Penny Barker has taken on some epic events over the years. We've followed her journey to the Race Across America, which included a 90-hour, 928-mile "warm-up" in the Race Across the West and from time to time she's shared things she's learned from training and preparing for the race of her life, such as why training indoors is more important than you might think.

Her latest thoughts are from her experiments with a low carb/high fat approach to fuelling. Her approach on this issue is different to ours, but it's certainly a hot topic, and not to mention one with lots of interesting views, so it's well worth a read if you're deciding what approach to fuelling you should be taking.

Over to you Penny...

Why should I try the LCHF approach?

There are two classes of athletes: the smart ones who understand that they need to manage their sodium intake, and the others who don’t.”

“The Art and Science of Low Carbohydrate Performance”, Jeff S. Volek PhD, RD and Stephen D. Phinney, MD, PhD

If you're reading this then chances are you already fall into the former category. Minerals matter and far more than you might think. They matter in a standard diet, particularly to athletes, but their importance is magnified when you ditch carbs. If you get your mineral balance right, you can reap the rewards; get it wrong and at best you’ll feel listless and unable to perform and at worst you’ll have raging muscle cramps, constipation and what is commonly known as “keto flu”.

I devoured various books on the low carbohydrate, high fat way of life in relatively quick succession (see my notes at bottom for which ones). I say “way of life” because for me, it’s not a diet. I need something that will work for me long term, both as an athlete and a human being.

The second bit of that is important – food is a massive part of our social system and that’s where many “diets” fall apart. I need to know that I can go out to restaurants, go round to friends’ houses for dinner and still enjoy what I eat without additional hassle. I also need a diet that is practical for training and racing as well as one that fuels my performance.

What's the theory behind the LCHF approach?

I’m going to explain some of the science behind LCHF very briefly before I go on. I’m also going to qualify this by saying that I’m a vet and evidence-based research is very important to me. There is a LOT of evidence behind this.

It’s not mainstream and it's still controversial but it is gaining momentum both in the medical profession and in the sports community, particularly amongst ultra-runners. I’m not aiming to court controversy here, just to explain enough about the principles so that the rest makes sense. If you want more evidence then I’d direct you to the works of Jeff Volek and Stephen Phinney.

I found enough evidence to suggest that a low carb, high fat (LCHF) diet is healthy, could sustain my athletic performance and put an end to my GI nightmares. The theory behind LCHF is that when you've gone through nutritional ketosis to flip the metabolic switch to fat burning you are accessing a much bigger fuel tank. You therefore don’t need to ingest large amounts of carbs when training or racing and this means that your poor, stressed intestines have a lot less work to do.

Your insulin sensitivity is raised back to normal levels as your cells are no longer swimming in sugar (as they are with a traditional Western diet), meaning that when you do have carbs, your cells are primed to receive them and it’s like having a bit of a turbo boost.

Can we live without dietary carbs?

Quite simply, yes. They are the one food group that it's not essential to consume exogenously (i.e. from external sources). We need protein for our essential amino acids and we need fat for our essential fatty acids. We are perfectly capable of synthesising carbohydrate from both fats and protein.

If you give the body of a person on a normal Western diet a choice of burning carbs or fat, it will burn carbs as a preference, because it’s quicker and easier. But, the brain actually prefers to run on ketones (the breakdown products of fats) and the body can synthesise the glucose it does require naturally.

A long time ago, doctors used to treat epilepsy with high fat, low carb diets. This died out when phenobarbitone was launched as an effective treatment, but there are now large scale studies showing the benefits of ketones on the brain in both epilepsy and dementia. This has also started coming into the veterinary world as our pets are also starting to benefit from our new understanding about the role of carbohydrates in disease.

Saturated fat is bad for you isn’t it?

The short answer is “no”. Eating too much fat alongside a high carbohydrate diet is bad for you (both for your health and your waistline), but high fat diets alone don’t raise cholesterol. Yes, you get raised level of fats in your blood stream, but they are also the first in the queue to be metabolised. Your body needs all the “natural fats” (not the man-made ones).

