Processed Stuff

Sugar v Fat – Horizon BBC Documentary


Dig yourself hole and then keep digging. That’s essentially what this BBC documentary did. They had two identical twins compare a high sugar diet versus a high fat diet for one month. How each faired then was based on body composition, cognitive performance, fitness and glucose testing.

They went off on the wrong front from the very beginning simply by the diets that actually were given. The “sugar” diet contained no animal foods of any kind and was allowed all forms of refined grains foods (breads, pasta’s) and sugars (fizzy drinks, cakes) as well any fruits and juices. The “fat” diet on the other hand contained only animal foods: meats, dairy, eggs, fish without the obvious carbohydrate food such as breads and pasta’s, but also excluded all vegetables and fruit. Just to look at this more, here’s another way of describing what they did

Refined grain, added sugar, low protein/low fat v No fruit or vegetables, high fat/protein, zero carbohydrates

So without even getting into the specifics, you have two diets that are not healthy in the first place. Anyone knows that eating no protein or no fats, and lots of sugary foods in not going to be good for you. Likewise, any low carb/sugar advocate will tell you that vegetables and certain fruits should be form a large proportion of a higher fat diet.

So right off the bat, you have two diets that no-one should be eating. It’s like trying to do a documentary comparing Marlboro vs Silkcut. Then just to really make sure neither is a healthy option, you make the Marlboro smoker drink only Jack Daniels everyday and the Silkcut smoker can only drink Jameson. I’m obviously over emphasizing the point but I’m simply trying to highlight how unhealthy both of these diets actually are in the first place. Thus, in terms of ascertaining whether one type of diet is healthier than the other, it’s almost an impossible task.

What actually is a high “fat” diet with low carbohydrates?

What the documentary is attempting to compare is the conventional food pyramid type diet with carbohydrates as the base against the more recent advice of a lower carb, higher fat (LCHF) type diet. The food pyramid essentially recommends that the majority of our daily intake should consist of carbohydrates. The LCHF diet however, says we should be eating fewer carbohydrates with the main bulk of our daily calorie intake coming from fats. However, what’s important to point out is that the “low carb” part doesn’t have any specific set values and it’s here where confusion starts. The thing is that “LC” can mean anything from 0 – 200g of carbohydrate depending on the individual. Let me give you an example

Person A – eats 2000kcal a day, 50g only of carbohydrate = 10% total carbohydrate

Person B – eats 4000kcal a day, 150g of carb = 15% total carbohydrate

So the first thing to point out is that LCHF still means you can eat carbs. The second point is that the amount can vary based on the individual. 50g of carbs is essentially a bowl of porridge. So if you are person B, you can still eat 3 bowls of porridge a day and still be considered as “LC”.

So the direction in which the documentary points you towards is very misleading. They are trying to suggest that the lower carb/higher fat way of eating excludes all forms of carbohydrates while binging only on meats/cheese/eggs (not to mention the fact that the quality of these foods is not controlled e.g. he eats fast food, processed meat/cheese/refined oils). And to make matters worse, he’s not allowed to eat any vegetables or fruit. Likewise, those advocating a higher carb/lower fat diet would still include meats, fish, eggs and dairy in their diet. So even this “sugar” diet is a misrepresentation of what a healthy higher carbohydrate should and would most likely consist of. But just to surmise, because it seems to be a very hot/controversial topic at the moment, a LCHF diet can be described as the following

  • “LC” or low carb does not mean “zero” carb
  • The low carb amounts, depending on the individual context can range from 0-200g
  • A LCHF diet is allowed to contain copious amounts of vegetables and lower sugar fruits
  • The source and quality of the fats from animal sources is key and processed meats/refined oils are not recommended. Animal products from pasture raised, grass fed, anti-biotic free, non factory farmed, free range etc are the recommended animal food sources
  • High fat does also not just mean animal fats, high fats from plant foods such as avocado, coconut, olives, nuts and seeds are also a large constituent of the fat portion.

So to put this into a clear perspective. A high “fat” diet or a LCHF diet in this context could very easily consist of the following: greens, tuber vegetables, fruits, nuts, seeds, yoghurt, eggs, avocado, fish and some meat….. which is still “high fat” and still “low carb” , but would be considered healthy by most and certainly far healthier than what this documentary portrays a “high fat” diet to be.

How to set the tone, confirmation bias

So based on the above, they are already set up for failure. Neither diet is healthy and neither diet would be recommended by anyone whether you are in the “carb” camp or the “low carb” camp. However, TV is a powerful tool and the general tone that the documentary sends out is that fats are really bad for you and carbs are not. A good example of this is here when they introduce Dr Robert Lustig. He is a paediatric endocrinologist researcher into child obesity that has spent several years investigating the mechanisms behind sugar metabolism and its affect on insulin and liver function. But just listen to his introductory music at 18mins and again when he talks at 19.30mins… it’s got the end of the world, “Darth Vader” ambience.

