Can eating more fat make you a better runner?

A new – and controversial – diet ditches carbs in favour of eating more fat. Can it really make you a better runner? RW’s AC Shilton tested the theory while training for a marathon; the results were surprising.

Photography by Mitch Mandel, illustrations by Serge Seidlitz

‘I just want to stand here for a minute,’ I say, pausing in front of the cake display. I’m three weeks into my training for a marathon. I’m also on a controversial low-carb, high-fat diet and although every proponent has assured me I’d lose my desire for sweet things after a week or two, I am now staring lustfully at the carrot cakes.‘ Do we need any sprouts?’ my husband asks, but I can barely hear him: I am transfixed by cake. How on earth am I going to make it through 13 more weeks of this?

Fat-adapted running is an emerging philosophy in the long-distance-running community. Some runners – especially ultra-distance runners – are trying low-carb, high-fat (LCHF) to try to teach their bodies to use fat for fuel.

The theory is that since the human body can store more fat than carbohydrates, becoming ‘fat adapted’ will enable you to go further and faster. A few elite runners, like 2:31 marathoner Zach Bitter, have switched to LCHF diets. Others only occasionally modify the approach to run low on carbs. Elite coach Ryan Bolton, for example, puts his runners through fasted long runs of up to 20 miles with the aim of helping them boost their ability to metabolise fat.

On an LCHF diet, 50-70 per cent of your calories come from fat, up to 20 per cent from protein, up to 20 per cent from vegetables, and just five per cent from fruits and starches. That ratio is, obviously, in stark contrast to the diet traditionally favoured by most runners (and regularly advocated in RW). Sports nutritionist Nancy Clark, for example, recommends that runners get 55-65 per cent of their calories from carbs, 25-30 per cent from fat, and 10-15 per cent from protein. The LCHF diet is similar to the better-known Paleo Diet in some ways. Grains, sugar and legumes are off the menu on both, and both allow ample amounts of eggs, grass-fed meats, nuts and low-starch veggies. But Paleo allows some fruits (in moderation), which would break the carb bank on LCHF (which does allow coconut and fruits such as berries). And Paleo has more of an emphasis on protein (up to 35 per cent). The LCHF diets allow saturated fat. In fact, Dave Asprey, author of The Bulletproof Diet, says saturated fats even count as ‘good fat,’ since they are used to build cell membranes and to make hormones. (Asprey is a biohacker: ‘You can hack your own biology and gain control of systems in your body that you would never have access to,’ he says.) Proponents of LCHF argue that links between saturated fat and coronary heart disease are based on science that is shaky, at best.

Despite turning traditional wisdom about the runner’s diet on its head, advocates of fat-adapted running swear you’ll both nail a PB and lose weight. After a few weeks, the only thing I felt I was losing was my sanity.

How did I get here?

I do not believe in fad diets. I’ve watched too many friends try very low-fat diets, grapefruit diets, cabbage soup diets – you name it – with no lasting results. Plus, an eating disorder in my teens and early 20s made me realise that eliminating entire food groups is – at least for me – a dangerous proposition.

Since recovering, I’ve been a moderation evangelist. I eat biscuits, just not the whole packet. But while I felt my moderation diet was working, I was not so enamoured with my moderate running times. I’ve been running for over 15 years and over four years, starting in 2008, I ran four marathons, all between 4:23 and 4:30. I’ve also done two Ironmans, both in almost exactly the same time. I sought help from a local coach, Matt Reedy, but when he said his marathon plan required eating no carbs two hours before or after training runs, I told him he was a few spaghetti strands short of a carbonara and moved on. A short time later I added a fifth 4:20-something marathon to my collection.

