyle: The ZOE PREDICT Study by Kate Bermingham, Inbar Linenberg, Wendy L. Hall, Kirstin Kadé, Paul Franks, Richard Davies, George Hadjigeorgiou, Jonathan Wolf, Francesco Asnicar, Nicola Segata, JoAnn E. Manson, Louise Newson, Linda M. Delahanty, Jose Ordovas, Andrew T. Chan, Tim D. Spector, Ana Valdes, Sarah E. Berry :: SSRN

[7] Current Sports Medicine Reports (lww.com)

[8] Carbohydrate Loading | Benefits, Tips and Side Effects - (scienceinsport.com)

[9] Tim Noakes: "If you've got Lore of Running, tear out the section on nutrition" | Gone For A Run

[10] Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon? | Sports Medicine (springer.com)