Whatever happened to the good old days, when everyone agreed that endurance athletes thrive on a high-carbohydrate diet? Regular readers of PP may be aware that some people are now advocating fat-loading, in an attempt to train the muscles to store away more readily-accessible fats. Although there are some research results suggesting that this warrants further investigation, it's still a contentious claim, running in the face of current understanding of exercise physiology.

Another carbo iconoclast is American diet guru Dr Barry Sears. He has developed a dietary system (handily backed up with food bars made of just the right ratios) which advocates a low carbo, low fat, high protein intake (40:30:30). This ratio is actually quite hard to achieve eating standard foods. Consuming such a high protein intake without taking in more than 30% fat is difficult, as many high-protein foods are also high in fat. This is where the Eicotech bars come in - Sears states that he developed them because of the difficulty of achieving the 'perfect ratio' of carbs, fat and protein through diet alone.

Reading some of the anecdotal evidence for his dietary system is compelling. 'Improved recovery and performance that were amazing', 'Swimmers who adhere closely to the diet show visible differences in the pool' (exactly what, we wonder). But - if you come across glowing testimonies to this diet, hold back before importing plane-loads of the bars.


Hormones misunderstood
Firstly, there's no research evidence to back up the claims - just subjective reports. This isn't said in order to be a killjoy - controlled studies really are needed to validate performance-related claims, as the 'placebo effect' is so strong. Basically, if you think that something is going to do you good, sometimes it will, purely because you believe it will! Secondly, Dr Sears rationalises the use of this dietary system with pseudo-scientific arguments based on faulty endocrinology ('Debunking the Eicotech Myth', Sports Medicine Digest, October 1993, p6).

Let's take a look at some of his assertions, and then assess them in the cold light of current knowledge.

Quoted in Swimming World and Junior Swimmer, Sears asserts that 'You must control the composition of every meal you eat to create the optimum hormonal responses from your body.' By doing this, Sears contends that you can control those hormonal systems which are '100 per cent under dietary control'. The main targets are insulin and glucagon, together with the ecosanoids (more commonly referred to as corticosteroids).

The Sears diet attempts to keep circulating insulin low, and boost glucagon and corticosteroid levels. This appears to be based on a fundamental misunderstanding of the way these hormones are regulated. Here' s a brief guide.


The ABC of hormones
The key hormones involved in the control of fuel use are: insulin, glucagon, and various hormones produced by the adrenal glands (catecholamines) - these include adrenaline, noradrenaline (American terms epinephrine and nor-epinephrine), and corticosteroids.

Insulin is secreted by the pancreas in response to carbohydrate being eaten. It then travels through the bloodstream broadcasting the message to cells that there' s food coming in, encouraging them to 'open their doors' and store the food. In particular, glucose uptake into cells is encouraged, as is the formation of glycogen. Fats and glucose are also taken into adipose tissue cells and stored as triglycerides.

Glucagon, in contrast, is secreted by the pancreas when blood glucose levels fall. It turns the system from 'plentiful supplies - let's get hoarding' mode to scarcity mentality, where the stores need to be cashed in. So, as glucagon levels rise, the message goes out to cells to break down fats into free fatty acids which are released into the bloodstream. In addition, the liver is instructed to break down its glycogen stores.

Adrenal hormones come in two main groups. Those made by the adrenal cortex (outer part of the gland) are the corticosteroids, including hydrocortisone (cortisol), which encourages mobilisation of fats and glucose. Hormones made by the adrenal medulla (inner part of the gland) are adrenaline and nor-adrenaline - these put the body into 'fight or flight' red alert, and also mobilise fuel stores. There may be some dietary impact on the amounts of corticosteroids; however, adrenaline and nor-adrenaline are released thanks to messages transmitted via the nervous system from the brain.


Exercise brings insulin down
Sears believes that the insulin produced in response to high-carbohydrate diets reduces performance by interfering with free fatty acid mobilisation and by lowering blood glucose levels. He also asserts that, since the body has a limited capacity to store glycogen, any extra carbo will be converted into fat, as the insulin response from a high-carb diet facilitates glucose conversion to triglyceride and storage in adipose tissue.