Then I spoke to Professor Tim Noakes, one of the world’s best-known exercise science researchers and author of the training bible Lore of Running. I was interviewing him for a story about muscle cramps when he told me he was now advocating running on a low-carb diet, something he talked to RW about in an interview in our January issue this year. ‘It’s totally changed my running,’ he told me. ‘I’d become lethargic and I wasn’t really enjoying it.’ Noakes added that he had developed type 2 diabetes, despite exercising regularly and eating what he’d thought was a healthy diet. When he shifted to a LCHF diet, he says he ‘went from running like a 60-year-old [which he is] to running like a 40-year-old’. So enthusiastic was he about his new approach that he wrote a book about it, The Real Meal Revolution, which was a bestseller.

That night, I pushed my post-speedwork pasta around my plate. I trusted Noakes. And I also had friends who had reached their goals using Reedy’s carb-free plan. What if there was something in this?

The big fat idea

So I went back to Reedy and he talked me through the changes I’d have to make and how my 16-week training plan would be more about boosting my metabolic efficiency than my aerobic capacity. So I bid a sad farewell to pre-run cereal and committed.

Metabolic efficiency is a term coined by sports nutritionist and exercise physiologist Bob Seebohar. A decade ago he realised that he kept hearing the same two complaints from runners: one, I want to lose weight; and two, I have GI issues when I run. He wondered if he could alleviate both issues by altering his runners’ metabolisms so that they thrived on fat. ‘I knew the physiology involved, I just didn’t know if it could be done through dietary manipulation,’ he says.

According to Seebohar, who outlines his theory in Metabolic Efficiency Training: Teaching the Body to Burn More Fat (Fuel4mance), the average body can store 1,400-2,000kcals of glycogen. These glycogen stores come from eating carbohydrates – fruit, pasta, bread and, yes, Hobnobs. Glycogen is your body’s go-to fuel source, and lots of evidence shows it’s the best fuel for performance. But when you run out of it, you hit the wall.

Now don’t take this next bit personally, but Seebohar says you have almost unlimited stores of fat calories. A 10st 10lb person, for instance, may have 80,000kcals of stored fat. Teach your body to run on fat and you can run, at least in theory, pretty much forever. Which is a mouth-watering promise.

Day one: please send pasta

I started on a Monday morning. I drank a coffee and tried to reconcile myself to a world of unsweetened things. I like my coffee sweet. Actually, I like everything sweet, and sugar is off-limits on LCHF.

The day went downhill from there. Out were my normal fresh fruit and snacks of peanut butter on toast. In were roasted almonds, chunks of cheese and bits of salami. My Monday evening run started slow and ended slower. By dinnertime I was morosely poking my chicken breast around the plate.

The next morning’s hill session felt like trying to summit Everest. And the following day’s spin class was no better. Reedy had designed my schedule so I was running just a few miles at a time during the transition. I’d finished a full Ironman before starting the diet, but even three-mile runs were hard now.

My husband had promised to try fat adaptation with me. Two miles into his second carb-free run, he slowed to a walk and exhaled a stream of language so fruity it was definitely contrary to LCHF guidelines. The next day I found a Ritz Crackers box in our recycling bin. I was on my own.

Then, after two weeks of doubt and desire, on Day 16, I ended up on a group run with my club’s ‘fast guys’. ‘There’s no way I’ll be able to keep up,’ I said. But when the time came to accelerate, my legs didn’t complain. I reckoned they’d give up on the next surge, but the crash never came and I was still tucked in there at the end. It was a revelation.

As was standing on the bathroom scales. Both The Bulletproof Diet and Noakes’s The Real Meal Revolution promise fast weight loss. I wasn’t carrying much extra weight in the first place, but over the 16 weeks I dropped 13lbs, almost a stone.

Now the bad news…

Most LCHF diets suggest very limited fruit, but after I suffered a week of insomnia, Asprey recommended adding more carbs before bed. I went for grapes and slept soundly thereafter.

I also suffered debilitating muscle cramps, particularly in my calves, both during the day and through the night. This may have been down to low magnesium levels. Low-carb diet plans eliminate wholegrains and beans, which are two of the best dietary sources of the mineral. And although dark leafy greens such as spinach contain it, you have to eat a lot to get enough. I started taking a daily magnesium supplement, which helped, but it also made my worry about just how healthy this all was.