Keeping with my analogy, now the two types of cigarettes they are comparing has some bias towards the Marlboro brand. Any type of bias can put a slant on the debate and television can use various different methods to convey that bias, background music being just one of them.

But rather than me just assuming that there is a bias, lets actually look at the science

Cognitive Function

As a measure of the affect on the brain the two diets have, the twins are put through a cognitive test. The outcome is that the twin on the high sugar diet scored higher and this is then attributed to his diet supplying the brain with glucose. It is then stated that glucose is the preferred fuel for the brain and improves memory function.

There are several holes with the logistics and design of this test in itself. Firstly, the test they actually use is some stock trading assessment not a validated method. The tests are also not performed under the same conditions. One twin is in New York, the other in London. There are several other factors that could affect mental clarity and focus such as their sleep quality the night before, their stress levels, their surroundings, noise etc.

However there is one crucial point that they fail to mention. While glucose can be used by the brain, there are also other compounds called ketones that can pass the blood brain barrier. These ketones are produced when liver glycogen levels are low as a result of a very low carbohydrate diet and/or fasting. Ketones are known to be more efficient compounds for the brain to use as they cause less neuron firing. Over excitation of brain neurons is known to cause epileptic seizures and a ketogenic diet (which produces ketones) is a treatment for epilepsy. In fact, there are several neurodegenerative disorders that a ketogenic diet has been shown to be effective for such as Alzheimer’s, Autism and Depression. A review here for those interested

Suffice to say that glucose is not the only brain fuel. In fact, due to the efficiency of ketones and how they have been shown to improve cognitive function, they are being researched for their use in the military and athletes.

So yes, glucose can be used by the brain. A LCHF diet, as mentioned above, can still contain glucose which can supply the brain. On the other hand, a very low carbohydrate diet where no glucose is available to the brain means that ketones can be produced. These ketones are now being used to treat brain disease and are also potentially a more efficient brain fuel source for healthy individuals.

Sugar and the Insulin Hypothesis


Obesity and many other modern diseases are being linked to high sugar diets. This is mentioned in the documentary and the statement made is that the quantity of sugar required to cause adverse effects is far greater than what would be considered an average daily intake. This is a huge debate in itself and there are books, journals and review papers that have been thrashing out this debate for years. The mechanisms by which sugar can have toxic effects are laid out here in this talk by Dr. Lustig


The sugar industry’s influence on research and government findings is clouded in controversy. This is unfortunately how many of today’s nutrition guidelines are flawed since commercial bias is what determines their outcome. Sugar is big money, it’s used in lots of today’s foods, we like it and there are massive corporate bodies that manufacture these foods for us. Hence, they don’t want studies to be conducted or health guidelines to be made that condemn this precious commodity. This isn’t just a personal opinion either by the way, you can read the proof here:


As with any nutritional guideline , it comes down to individual context. For example, a professional cyclist might consume 100g of sugar a day without it causing any health problems. A sedentary person consuming 100g of sugar a day is likely to suffer from insulin resistance leading to obesity and other health problems. This is probably the point the documentary was trying to make. However, they simply stated that we don’t yet know the level of sugar intake that causes health issues. However, given that the mechanism by which sugar works and the studies which show it cause adverse affects, it would seem obvious that the majority of the population should be severely limiting their intake of sugar.

They next briefly discussed the “Insulin Hypothesis” which was described as how high levels of insulin (caused by high carbohydrate consumption) leads to insulin resistance. Although this was mentioned, it was quickly refuted by them stating that there are no clinical studies yet that provide evidence.  Yet, since 2002 there have been over 20 clinical studies that have investigated low carbohydrate diets showing it to improve weight loss, blood lipids as well as neurodegenerative disorders. Here is a meta-analysis on low carbohydrate diets:;jsessionid=7396D07C42DB2B84642D35158B132AF9.d04t04

I’m not sure how or why they did not mention or make a reference to the fact that there have been many low carbohydrate clinical trials conducted . Again, it’s possibly another illustration of the bias and tone that the documentary set out with.

Exercise: Sugar makes you go fast up a hill, fat slows you down

I don’t know where to start with this one. Let’s just first describe what they did. They put the two twins on turbo trainers (bikes) and had them cycle for 1hr after a 12hr fast. It was described by one of the twins as exercise that “pushed them to the edge of our capabilities”. After this 1hr of cycling, they then were told to race up a hill at full pace. Prior to the “race” the sugar twin was given a sports gel and the fat twin was given a small square block of butter. They then cycled up an inclining stretch of road and the sugar twin won. No distance speed or times were given. The conclusion assumed and portrayed in the documentary was that this must show that sugar makes you go faster and fat slows you down.