This may all sound very convincing if it wasn't for a rather crucial factor. When is it that you need the availability of free fatty acids and mobilisation of glycogen? When you're actually exercising. And exercise itself has a major impact on the levels of circulating hormones. Firstly, it's been recognised for some time that exercise actually has the effect of bringing insulin levels down. This is why it's OK to consume simple sugars during exercise without running the risk of a subsequent blood sugar 'low' - which you might get after a sugar binge at rest.

Another hormonal change which occurs during exercise is that blood glucagon tends to rise. So do levels of the adrenal hormones - both because of a response to the levels of circulating fuels and as a nervous-system mediated response to 'stress'.


Enlisting the help of sympathetic nerves...
A lot of attention tends to get focused on the effects of the different dietary fuels on hormones (particularly insulin), with accompanying assumptions being made about the likely effects of different diets. What has been relatively neglected, however, is the role of the nervous system. A branch of the nervous system called the sympathetic nervous system wires your body up for all systems go when it perceives 'stress' (this includes taking part in exercise). Heart and breathing rate increase, blood flow to the muscles goes up, and ready availability of fuels (ie, blood glucose and free fatty acids) increases too. Some of this is achieved by direct nervous stimulation of the hormone-producing (endocrine) glands, such as the adrenal glands.

At the onset of exercise, impulses from motor centres in the brain ('central command'), as well as from working muscles, elicit a work-load-dependent increase in sympatho-adrenal activity, and in release of some pituitary hormones (eg, growth hormone). These changes in turn control the changes in secretion of subordinate endocrine cells: sympatho-adrenal activity depresses insulin secretion and possibly stimulates the secretion of glucagon.

In fact, some research indicates that the rise in adrenal hormones actually has MORE influence on fuel use than the changes in insulin and glucagon levels which occur in response to exercise-induced changes in blood glucose - ie, the mechanism is a feed-forward (proactive, nervous-system mediated) system rather than relying primarily on blood fuel feedback.

There is evidence that endurance training over a long period of time increases adrenal medullary secretory capacity. Theoretically, high sympathetic activity may induce many of the adaptations to training ('Endocrinology and Metabolism in Exercise: Future Research Directions', Galbo and Kjer, Canadian Journal of Sport Science, Vol 12 (3) 51, pp 102 -107). If this is the case, even if athletes consuming a habitually high-carb diet had higher levels of blood insulin while exercising (itself unproven), any effect on free fatty acid availability would be overridden by the sympatho-adrenal activity.


Low-carb is a disadvantage
The long and the short of all this is that the body is very well-designed in terms of being able to change its internal milieu in response to the demands of the environment. Thus, when you've had a high-carb meal and are lounging around on the sofa, your body will be concentrating on storing away the incoming carbs as glycogen. Which you'll be grateful for when you're next doing any serious exercise.

Once you start exercising, the emphasis changes to breakdown of fuel stores and making them readily available to the muscle cells. These changes will come into effect regardless of whether you usually eat a high-carb or a high-fat diet, and regardless of what your resting circulating insulin levels are. The only difference will be that if you' ve been subsisting on a high-fat diet, you won't have much glycogen to cash in, putting yourself at a disadvantage for exercise that lasts longer than an hour.

There is a school of thought that argues that athletes who train on a high-fat diet will be better at burning fat when the glycogen runs out, purely because their muscles are more used to it. This has never been effectively proven.

In an ingenious experiment at the University of South Carolina, sports scientists attempted to test whether having higher blood fat levels would improve endurance performance. By means of IV lines, they infused fat directly into the blood vessels of competitive cyclists as they exercised; thereby providing a direct test of whether increased blood fat could actually boost endurance. In fact, IV administration of fat quintupled blood fat levels, but had no positive impact on endurance. In contrast, when given carbohydrate orally while riding, the cyclists carried on for an average of 52 minutes longer before exhaustion set in.

When carbohydrate was not taken orally during exercise, fat oxidation did increase, but blood glucose levels fell, and fatigue occurred quickly. ('Increased free fatty acid and glucose availability and fatigue during prolonged exercise in man', Medicine and Science in Sports and Exercise, vol24(5), p570, 1992.)