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David Thompson reports on his training and preparations for a planned trip for a group of McArdleites – climbing the UK’s “Three Peaks”. This challenge involves the highest peaks in: Wales – Snowdon (Yr Wyddfa), England – Scafell Pike, and Scotland – Ben Nevis. He made a trial run with non-McArdle friends this year.


David Thompson




McArdle’s (GSD5)


Age 42


Age 57



You can read David's personal story of childhood symptoms, an unnecessary operation, and finally achieving his McArdle’s diagnosis in adulthood, on the AGSD-UK website. Go to David’s story.

It becomes clear that a trial run is needed

Having attended the Future Leaders Walking Course in Snowdonia this July, it became clear to me that to lead such an event as the Three Peaks comes with significant responsibility. A key part is to undertake reconnaissance of the planned routes for each mountain, and to assess the time commitment and fitness demands that climbing these mountains would pose for somebody with McArdle disease.

I planned a trip for a week in September with three friends, two were experienced mountain walkers and all three knew about my condition. A few weeks earlier the challenge might have been sunstroke, but in that week, the challenge was high winds and heavy rainfall!

Building fitness and losing weight

In preparation I increased my attendance at the gym and from 12 August I adopted a vegan ketogenic diet. I’m vegan anyway but my previous experience of a high fat ketogenic diet had shown that it would increase my exercise capacity and allow me to shed some surplus weight which I was keen not to carry up these mountains.

In the couple of weeks leading up to our trip I started to change the focus of the fitness regime away from the gym and towards long hilly hikes. In the last week I completed three 10 mile hikes each, with >400m ascent (finding 400m was quite a challenge in my part of Wiltshire). Each of the walks started with a steep hill, but when I was strictly following a ketogenic diet (70% fat intake, 20% protein and 10% carbs) I seemed to be able to walk up it with minimal effort, without the need to wait for second-wind and with no sign of muscle cramping. 

However, at the 3 hour mark on each walk, and confronted by another big hill, I started to feel quite rough. I initially put this down to dehydration due to the hot weather. Confident of that assessment I ensured that I had electrolyte drinks with me for each of the mountains.

The first peak goes well, despite the weather

The first mountain was Yr Wyddfa – Snowdon – on 18 September. Torrential rain and high winds whistling up the Llanberis Pass put the first day in doubt, but a quick call to Andrew Wakelin offered a solution by changing the planned route from the PyG Track to the Miners Track which would offer more shelter until the wind abated. We reached the summit in very high winds just as the clouds cleared for an amazing view out over to Anglesey and beyond. We descended down the PyG Track and completed the 874m of ascent and descent in 7 hours, only 1.5 hours more than the guide time for a non-McArdle person.

Scafell Pike in wet boots!

Next day we drove up to the Lake District ready for the ascent of Scafell Pike. Wednesday brought more high winds and heavy rain. We had to wade across the flooded Lingmell Gill and then climb the mountain in wet boots. Visibility at the summit was less than 100m with wind gusting up to 110kph / 70mph, but on the way down the clouds cleared and we were offered a beautiful view across the Lake District and enjoyed the warmth of late afternoon sunshine. We completed the 906m of ascent and descent in 6.5 hours, 75 minutes longer than the guide time.

Scroll through a few photos of our week. Click to enlarge.

Reviewing the first two peaks

The ascent of both these mountains had gone well and I was pleased with the elevation that I achieved before I needed to rest. The ketogenic metabolism was clearly helping. The limit of my ability seemed to be my aerobic capacity with the need to stop being brought on by being out of breath rather than by muscle cramps. However, just as on the hikes the week before, at 3 hours in I started to feel rough even though I was fully hydrated. For Scafell Pike I had packed a glucose sports drink and an orange with me just in case it was caused by hypoglycaemia (a drop in blood sugar) and the intake of sugar certainly seemed to make a big difference. Interestingly, despite taking on board all that sugar I was still in Ketosis when I got back to our hostel.

On the Thursday we drove from the Lake District to the Scottish Highlands in readiness for the ascent of Ben Nevis. The forecast was showing <100m visibility at the top, 1°C with high winds giving a -10°C windchill and a risk of heavy snow showers. 

Concerned about diet

I was getting a bit concerned with the keto diet. As well as the hypoglycaemia, the high fat intake was dulling my appetite even when climbing high mountains and burning lots of calories. Before the trip I’d lost 9 kg (20 lbs) but I didn’t want to lose any more. I decided to stop the keto diet that evening but that would have implications for the following day. 

I was awake all night with acute stomach cramps which didn’t abate until I glugged a can of fizzy drink and then proceeded to bring up the previous evening’s meal. I can only assume it was caused by the abrupt change in diet. I was in no fit shape to climb Ben Nevis the next day and I went back to bed as my friends set off for the mountain. Two to three hours later I pulled myself together, had some breakfast and set off after them. I texted them to say that I would meet them on the way down and I ended up meeting them at an elevation of 450m, about a third of the way up. Interestingly, for that ascent I was powered by glucose sports drinks rather than ketosis. I felt quite energised but the marked contrast was in having to stop in order to prevent muscle cramping rather than being out of breath. I’ve certainly come to the conclusion that mitigating the symptoms of McArdle disease with diet is a difficult path to navigate.

2.3 peaks achieved!

So that’s how the trip ended, my friends climbing the 3 peaks and me climbing 2.3 peaks! The week certainly had some challenges for me, but hanging out with friends I’ve known since university was a lot of fun. I’m hugely grateful for their patience, having to hang around in the wind and rain waiting for me to catch my breath. 

Ben Nevis remains a challenge

As far as I know, Ben Nevis is yet to be conquered by a McArdler. That is a challenge I have set for early summer next year. I would say it is the least technically challenging of the mountains but with an ascent of 1325m, it is 50% more to climb than Scafell Pike, so not to be underestimated. Anybody tempted to join me? If so, you can email me on:


The plan is to get together a group of McArdle people to undertake the Three Peaks Challenge, led by David Thompson. This would be over a one-week period and the individuals would need to train up ready for the event. It is hoped to make it an awareness-raising and fund-raising effort. Anyone who is interested please contact David on

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