Fats vs Carbs: is truth really on the Horizon?

The Horizon programme that aired last night was designed to get at the ultimate truth behind the fat vs carbs debate raging in the media at the moment. I was on the edge of my seat. But it failed to deliver for 3 major reasons:

1. The identical twins gimmick was a gimmick,

2. The trial wasn’t long enough to support any conclusions

and, most importantly,

3. They weren’t testing carbs against fats.

Read on to out why this was a first class of example of pop science gone wrong.

First let’s look at what they were trying to show.

Except that wasn’t at all clear. They were supposedly pitting carbs against fats but to what end: weight loss? health? exercise potential? The programme was zig-zagging all over the place and simply seemed to be having fun. There’s nothing wrong with that, except it was posing as education.

Nature or nurture?

The central characters were a pair of media-friendly identical twins, which supposedly eliminated any genetic bias in the study. Both twins are doctors who admit to knowing very little about weight loss and diets.

For the experiment, one twin was put on a ‘pure carb’ diet and the other on a ‘pure fat’ diet for four weeks. At first, no mention was made of the protein content of either diet. Despite their identical genes, one of them has a history of weight problems having reached a maximum of seventeen and a half stone while his slimmer brother has apparently stayed around twelve and a half. When they weigh in for the first time the more stable twin is 22% body fat, while the other is 27%. So we already know that one has a greater propensity to store fat than the other. Not identical in terms of their biochemistry then. Of course not. We know that diet and environment have an important role to play in metabolism and thus biochemistry.

So they put the fatter twin on the fat diet, and the slimmer twin on the carb diet. I’m disappointed about that because I think we would have seen more interesting results the other way round. No doubt they have their own reasons for this and I’m hoping it’s not because they had already decided what they wanted the outcome to be.

For the purposes of our discussion we need to differentiate them. Let’s call the New York twin, on the fat eating diet, with the bigger body fat index the US/Fat Twin. And so we’ll call the London Twin on the high carb diet with the lower fat levels the UK/Carb Twin.

The Carb Twin is allowed 2% fat. The Fat Twin gets absolutely no carbs. The logic behind this is that you ‘cannot survive’ without fat which immediately helps us with another fact: the programme advisers agree that it’s impossible to survive on zero fat. That’s not the case for carbs; we have no problem surviving without them. The carb eater doesn’t even get any non-starch polysaccharides, or NSPs.

NSPs are what we call ‘dietary fibre’. Technically carbohydrates, they are completely indigestible to the body and therefore nutritionally inaccessible in energy terms. They provide bulk to the stool and food for our gut bacteria. Fat Twin knows from the start that he’s going to suffer from constipation and bad breath. More on this later.

The power of research

One thing the programme does very well is keep referring to research. It’s a clever new trick the experts use. They refer to it without citing it. It has the effect of adding weight to their opinion without adding evidence. Experts frequently use this phrase when what they really mean is ‘from my experience’ or ‘from the two or three papers I’ve read’ or ‘I’ve been told that’ or (worse) ‘in my opinion’. It would be impossible to hold all the research in this field in your head, the people who have attempted to review it have written books and books.

The science is contradictory, and for good reason. The way we metabolise our food is an individual thing. Large studies are unlikely to find answers. What works for one body doesn’t for another. I have identical twin sisters and I know that they are physically very different in their brains, their moods, their bodies, their energy levels and their appetites. Their whole biochemical soup is different. They are different. We are all different.

So that’s Point One covered off: the twins’ biochemistry is different enough to undermine the findings right from the start. But that was glossed over.

It turns out that ‘glossing over’ is a feature of the programme. By stringing together a series of poorly designed ‘experiments’  with a bunch of non sequitur facts and opinions around fats and carbs, while failing properly to analyse or criticise their findings they arrive at a jumbled but emphatic conclusion that processed food is the baddie, and the fat diet failed because it didn’t preserve muscle mass. Let’s look at how they arrived at those conclusions.

The twins take part in a series of tests. First a blood analysis to measure cholesterol levels, then an insulin sensitivity test where they drink pure glucose and then measure blood sugar levels and infer insulin response from those data. Then they go on to test the two different fuels in different ways, starting with the brain.

Time to adapt

For the brain test each twin is put in front of a stocks and shares dealing screen, given a tranche of money, and told to start trading. One is in New York, the other in London. The Carb Twin flies, the Fat Twin fumbles. The difference between their performance is down to brain fuel we are told by a whispering ‘expert’ who is ‘observing’ the experiment in a room with the Fat Twin and therefore, clearly, not in a room with the other one.

Come on! As an experiment it is rubbish. It’s perfectly possible that one twin is good with software and the other less so. One twin might have enabling money beliefs while the other struggles. If that were the case, I’d be prepared to hazard a guess, simply on the basis of demographics, that the one more comfortable with money and success is the one living in New York. Guess which one wins?

The Fat Twin admits to forgetting things and finding the exercise difficult. The Carb Twin says he can ‘feel’ when his brain is working hard and ‘knew’ it was firing on all cylinders during the test. Great data then.

