For many years the accepted wisdom for those participating in any sports that required endurance was that carbohydrate-loading was essential in order to meet the demands for an energy supply. This view was reinforced by the results of investigations designed to compare the effectiveness of fat and carbohydrate as energy sources which apparently demonstrated the superiority of the carbohydrates. However the recognition that ketogenic diets (those which are low in carbohydrates and high in fat (LCHF)) are effective in overcoming Insulin Resistance (IR) has stimulated a re-think about the roles of fat and carbohydrates in sports performance. Two of the leading researchers in this field are Jeff Volek and Stephen Phinney who have summarised their conclusions in

“The Art and Science of Low Carbohydrate Performance” (1).

Their starting point is that the body can store over 40,000 Kcals as fat whereas with carbohydrates the total is about 2,000 Kcals. This means that there a limit to what can be achieved when the carbohydrates are the main energy source. Eventually the body runs out of fuel, even if this can be delayed for a while by consuming sugary drinks, a phenomenon referred to by marathon runners as “hitting the wall”. Despite this a very high proportion of the research done over the past 40 years has been about enhancing glycogen levels while there has been little effort devoted to finding out how to decrease the body’s dependence on carbohydrates during physical activity. Nevertheless the different fatty acids can be broken down to ketones, which can be utilised efficiently by all the organs in the body, including the brain, as a source of energy.

The factors which control the “fat-burning” capability of the body are rather more complicated than for carbohydrate utilisation. Firstly, a diet which has a high carbohydrate content, will stimulate the production of insulin, which in turn will act to inhibit the fat-burners. Secondly, it takes at least several weeks for the fat-burning to become fully activated. Hence the LCHF diet must be maintained continuously for this to happen. Incidentally this explains why early experiments reported that fat was less effective than carbohydrates as a source of energy. The investigators had not appreciated that the fat-burning capability takes time to develop and become fully operational. To make matters even more difficult there is considerable variation between individuals in their ability to burn fat.

As long ago as 1983 Steve Phinney conducted a study with lean highly trained cyclists who had been used to consuming a diet which was high in carbohydrates. At the outset, they did an endurance test to exhaustion. Then they spent 4 weeks adjusting to an LCHF diet described as keto-adaptation. When they did the endurance test, the results were just as good as those done originally. However this was achieved by almost complete reliance on fat rather than on carbohydrates.

Athletes who are not keto-adapted reach a point at which the availability of the carbohydrate starts to fall off which results in a deterioration in both mental and physical performance. This is unlikely to occur when the athlete is utilising the fat as a source of energy. This means, for example, in a marathon, the runner would not experience the “wall”. Hence it would be expected to be especially beneficial for those whose active participation is for more than about 2 hours. In addition even for events of shorter duration, training sessions would be facilitated. All-round improvements in general health and a reduction in proneness to injury all combine to have a positive impact on performance when the finest margins can make a huge difference to the final result.

Anyone with a tendency to excess weight will almost certainly lose adipose tissue (fat) by adjusting to an LCHF diet. At the same time, there is likely to be a build-up of muscle mass, which may translate into improvements in speed, agility and performance. This also improves power to weight ratios, which play a key role in acceleration and endurance. This can be crucial in athletes involved sports which demand high intensity and explosive bursts.

During physical activity, oxygen free radicals are produced which can cause damage to the various organs. When fat is utilised as the source of energy, there are less free radicals produced than with carbohydrates.

The advantages of a ketogenic diet were amply demonstrated by the experience of those who participated in the Schwatka expedition in 1879-80. A total of 18 people led by Lt Frederick Schwatka departed from the west coast of Hudson’s Bay in April of 1879 with 3 heavily laden dog sleds. They had a month’s supply of food (mostly walrus blubber). Apart from that they had to rely on their own ability to acquire food by hunting and fishing. They succeeded in covering over 3000 miles on foot over ice, snow and tundra. All 18 members of the original party plus their 44 dogs returned to the starting point in March of 1880. This remarkable feat of physical endurance was achieved on a diet which was virtually devoid of any carbohydrate (2).

Some people can cope with a reasonable amount of carbohydrates in their diet, which is fine. But there are many who are carbohydrate resistant (CR) which means that the presence of carbohydrates in their diet is damaging to their health, which is manifested as T2D and related diseases. It is well established that in such individuals, almost invariably good health can be restored by reducing the intake of sugar/carbohydrates and replacing them with the good fats. However it also means that the sporting performance can be improved or recovered. Professor Tim Noakes is a distinguished scientist who was the professor of Sports Science at the University of Cape Town for many years. Because he developed Type 2 Diabetes (T2D) he studied the research and discovered that it was almost certainly caused by his habitual diet that pretty well complied with the conventional dietary guidelines (Low Fat and High Carb). As a result he switched to a LCHF diet and within 8 weeks he had lost 11kg. Over the same period, many of his minor ailments disappeared, he had energy to spare and his running was back to where it had been 20 years earlier. As he says himself:

“it was astonishing” (3).

He is convinced that the LCHF approach will definitely be beneficial to all those who are CR and probably overweight. He goes on to give a number of examples. Bruce Fordyce is one of the best marathon and ultramarathon runners in the history of South Africa. During the late 1970s and early 1980s he worked with Tim who put him on a high carbohydrate diet. Bruce won repeatedly over a 10-year period but then things began to go horribly wrong. He started putting on weight and was no longer enjoying his running. When he had gained 16kg he tried the LCHF diet and within a short time at the age of 56 years he had reduced his time for 5k from 23 to 17 minutes. Another athlete who Tim helped improved his time for 56k by 3 hours, largely because he was able to double the amount of training he could do.


Clearly there is convincing evidence that the LCHF approach is beneficial for athletic performance. Some more examples will be posted in the next blog.


  1. J S Volek and S D Phinney (2012) “The Art and Science of Low Carbohydrate Performance” ISBN 13 978 0 9834907 1 5
  2. S D Phinney (2004)