The BBC recently aired a Horizon documentary titled Sugar v Fat’. During the show two doctors (identical twins) embarked on a month long study comparing the differing effects of Fat and Sugar in our diet. It was thought that since the twins have similar DNA the results of the study will be down to diet alone, as genetics (which can play a big part) would not be a confounding variable.
One of the twins followed a diet where he consumed only fat, whilst in stark contrast the other twin followed an extremely high carbohydrate diet (HCD), with only the negligible amounts of fat that were found in the high carbohydrate foods that he was eating. You may be wondering how your diet can consist of only fat, in truth it is very difficult. The doctors’ diet should really be referred to as low-carbohydrate, since it also contained a substantial amount of protein in addition to the fat that he was eating.
Over the course of the month the twins undertook a variety of tests and tasks to examine the effects that the extreme diets had on their bodies.
Mental Performance
One such challenge involved them both undertaking the role of stockbroker in a busy office, their performance was measured based on how successful they were in their roles. The twin following the High Fat diet was outperformed by the twin following the High Carbohydrate diet. The documentary came to the conclusion that (HCD) for mental performance as the brain relies on glucose as a fuel source. Following a low carbohydrate diet would mean that the brain is deprived of glucose once the bodies’ glycogen stores have been depleted (which would happen relatively quickly on such a low carbohydrate diet).
Physical Performance
The twins also performed a task reliant on their physical capabilities. Firstly they were guided through 60 minutes of cycling on a fixed bike designed to deplete the (HCD’s) glycogen stores, they were then both given a source of energy. The High fat twin was given a serving of butter which was equal in energy content to the carbohydrate gel given to the other twin. They were then asked to race up a hill on a bicycle, the (HCD) twin won the race quite comfortably. This is down to the fact that carbohydrate is a much more suitable fuel for high intensity exercise than fat due to the rate that the different fuels can be broken down and used by the body.
Hunger and Food Consumption
On one day of the study the twins were given breakfasts identical in calories that corresponded to their dietary conditions, 3 hours later they were taken to a buffet and given freedom to eat as much as they want for free. The (HCD) twin ate more than high fat twin; this was suggested to be due to the satisfying effect of fat as a macronutrient.
Physical Condition Tests
As well as performance related tasks the twins were tested on a number of physical parameters. Both twins undertook a fasting blood glucose test which is a basic measure of your blood glucose level in a fasted state. The results of the (HCD) twin are not presented, however the twin following the high fat diet was shown to have a reading of 5.9mmol/l (considered normal) compared to a pre-experiment of 5.1mmol/l (also considered normal). The twins were also taken through a glucose tolerance test which is a measure of how well their body responds to a dose of glucose. Both twins were given an identical amount of glucose and then the rate at which the body disposed of this sugar from the blood into storage in the liver and muscles was measured. The faster this happens the better the body is considered to be functioning in terms of its ability to tolerate glucose. The twin following a (HCD) was found to be able to tolerate glucose better than the twin following the high fat diet.
Changes in Body Composition
During the 4 week study both twins experienced a change in body composition. The twin following the high fat diet lost 4kg in weight, 1.5kg of this was deemed to be from fat tissue, whilst the twin following the (HCD) lost 1kg in weight, of which half was deemed to be from fat mass.
Final Conclusion
The documentary concluded that it was neither fat or sugar alone were contributing to obesity but the combination of excessive amounts of either amount of macro nutrient will lead to obesity and other related health issues.
So there are the major results of the study but let us take a little closer look to see if we can dig out some further information.
Mental Performance
So it was concluded that a (HCD) was better for mental performance tasks based on the results of the stockbroker task. However this may be a little premature, since the task could have been influenced by a number of factors other than the diet that the two twins were following. For a start most stockbrokers will confirm that performance in a one-off short term task can largely be down to luck rather than concentration or skill levels. It also has to be noted that the twin following the low fat diet had very little, if at all any dietary fibre in his diet, this would result in his gut bacteria becoming starved of nutrients due to the absence of indigestible food. These bacteria will start to die and decompose releasing toxic substances which may affect concentration and mental performance. In reality you would not follow a diet containing no dietary fibre, even following a low-carbohydrate diet you would include some low sugar vegetables to ensure healthy digestion. It should also be remembered that the brain can adapt to using fat as a fuel through a process known as ketosis, however this adaptation would take longer than the four week study. To fully compare the effect of whether fat or sugar is more effective as brain fuel would need an acclimatisation period to adapt to the high fat diet which the experiment did not do.
Physical Performance
As mentioned the (HCD) twin performed better in the high intensity cycling task, this is because glucose is more readily utilised as an energy source than fat. Although we can adapt to using fat as a fuel (even at high intensities), however 4 weeks is probably not enough time for these adaptations to take place and this also relies on an element of physical training. However it is important to question the purpose of this test, are we looking at the effects of the two diets on general health and wellbeing or on an almost sports specific physical task?