I worried about my kidneys, too. When your body burns fat, it releases acid molecules called ketones into the bloodstream, which are then filtered out by your kidneys. Some researchers believe the extra ketones, along with high protein consumption, could cause increases in kidney stones or renal disease. However, a study published in the Clinical Journal of the American Society of Nephrology found no difference in kidney function in those on low-carb diets versus those on low-fat diets. And a study review published in Nutrition & Metabolism found no correlation between an increase in animal protein consumption and incidences of renal disease.

I was also concerned about my cholesterol. At the height of Ironman training, a doctor said my levels were borderline high. I’d been watching my fat intake, so when I started loading up on bacon, every interval run made me imagine my heart was a ticking time bomb of renegade lipid particles about to explode.

My cholesterol did go up – my ‘good’ HDL from 67 to 87 milligrams per decilitre and my ‘bad’ LDL from 122 to 145 millilitres per decilitre. My triglycerides, however, dropped. ‘I’m not concerned yet,’ my GP told me. ‘In a healthy, young person like you, it’s not really something you need to worry about. However, you may want to consider cutting back on the bacon.’

Expert opinion on cholesterol and saturated fat is shifting. A growing body of research is showing that the HDL/LDL picture is incomplete, and that conventional wisdom and testing may not give us the actual data we need to detect real cardiac risks. The size and density of LDL particles seems to matter more than the total number, and some research has shown that LCHF diets produce the worst kind of LDL particles – the small, hard kind.

However, other studies have shown LCHF diets produce more large, low-density LDL particles, which are the least dangerous.

In defence of carbs

When you tell people you’re training for and running a marathon without carbs, you get a lot of sidelong glances. But when you’re a well-known researcher who has spent his career talking about how athletes need carbs, that’s a whole other kettle of fish (although no chips, obviously). Noakes has taken a lot of flak for his shift in thinking, being called a charlatan and snake oil salesman. ‘Noakes has gone out on a limb, and it’s good to challenge dogma, but I don’t think he has the science to back it up,’ says Noakes’s former student and collaborator Professor John Hawley, head of the Centre for Exercise and Nutrition at the Australian Catholic University in Sydney. Hawley points to research that shows long periods of fat adaptation don’t lead to athletes saving glycogen as they hope to.

Instead, long-term fuelling from fat can actually impair the body’s ability to process glycogen. And the research on glycogen is clear: it is the far superior fuel for short, fast efforts. ‘Ask the muscles to fire rapidly and power will be dramatically reduced if your glycogen stores are depleted,’ says Hawley. ‘Muscles prefer using carbohydrates.’ But he adds that ‘the longer the event, the more fat adaptation is an advantage’.

Sports nutritionist Nancy Clark isn’t sold on the Noakes approach, either. ‘How long do you really want to not eat carbs?’ she asks. ‘I always wonder what the [post-diet] backlash will be.’ For people who try a low-carb diet, reintroducing carbs can be particularly discouraging. Clark says that for every gram of carbohydrate you eat, your body stores three of water, causing your weight to shoot up.

There are other, more moderate, approaches to fat adaptation. Seebohar advocates a one-to-one carb-to-protein or carb-to-fat ratio, using what he calls the ‘hand method’, meaning if you eat a wrist-to-fingertip-sized portion of protein, you need to eat the same portion of vegetables.

The proof is in the (low-carb) pudding

The pre-marathon consensus at my running club was that I was running so well I was on for sub-four. Three weeks before the race, I began reintroducing carbs to my long runs. I did a trial half marathon and took a gel at mile 10. I hadn’t felt like I’d needed it, which is the beauty of this system – you never feel low on energy – but I slurped it down anyway.

Reedy had told me eating carbs after going without them would make me feel superhuman, but I got to the finish feeling no better or worse for the maltodextrin boost. Perhaps my body was already losing its ability to process glycogen, as Hawley had warned.

