The studies are looking at genuinely low-carb state where the person has been low carb for some time and their glycogen stores are significantly depleted.

For long-distance walking and cycling, which I am well-adapted to, my muscles are very efficient and don’t usually use much glycogen for the effort (a bit like the effort required for a generally active individual to stroll around the shops) – for a short walk or ride they may even use little enough glycogen that I can replenish it simply through the vegetables I eat with my dinner. However if you do something excessively fatiguing to the muscles (so that you have used a significant amount of the glycogen store in the muscles which was created by the carbs) then it is important to consume something that replenish the stores in due course, and certainly before they run out, ensuring that when they are next needed there is glycogen available.

As an example, when I’m on a walking trip I may be walking for 8-10 hours a day. No matter how efficient I am that will affect my glycogen stores noticeably, although it won’t deplete them completely as my body will use a combination of fat and glycogen for fuel depending on the muscular effort involved. As a result I’m not yet destroying muscle to release the energy in protein or relying solely on fat for fuel (therefore resulting in excessive release of fat). Those trips tend to involve a big pub dinner each day full of potatoes and similar high-carb foods to replenish the stores.

As a result of replenishing glycogen stores through consumption of carbs after any significant effort that uses those stores, it will mean that when you next tax the muscles while you may be perceived to be training in a low-carb state, you actually still have the glycogen stores available in the muscles and liver to provide energy for that effort. In other words you are not genuinely in the low-carb state that these studies refer to (where glycogen in no longer available as an energy source).

The situation of being adapted to burn fat for fuel however is also invaluable as an emergency aid. If glycogen stores become very low the body will try to preserve them as glycogen is the only thing that the brain can use as energy. At that point it will move over to using fat and protein for energy to keep you moving.

Every so often I will run low on glycogen if a route has been more taxing than expected or (as happened last winter) we arrive at our destination and find that there is nowhere to get any food. In those cases I know my body will know how to revert to fat and muscle breakdown to get me to somewhere where I can feed and replenish the stores. Agreed that in those situations there will be side-effects to having pushed yourself past the point of using glycogen stores such as breaking down protein for energy and oxidative damage from excess fat which has been liberated – I’ve certainly had that happen to me and it is an unpleasant feeling. However you would usually avoid this by knowing both your body and what effort you are going to be expecting of it before you will next get an opportunity to consume carbs.

Before adapting I did hit the glycogen-depleted wall a couple of times and was unable to continue walking or cycling. I’ve got strong memories of being sat on the side of a path in the middle of the Cairngorms, about an hour away from our final stopping point where there was some food available, feeling unable to walk, violently sick, light-headed, faint and only managing to walk a few metres at a time. It can actually be very dangerous. In comparison, when we arrived at our destination last winter to find there was no food available I was able to push through for another half hour to somewhere where we could beg a few slices of bread – a good thing considering it was raining hard, dark and the temperatures were low, even if it did have a detrimental impact on my body. As my long-distance running friend points out, he always has sugary drinks provided at feeding stations, so he’s not going to have that happen to him!

Somebody recently wrote to Robb Wolf and, reading through what he has written, he seems to have run a marathon fuelling on almonds and water. I certainly used to use mixed nuts for fuel on my walks when I still wasn’t able to go for the whole day running on the previous evening’s fuel input and body fat.

Depending on where you are on the fat-adaption process there may be an argument to put some basic carbs in partway through a marathon to get that extra boost (dried fruit broken up into small bits stuff which may be easy to digest – bananas?). However if you are already well adapted then you can probably afford to go for those high fat/lower carb options like nuts and chocolate. I would imagine that chocolate carries the benefit of having less digestion issues if you carry it broken up into quite small pieces and almost melt it in your mouth to a semi-liquid form before swallowing – avoids chewing and digesting – whereas nuts may be harder to handle while running.

You probably need to try a few things when training to see what settles most easily in your stomach and where you sit on the scale of fat-adaption.

Not sure how much help that is, but hopefully it’s given you some ideas of things to try out. Would be good to know what you eventually find works for you!

I converted to a low-carb diet for health benefits while I was in the Philippines. I felt AWFUL for a couple of weeks. I even stopped my run one day while I was attempting to run 8 miles. However, I stuck to the low-carb regime.

I am happy to report that I am currently reaping the benefits. I wake up clear and ready to run. In 2 months, I have yet to experience any “lag” or feeling of fatigue on my morning runs, despite the fact that I only drink a cup of coffee with cream before exercising. I do not get hungry until about noon, eat a sizable meal, and snack in the evening as I am not hungry enough to eat a full meal. I have lost 10 lbs (now weigh 110) and have great energy.

Interestingly, I have always sort of run “low” in blood sugar requiring that I eat frequently (usually high sugared fruits, healthy crackers, etc) to thwart symptoms. Since converting, I have yet to have a feeling of “low blood sugar.” Interesting, eh??

So. . .I can only report how my body has responded to burning fat over quick carbos.

I’ve found that I have performed better than ever in endurance sports on a low-carb diet. I’ve also found that my performance is, so far, not particularly hindered in the gym either (though there is a possibility that my progress would be a bit faster if I took in slightly more carbs) and would put forward Robb Wolf as another example of someone on a low-carb Paleo diet who continues to lift weights.

There may be an argument for being higher carb if your sport is particularly sprint-focused where the body tends to call on glycogen reserves, however the body is perfectly able to adapt to burn fat for longer, slower, endurance output and can keep going for longer by relying on fat rather than carbs. I strongly believe that you need to look at the sport you are referring to and adapt the body and the diet to suit the particular activity.