I’ve spent the last several years studying and researching fat adaptation for athletes. I’ve used it myself competing in ultramarathons and I’ve worked with other high level athletes that have used it also. I’ve written a separate article ( on this explaining the mechanisms behind it but all I will say here is that it can work, there is evidence to support it, and more evidence emerging (I know of a study being conducted by Dr S. Phinney in the US where they are assessing fat adaptation in ultra marathon runners). To give a brief outline of how exercise can be fuelled by fat rather than sugar it requires the following: a LCHF diet similar to what I described previously,  prolonged exercise in the fasted state and several months of implementing this before adaptations can have an effect on performance. The “fat” twin was not following a tailored LCHF diet nor was he doing any sort of fasted state exercise or even any sort of regular exercise at all. Finally, he was following this high “fat” diet for only a couple of weeks. Suffice to say that this does not effectively assess the impact of what a high fat diet has on exercise performance. However, that was the message that was essentially given. Just to emphasize how poor their observations were:

  • 1hr of cycling does not deplete glycogen reserves. An average male individual can store ~500g in the muscle and liver. Even given a high intensity of exercise,  with somebody who is a 100% carb burner, this is going to last >2hrs. Hence, the “sugar” twin would not have been “empty” of his glucose stores.
  • There are various other fuel sources that can be used to produce energy following the same pathway (Krebs or TCA Cycle) as exogenous carbohydrate i.e. glucose. Fatty acids stored in the muscle and adipose tissue can be used,  amino acid intermediates  can be used, and ketone bodies can be used.
  • When the body completely runs out of glucose and doesn’t receive any external supply, it can make its own, process called gluconeogensis. Glycerol from fatty acids can be converted to glucose, amino acids (from the amino acid pool) can be converted to glucose and pyruvate/lactate can be converted to glucose.
  • Butter before a “race” would not be recommended by anyone, even the LCHF advocates. The only fats that can potentially be used as an exogenous fuel source during exercise are Medium Chain Triglycerides. These fats can be converted to ketones but even still, there effectiveness during exercise, particularly any short duration exercise would be very limited. Butter doesn’t contain MCTs, thus giving an athlete a block of butter before a short hill race serves no purpose.(For the record, I know Nigel Mitchell, Sports Nutritionist Team Sky,, and I know he knows all of the above. The limitations of conducting this type of trial for a TV documentary would have severely restricted him from going into any of these details)


Body Composition 

After 4 weeks, the twins were then tested for weight loss.

High Fat twin – lost 3.5KG, 1.5Kg fat mass

High Sugar twin – lost 1Kg, 0.5Kg fat mass


The high fat diet lost more weight and more body fat. So even though the “fat” twin was eating unlimited amounts of fat, poor quality foods, no vegetables or fruits and no exercise regime, he still lost more body fat than the low fat/high sugar twin. However, the documentary twisted this result by pointing out that although the fat twin lost more weight, they equated the 3.5Kg weight loss/1.5Kg fat loss with an overall 2Kg of muscle loss. This was then pointed out as being dangerous as muscle is important for health. Yet, the method they used was a BodPod which works by using air displacement. This means that it measures overall volume differences, not tissue type. So the 2.5Kg of lost “volume” would have not just purely been lean muscle mass, but it would also contain water weight. This is a well known part of the weight loss process when individuals go on a very low carb diet. As glycogen is depleted, and since glucose is bound to water molecules in ration of 1:3, it means that if 500g of glycogen is depleted, it can equate to 1.5-2Kg of actual overall weight.


Insulin and Blood Glucose 

The final testing looked at insulin levels and blood glucose levels.

The sugar twin results showed raised insulin levels. This was commented on as being “better able to produce insulin”. It was made sound like a good thing. However, its known that when the pancreas needs to produce more insulin than before, it indicates insulin resistance. This is what leads to diabetes. Yet, all that is mentioned is that in the short term this is a good thing and that long term it “might” be a problem. In addition, his fasting blood glucose levels were not discussed. 

The fat twin’s insulin levels were not discussed. We can only assume that they are good or normal since if they had also increased this surely would have been discussed. Only his fasting blood glucose levels were discussed which went from 5.1 to 5.9mmol/L. This is commented on as being a concern and an indication of pre-diabetes. However, they fail to mention that a) the result is still within the normal range and b) that value varies on a daily basis and can be affected by other factors (e.g. cortisol levels affected by sleep/stress) other than diet.