Eating a high fat diet won’t kill you if you aren’t eating all of the sugar and carbs to go with it. We have been made to think of fat as bad and getting ones head around a different way of thinking does take some doing, especially when conventional dietary advice is still advising low fat options (many of which contain more sugar).

What about protein?

LCHF is about moderating your protein intake (1-1.5g/kg of bodyweight). I had increased my protein intake last year due to the conventional sports science wisdom on the need to take on protein to fuel your muscles, particularly alongside carbs after exercise.

The LCHF protocol is to moderate protein as your body will metabolise the protein it doesn’t need into carbohydrates and you won’t go into ketosis, or stay down there. I’ve dropped my protein intake since going LCHF; I’ll eat normal food at meal times but have stopped the post-workout protein shakes and the odd protein bar during long workouts.

Ketosis – isn’t that dangerous?

In order to properly 'flip the metabolic switch' from carb-burning to fat-burning, you have to drop your carbs to under 50g of carbs a day and go into what’s called “nutritional ketosis”.

This is very different from the ketosis that diabetics suffer – they have high blood sugar levels and very high levels of ketones. When you go into nutritional ketosis you'll have normal-to-low sugar levels and your ketone levels will never get into the same range as diabetics.

It’s this rise in ketones that shows you are in ketosis, and its these ketones that your body is then using as fuel. It takes a few days to get there and ideally you stay there for a least a few weeks before adding any carbs back in. To become truly metabolically flexible takes months, so ideally you should aim to stay low carb for much longer to allow your body to fully adapt. Whilst under 50g is the magic number, carbs sneak in everywhere so it’s actually easier to aim for 30g and give yourself some flexibility!

What does LCHF look like in practice?

To most athletes, 30g carbs looks like one serving of their favourite brand of energy bar/gel/ drink, but how many of us know what it looks like in “everyday life”? Here are a few examples...

  • An average medium latte/ cappuccino = 17g carbs
  • A bowl of porridge with water = 27g
  • A 200ml milk = 20g
  • A medium apple = 22g
  • A small banana = 30g
  • A medium-sized potato = 26g
  • Two carrots = 11g
  • A slice of wholemeal bread = 10g

We’ve not even touched on breakfast cereals, rice, pasta, quinoa, cakes, pastries; essentially one serving of any “pure carbohydrate” will put you over the limit without blinking as most are 40-50g of carbs. Milk, fruit and root vegetables are something else you have to watch for when in ketosis. Similarly, avocados and some nuts are a great source of fat but also proportionally quite high in carbs.

How much fat is “high fat”?

Lots! On the LCHF diet, between 65-80% of your calories are coming from fat (about 20% from protein and 5-15% from carbs). Fat tastes good and it makes you feel full, both of which are huge bonuses of this diet.

In practice, this means that you're eating fat with every meal. Gone are the low fat (and often high sugar) options. This is about cooking with butter, hard cheeses, lots of olive oil on salads, having the fat that you cooked your meat in mixed in with the gravy. It’s butter on veggies, cream with berries, cream in coffee and nut butters (sparingly!). It feels indulgent and, to start with, it did really involve getting my head round the “fat is bad for you” thinking that’s been hammered in over many years.

So what do you eat then?

As with anything, you need to focus on what you can eat, rather than what you can’t and be conscious about what you’re putting in your mouth.

It’s about unlearning some habits we have got into with food. I used to have porridge for breakfast most days, a banana or apple as snacks, sweet potatoes regularly with dinner and a treat was some nut butter on toast. I had also been a vegetarian for a few years. Whilst I love the veggie way of life, it doesn’t sit easily with LCHF, particularly not to get into ketosis because veg and pulses are all carbs. Being vegetarian was a lifestyle choice rather than a deeply ethical thing for me, so I decided to go back to eating meat as I had to commit to LCHF properly if I was going to do it at all.