Hunger and Food Consumption
Many studies have shown the satisfying effects that high fat diet (and high protein) diets can produce, the documentary further supported this. It is still unclear why high protein/fat diets have such a satisfying effect. It may be due to the lack of insulin release in response to a high fat/ protein meal as opposed to a high carbohydrate meal. Therefore you do not get a large spike followed by a decline in blood sugar levels which can trigger hunger.
Physical Condition
The twin following the high fat diet saw his fasting blood glucose reading rise from 5.1mmol/l to 5.9mmol/l. The documentary portrayed this as a major finding of the trial, with the suggestion that following a high fat diet was affecting the bodies’ ability to control blood glucose; however there are a few issues with this. Firstly 0.8mmol/l is not a large difference, in fact I doubt that this would be represented as statically significant if it was ran in a clinical trial. A fasting blood glucose reading may differ by this small margin from one day to the next, or even at different times of the day so this cannot be seen to be an effect of the study. Secondly 5.9mmol/l is still considered to be a normal reading.
The results of the glucose tolerance test indicated that the twin following the high fat diet was unable to deal with a dose of glucose as well as the twin following a (HCD). The show then went on to portray the image that eating a high fat diet could lead to type 2 diabetes. If we take a closer look at this we can start to question this idea.
The twin who followed the (HCD) suggested that his body had got better at dealing with sugar as he had been eating a lot of it and presumably following a high fat diet means that your body does not know how to deal with sugar.
He is corrected by the expert who said ‘no, you have got better at producing insulin. Insulin resistance is the pre-cursor to type 2 diabetes and is a scenario where the bodies’ cells become unresponsive to the effects of insulin, making it difficult to control blood sugar (as this is one of insulin’s primary roles). It is thought that over time the peaks and troughs in both blood glucose and insulin mean that we become less sensitive to the effects of insulin. As we become less sensitive we have to produce more to get the same effect until eventually we cannot produce the amount of insulin that is required to control blood sugar.
So is producing more insulin such a good idea? It seems that the effects of a high carbohydrate diet seemed to cause this.
Cholesterol levels were also tested (although they only received a brief mention); much to the twins surprise following a high fat diet did not cause cholesterol levels to rise. It has been drummed into us that eating a high fat diet will give you high cholesterol and as a consequence heart problems, despite the fact that there is little evidence to support this.
Changes in Body Composition
The high fat diet caused greater weight loss, 4kg compared to the (HCD) where 1kg was lost. In the high carbohydrate condition 50% of the weight loss was determined to have come from fat tissue, whilst in the high fat condition only 1.5kg of the 4kg total was said to come from fat tissue. The composition of the weight lost was measured with a ‘BOD POD’ which can only determine ‘Fat Mass’ and ‘Non Fat Mass’, so it is impossible to determine the whether or not more lean tissue is lost following a high fat diet.
What is likely is that a large proportion of the weight lost following the high fat diet is due to water loss. This is because carbohydrate is stored as glycogen in the body and it can only be stored with water, since the high fat diet was absent of any substantial amount of carbohydrate the bodies stores of glycogen will have been broken down and used in the early stages of the experiment and along with that the water that was stored with it.
So whilst the conclusion from the show is that neither fat nor sugar alone is worse than the other, the evidence from the experiment is presented in such a way that does seem to demonise fat over sugar which is slightly unfair. If we look at the data from the study the high fat twin lost 4kg (1.5kg from fat tissue) in 4 weeks which is not to be discredited despite the fact that there was no restriction on his dietary intake. The study also showed that a high fat diet also promotes satiety, and there are numerous other studies which support this idea. In essence despite the fact that foods high in fat are more energy dense (as fat contains 9 kilocalories per gram which is double that of carbohydrate and fat) eating foods high in fat often means that we eat less over the course of the day as they fill you up for longer, Since weight gain is caused simply by a positive energy balance this can be considered a positive. Fat as a macronutrient is quite often accompanied by protein in food, typical examples being eggs, most meats, dairy products and nuts. Protein is also known for its satisfying effects, as well as the fact that it costs energy to break it down and utilise is a fuel source. This will result in an increased metabolic rate meaning that energy expenditure will rise without any physical effort. Eating a diet high in fat (and protein) also means that the carbohydrate content of the diet is usually quite low meaning that you do not experience a large peak, followed by a decline in blood sugar which can lead to feelings of tiredness and hunger. In fact once the body has adapted to using fat as a fuel source people often that they feel more energised.
Obviously following an extreme diet is never good and I am by no means suggesting that we all eat a very high fat diet as practiced in the study as any extreme is not healthy. However I do think that we need to be a little less fat phobic. Most people would do well to reduce their sugar content in their diet, and I do not believe that there are many people who eat too much fat since the recommended intake is 30% of your daily intake. You must remember that fat has a lot of essential roles within the diet, some of the essential vitamins that we need cannot be absorbed without the presence of sufficient fat. Fat is also particularly important in maintaining hormonal balance and healthy skin. I believe that we should be aiming at a Carbohydrate, Fat, and Protein ratio of 40:30:30, with the majority of the carbohydrate intake coming from fresh fruit and vegetables in addition to whole grains.
So yes whilst the main message of the program is true but maybe fat does not deserve all the bad press that it sometimes gets.