The doctor conducting the tests on the twins states that the high fat twin is on his way to becoming pre diabetic. He is told that he should stop his diet immediately. Nothing is mentioned about the high sugar diet in terms of the results. The twins then discuss this (with gloomy music in the background at around 42mins). They conclude that high sugar diet makes you better at producing insulin (when this is actually signalling insulin resistance) and that the high fat diet makes you have higher blood sugar levels (from 5.1 to 5.9, which indicates nothing) and causes insulin resistance (yet the results for this test on insulin levels were not discussed).

This particular section of the documentary is a prime example of how a debate can be slanted. They don’t discuss all of the results, they twist the interpretation of some of the data and they make inaccurate claims.


Fat + Sugar is bad for you

The final part of the documentary looks into self regulation and reward theory. They discuss studies done on rats where foods with 50/50 sugar + fat content drive appetite and increase weight gain. They state that eating ice-cream and sugary/creamy donuts make us eat more and cause weight gain as these foods consist of both sugar and fat. Hardly a revelation.  There are no health professionals out there that would recommend a high sugar + high fat diet for obesity and general health.



 To call this an experiment with justifiable evidence of any kind is difficult. They use two diets that are unhealthy and that don’t reflect the benefits of eating a higher fat diet or a higher carbohydrate diet. They use a cognitive test that is uncontrolled, an exercise trial that uses a weak protocol and then they misinterpret results and data.

Yet, through the tone and wording, the message given is that high sugar improves mental focus, it improves exercise performance, it makes you lose less muscle and it reduces your risks of becoming diabetes.

On the contrary, I’ve presented strong evidence to show that ketones are more efficient than glucose for cognition, fat adaptation can improve athletic performance, low carb diets improve weight loss and prevent diabetes. Plus there is also strong evidence to support how commercial interests can govern public health recommendations.

In conclusion, this is just another one of the many ways in which the public can be miseducated and misdirected when it comes to nutrition. It also proves how powerful a role confirmation bias and commercial bias plays.

“You can fool some people sometimes, but you can’t fool all the peoples all the time”

Bob Marley

  1. Ron Stuart02-05-2014

    Sorry Mr ON4S the diets were not supposed to be healthy they were about comparing a high sugar diet verses a high fat diet. The twins were acting as Guinea Pigs not trying to find the perfect diet! It was about what is making the western world obese and trying to put some indicator on what that may be. Experts with something to sell and by the way it’s the food industry that is killing people and it’s full of EXPERTS!!!

    • Barry Murray02-10-2014

      Dear Mr Stuart

      so who are you working for ??

      “Dig yourself a hole and keep digging” was how I started my review. I wanted to explain how and why they dug the first hole for themselves. I think I made that quite clear. It was done to highlight the complete futility of the experiment from the very beginning.

      Hence, your reference to their objective of “trying to put some indicator” on the causes of obesity” makes no sense because they never had the correct protocol in order to do that in the first place.

      Its incorrect science, false dogma’s and commercial interests that are damaging public health. This documentary exemplified all three.



  2. Keats02-06-2014

    Great review baz! I like the unbiased approach of your defence. I’m still learning a lot about the carb hypothesis and it’s relation to metabolic syndrome, atherosclerosis, diabetes…. Etc. Almost finished a book called “good calories, bad calories” by Taubes. Any other reading you would recommend?

    • Gavin02-08-2014

      Nice review!

      I read Calorie Myth and thought it was great, found Taubes hard enough work

    • Barry Murray02-10-2014

      Cheers Brian

      have a look at the resources page on my website

      also, more recently

      – The Perfect Health Diet.. Paul Jaminet
      – Peronal Paleo Code… Chris Kresser
      – Food Pyramid… Denise Minger
      – Art and Science of Low Carb Performance. Phinny and Volek


  3. Caroline02-06-2014

    Great article Barry; terrible so called scientific doc from the BBC

  4. Doug03-08-2014

    Your quote is one of my favourites and I would attribute it to Bob Marley as well but it seems that he was (unsurprisingly) not original source 🙂

  5. Philip Prendeville03-12-2014

    Top article. Like the simplistic analogies to smoking/drinking. Haven’t seen the documentary but from the sounds of it it is obvious to anyone with a bit of cop on that from the protocols used not much could be determined apart from the fact that those types of diets were clearly unhealthy.

  6. chris adams03-16-2014

    Nice summary, Barry


  7. Conor McCauley04-10-2014

    Great read Barry, saw the doc and was really surprised by the results they presented as I’ve been a firm believer in your approach for some time now, it’s worked wonders for my training. I couldn’t quite explain how they came to their conclusions so It was good to read your scientific analysis and get a better understand of exactly why the experiment was flawed from the start. This misinformation is unfortunately inline with the same conventional “wisdom” I read all the time relating to exercise and nutrition, people need to start questioning more. Great podcast on Marathon Talk recently also.

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