My diet is still very varied but to give you a rough idea, this is what it looks like on average:


2 egg omelette cooked in butter / scrambled eggs, spinach and cheese.


Full fat natural yoghurt, dessicated coconut, berries, chia seeds, medium chain triglyceride (MCT) oil (flavourless).


Big salad with seeds/ walnuts, lots of olive oil and either some cheese or tuna/


Chicken/ pork chop/ salmon/ steak with green veg and butter.


Homemade curry made with coconut milk and cauliflower rice.


Brazil nuts, celery, almond butter (not all together!)

100% dark chocolate – about 6 squares are fitted into my daily calorie allowance; low carb AND a superfood!

When you restrict carbohydrates, your body changes from retaining both water and sodium to discarding them both. If you don’t replace the sodium then you get headaches and feel faint. You can’t perform physically or mentally, you feel sluggish and constipated. Your kidneys also dump potassium to try to preserve sodium. Potassium is needed for building and maintaining skeletal muscle so this has a knock-on effect on performance and is also likely to lead to cramp.

All of the books I read talked about increasing my salt intake, but I didn’t realise until too late exactly how important this was. I was adding salt to my food and having my normal PH drinks whilst exercising so, apart from a couple of days, I didn’t suffer too much from signs of “keto flu” but my intestines did slowly grind to a halt and a couple of months in I started suffering from the most horrendous cramps in my legs.

I’d frequently be up for hours in the night, kneeling on the landing or lying on the floor downstairs unable to move my legs in one direction or another because it all made one muscle or another cramp. After re-reading Volek and Phinney’s books, I started supplementing magnesium and upped the salt in my diet by having 1-2 cups of vegetable bouillon a day. The cramps stopped, the guts got moving and my performance returned.

Can you really perform without carbs?

There are plenty of advocates of the LCHF way of life across all sports, both endurance and strength/ speed-based and there is plenty of evidence to back it up. Once your body has adapted and you have sorted your mineral balance out, you can perform just as well as you could with carbs. For me, the proof came in two slightly different forms.

In September, after 3 months on LCHF I did a 100 mile sportive. I had an omelette for breakfast about 2 hours before the start and during the event I had 7 brazil nuts, a piece of cheese and a mini pork pie.

It was a hilly course and I worked hard, virtually time-trialling the last 20 miles because I felt good. I had a cup of tea at the end, did the weekly shop and only then stopped for a coffee and a biscuit. I refuelled properly with salmon, veg and sweet potato in the evening. No bonking, no “hungry puma” moments afterwards and certainly no weakness on the bike. This for me was the first proof that this diet really could work for me and I was buzzing.

Shortly after that event I subjected myself to the sports science testing that I do up at the Endurance Store with my coaches. It’s a VO2 max and lactate threshold test and, because your inspired and expired O2/ CO2 are being measured, it's possible to see the exact point at which you stop burning fat and start burning carbs. My testing showed that my aerobic power: weight ratio was the same as before Race Across the West, my anaerobic power: weight ratio had actually increased and the point at which I switch from burning fat to burning carbs had moved by 40W. At all intensities apart from a full-gas anaerobic effort, I am still able to burn fat.

But no cake….ever??

If you're a healthy individual (i.e. not doing this to treat diabetes or other conditions), once you've made the switch, it's then up to you how “low carb” you want to stay. Some people stay down in the “no carb” zone, others go for a “train low, race high” approach.

I’m going for something a little in between, which is based on a strategy called “Optimised Fat Metabolism” pioneered by Peter Defty, who runs a company called VESPA. He works with a number of athletes, including Romain Bardet (2nd at the Tour de France last year) using carbs strategically on a strong LCHF base.

It’s a concept called “metabolic flexibility” and means that you get the health and performance benefits of fat metabolism but with the added performance boost from carbs when needed. It also means that you can indulge a little if you want to. I do fasted rides first thing in the morning when I’m doing aerobic base work, but for longer and harder rides I do fuel and I do have some carbs.

