I’ve looked at the effects of high fat vs high carb diets on endurance activity. Here’s an exert from a book called Survival of the Fittest: Understanding Health And Peak Physical Performance, by Dr Mike Stroud.
Dr Stroud is an amazing man, In 1992/3 Stroud and Sir Ranulph Fiennes made the first unsupported crossing of the Antarctic continent during which by Drinking isotope labelled water and collecting regular blood and urine samples, Stroud discovered that their energy expenditure exceeded 10,000 calories per day. So he should know a thing or two about what happens to the human body during endurance activity.
In the 1990’s Dr Stroud was working on research for the ministry of defence, looking into the best diet for infantry soldiers during military operations.
There is a difficult balance to be struck here, for troops must often carry their food on their backs, and so eating more to feel better and stronger has to be offset against the increased weight of their loads. Using high fat rations would allow more energy to be carried for the same weight….But sometimes soldiers must work at high intensities, the sort of levels at which muscles prefer carbohydrate for fuel. My research examined whether using high fat rations to save weight might reduce their working capacity.
So what did they do?
To answer the question we took two comparable groups of soldiers and made them compete against each other in performing hard military excercise over bogs, moors and fells for many hours every day. The training continued for a whole week, and when they arrived back at the training camp each evening, we took them into a temporary laboratory and made them work on excercise bikes until they were so utterly exhausted that they could do no more. The time that they could keep going on the bikes in the evenings was used as a measure of their endurance reserve at the end of each day’s efforts.
What did they find?
Muscles burn fat at all but the highest intensities of work, and regular activity modifies the fat profile of your blood and eliminates a good part of any excess intake. Furthermore, when endurance trained, the muscles have enlarged fat stores within them, ready for use as an immediate source of fuel. As part of this adaptation, the HDL transport system becomes more efficient in order to carry fat to the muscles where it will be needed. Regular exercise will therefore produce a healthy ratio of HDL to LDL and so reduce the risk of fat being deposited in the wrong place. Such benefits are evident even on lazy days. Studies of volunteers who ate a fatty meal on the day following a bout of vigorous work showed that fat levels in the blood an hour after the meal were 30 per cent lower than when eating the same food on the day following no such activity.
A similar but more extreme example of the effects of exercise on fat metabolism wer to be seen in the blood samples taken from Ranulph Fiennes and myself during our Antarctic crossing. As explained earlier, we ate a very high fat diet during our walk in order to minimise the weight of the sledges we dragged, and the fat source used was mainly butter. Our food was not only high in total fat, but very high in saturates. Our Antarctic rations contained 57 per cent fat, and that was a proportion of a diet containing more than twice the normal adult consumption. The result was an intake of more than four times the saturated fat normally considered wise, yet our total cholesterol stayed at healthy levels and the good HDL type went up while the bad LDL went down. The message wa clear. If you exercise enough, the fat gets used for the purpose that nature intended.
The conventional wisdom (and you hear it everywhere) is if you want to do endurance activity you need carbs and lots of them. Yet here are soldiers being run up and down mountains for days on end, and men walking 10 plus hours per day, pulling 500 lb sledges and burning in excess of 10,000 calories a day, and they are doing it on low(ish) carb/very high fat diets, without any clear loss of performance and at the same time having positive effects on their HDL/LDL ratios. Sure you can’t burn fat when your running the 100 meters, but how many calories do you burn in a 12 second sprint? Not many. So the big question for me is do we really need to base diets for so many sports around carbs?