On marathon morning, 16 weeks carb-free and 13lbs lighter, I drank coffee blended with butter and ate full-fat yoghurt. I lined up with almonds and gels tucked into my shorts, convinced a 3:58 marathon was within my reach.

At mile six I sipped at my gel, as instructed. At mile 12 things were still looking good, and though I felt no difference whether I took in a gel or not, I swigged anyway. The miles ticked by and I was right on pace. Then, at mile 16, I fell apart. It was a total mental meltdown. My iPod broke. My asthma flared. I had to make a pit stop. There were two sun-seared miles without a water stop. I found every excuse why I couldn’t run 3:58, descending into that deep, dark brain crevasse that every runner knows and fears.

The one thing missing from my expansive list of reasons I wouldn’t hit 3:58 was a lack of fuel. I felt topped up the entire race. There was no physical ‘bonk’, not even the threat of one. My meltdown was purely mental.

I finished in 4:07. Not the time I wanted, but it was a 16-minute PB, and after five years stuck at pretty much the same time, it felt good. However, I’m not sure I’d stick with a high-fat diet. Noakes argues that this is a lifestyle change that someone must be committed to forever in order for it to really work. Reedy lets his runners go back to a normal diet between races, but that means re-fat-adapting every time you train for a race. Seebohar advises his runners to ‘periodise’ their diets depending on the type of training they’re doing, letting some eat more carbs at certain times, but advising against yo-yo-ing between high-and-low-carb diets.

Looking back on my experiment, I can say that I loved the steady supply of energy that fat adaptation gave me, and it got me leaner than I’ve been in years. But I missed beers with friends and sharing pizzas with my husband. I hated feeling guilty about grapes. And I pined for cake.

As I lay in the grass just past the finish line, my husband asked if I was ready to go home. ‘Home?’ I replied. ‘I have a carrot cake to buy.’

A day of eating low carb, high fat


Coffee with two tablespoons of butter, one teaspoon of stevia and one tablespoon of Dave Asprey’s MCT Oil (£22.95 for 946ml,, plus one egg scrambled with one egg white (430 kcals, 43g fat, 1g carbs)

Mid-morning snack

Serving of salted mixed nuts (172 kcals, 17g fat, 6g carbs)


Salad of mixed greens, shredded carrots, red onion, and half an avocado topped with 115g of smoked salmon, one teaspoon olive oil and one teaspoon balsamic vinegar (427 kcals, 28g fat, 13g carbs)

Afternoon snack

Tea with half milk\half cream and two tablespoons almond butter (200 kcals, 18g fat, 6g carbs)


Courgette spaghetti with marinara sauce and turkey meatballs, topped with Parmesan cheese (451 kcals, 17g fat, 24g carbs)


One grapefruit and four dark-chocolate-covered almonds (120 kcals, 5g fat, 20g carbs)


1,800 kcals, 128g fat, 70g carbs

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Discuss this article

without knowing this existed as a plan, I did something similar foe the marathon training.  In Feb this year I was pushing my weight limit to (my) maximum. Only because I wasnt quite as active, but fathomed I'd need to trim down half a stone to be efficient during the marathon. (My first). 

I binned most of the carbs. 

I lost the half stone, I ran the 26.2. I didn't hit any wall and felt like I could have carried on running. 

Posted: 04/05/2016 at 08:32

Nah it's complete rubbish. Just eat normal and have a balanced diet. 

Jan. Your marathon was 4.20 and you did finish will quite strongly. Fair enough

Posted: 04/05/2016 at 09:43

It's not complete rubbish. There is certainly some evidence that it works ... but more is needed.

Some discussions here over on The Dark Side of the forum...

Posted: 05/05/2016 at 17:13

I have done it and it works a treat  

A full fried heart attack breakfast every morning and no food cravings for the rest of the day!

Posted: 06/05/2016 at 16:59

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