I’ve ditched gels and energy bars and now go for less processed alternatives (natural oat/ fruit/ nut-type bars) or just real food; a cheese roll and a cheese twist from the Co-op fuelled my last hilly 7 hour ride!  It’s also higher in fat which keeps me feeling more full. I’ll have some sweet potato or other carb in the evening before and after a hard ride and will now have the odd cappuccino or a piece of toast with some nut butter. Some days I will even have pudding!

Most days I’m still around the 50g of carbs mark, on long training days it’s probably closer to 150g but that is a far cry from where I was as a "normal" endurance athlete. I’d have been having around 100-150g of carbs on a normal day and if I was doing a long session, I’d have been having a minimum of 30g carb/hour and more for a very long or hard ride. 

Do you count the carbs in everything?

Yes to start with, then you just start getting a feel for it. MyFitnessPal has been a godsend. It helped me keep a track of my macros until I was comfortable that I knew what an LCHF day looked like.

I got very obsessed about carb counting, particularly when I started adding carbs back in. I’d found cutting them out quite easy but then stressed that I was going to reverse the whole fat adaption process in one meal. This was very negative and counter-productive as stress inhibits fat-burning (cortisol loves carbs). I spoke to Pete Defty and took a lot of reassurance from him and some of the podcasts he’s done with his athletes.

My husband went LCHF with me and has been far less stressed and rigid about the whole thing. It’s a learning process and like anything we do in life it's taught me something about myself (in this case that I’m very data driven which isn’t helpful given that this is a very individual process!)

Any other pros and cons?

A lot of the literature talks about athletes suffering from less DOMS (Delayed Onset Muscle Soreness) with the LCHF diet, due to there being less free-radical production from fat burning and therefore less oxidative damage to the muscles. Whilst I still get sore from doing heavy sessions in the gym, I rarely get the DOMS I used to from a hard ride on the bike; either I’m not working hard enough or the literature is true!

Fat-burning does take a while to crank up so you do need to warm up to perform at your best. I tend to do at least a 30-40min warm up before doing a hard interval session now, and on a long ride I often don’t feel that I’m fully working until an hour or so into the ride. I can still go hard if I need to and reach the required power, but it feels harder.

So will it work for Race Across America?

The honest answer is I don’t know. There are lots of reasons for “GI distress”, including nerves, heat and dehydration. My old strategy didn’t work, so it was worth trying something new. I am convinced that this is a healthier lifestyle, so regardless of what happens on RAAM, I won’t go back to my old carb-based diet.

Are you still using PH electrolytes?

Absolutely. As I need to take in more sodium, I’ll have my vegetable bouillon before exercise to ensure my circulatory system is fully primed but then have my PH 1000s on the bike. Indoors I will often use the PH 1500s as I sweat much more. I top up on longer sessions with the SweatSalt capsules as they are a handy way to carry more salt and I’ve usually got a bottle of PH 500 to hand particularly if I’m out and about.

A LCHF reading list

“The Art and Science of Low Carbohydrate Performance” Jeff S. Volek PhD, RD and Stephen D. Phinney, MD, PhD

“The Art and Science of Low Carbohydrate Living” Jeff S. Volek PhD, RD and Stephen D. Phinney, MD, PhD

“The Real Meal Revolution” Prof Tim Noakes, Jonno Proudfoot, Sally-Ann Creed

“What the Fat? Fat’s IN, Sugar’s OUT” Prof Grant Schofield, Dr Caryn Zinn, Craig Rodger

“What the Fat? Sports Performance – Leaner, fitter, faster on Low-Carb, Healthy Fat” Prof Grant Schofield, Dr Caryn Zinn, Craig Rodger

“The Low Carb Athlete” Ben Greenfield

Pete Defty's website for his stuff on OFM.