Meanwhile the ‘expert’ they roped in doesn’t bother to present us with any data she just tells us that the brain, especially the memory, prefers to run on glucose. What she means by this is something that no one would argue: given a limited amount of glucose in the blood, the brain gets priority because other organs and muscles can more easily burn other fuels.

No one challenges the brain expert’s analysis. I could live with this if they didn’t, later, report the Lustig interview with a sceptical commentary. There was clear bias here and many times during the programme when dogma was presented as fact due to a lack of critical thinking.

Cold turkey or hot potato?

Back to the brain expert. What she fails to mention is that a brain accustomed to a lifetime of running on glucose struggles to make the switch to running on fat (via ketones). So if you force your body to make a sudden switch to burning fat there will be a transition period while the brain becomes keto-adapted. It’s well documented that during this period of adaptation the brain feels slow and foggy. Given that they carried out this test during week two of the diet, the results hold no surprises. The Fat Twin loses.

Which brings me to my second major criticism. The nutritionist devising the diets didn’t account for the fact that one twin would undergo a radical switch in biochemistry. The US/Fat twin had his normal fuel completely removed while the UK/Carb twin was simply given higher levels of his ‘preferred’ fuel. In effect, the Fat Twin’s body was going cold turkey while the Carb Twin sailed merrily on. To have a chance of being fair, the data should have been measured after the initial period of adaptation, about 3 to 4 weeks into the diet.

You’ll remember that I referred to NSPs, or dietary fibre? The lack of it in the Fat Twin’s diet is significant. He knew he would suffer from constipation and bad breath but he obviously didn’t know why.

Proteins and fats are low residue foods because they have the potential to be completely broken down and absorbed. Stool bulk is provided by carbohydrates – particularly NSPs, or fibre. Fibre provides food for the friendly bacteria in our gut. The fibre in our stool and the metabolites of our gut flora play a vital role in our gut motility and thus bowel frequency.

We have a surprising ±2kg of bacteria in our digestive tract and when they are starved they die. In the process they decompose and give off noxious gases. Hence the bad breath. They also stop facilitating the movement of food through the digestive tract. Hence the constipation. If you can visualise the effect in the body of 2 kilos of rotting bacteria you can, perhaps, also imagine that the toxicity would lead to brain fog and feeling a bit slow and under the weather.

A bad gut feeling

Fat is not the cause of the problem, it’s lack of fibre. It would have been perfectly possible to include NSPs in the Fat Twin’s diet without compromising the experiment. But the nutritionist didn’t do that, which means once again that the Fat Twin’s body was coping with side-effects which would have made him likely to perform worse than his brother.

Throughout the testing period the Fat Twin showed unpredictable blood sugar levels, which is exactly what you would expect during early adaptation to ketosis. And while the twin overdosing on carbs made a pile of money, no mention was made of the adverse effect of high sugar levels on our neurotransmitters and mood, nor of the difficulty of sustaining energy levels for any period of time on a high sugar diet, nor of the damaging effects on the brain (and eyes, and kidneys, and arteries, and nerves) of a lifetime  of high blood sugar.

Point two: The results would have had more validity had they run the test for a longer time or at least waited until they could measure that the Fat Twin’s brain and body were keto-adapted.

Farcical on a bicycle

For our next trick… the twins exercised solidly for an hour until their blood sugar levels were similarly low then they gave the Fat Twin butter and the Carb Twin a sugar gel. Guess what happened? The one given sugar (who was also the one with the greater muscle mass) outperformed.

I’m not even sure what this experiment was aiming to prove. It showed that if you start exercising from low blood sugar then top up your sugar levels you will go faster. Doh.

What it didn’t show is whether exercising on sugars was healthier: speed isn’t the only reason people exercise. And, just to be clear, I have ketogenic marathon runners among my clients, so I know it’s possible to perform very well on a high fat diet. The person conducting the test concluded that carbs provide a competitive advantage for intense exercise, which may be true, but I don’t think these data proved that.

Again, a poorly designed test, with poorly measured data and an opinion, not a result.

Full of protein

Then we came to the hunger challenge which, if you were watching carefully, gave us the information for point three.

The twins ate their respective carb and fat breakfasts and a few hours later were faced with their carb and fat lunches. The Carb Twin had no appetite control while the Fat Twin knew when to stop. Interestingly (which is code for infuriatingly) the nutritionist put this satiety effect down to the high levels of protein in the fat diet because protein ‘is well known to cause satiety’. No mention that fat is ‘well known to be’ the most sating macronutrient of all. So no credit to fat for playing a major role in appetite regulation. You may think that shows a bias.

But in admitting this we get a vital piece of data. The Fat Twin’s diet was high protein. Rewind to the beginning and watch her showing the Fat Twin eggs, chicken, steak, pork chops, as well as cheese, cream, butter and oils. The Carb Twin has no proteins or fats. Which has profound implications for the study. If protein levels are high enough we can use it to make glucose. Indeed, if protein levels are high enough we never need to go into ketosis which means we don’t become keto-adapted. Instead the body burns amino acids from proteins we eat and then from our own muscle protein when the stores run out. Which is exactly what happened.

They weren’t testing carbs vs fats, they were testing carbs against protein + fat. Because of the poor design of the diets it turns out that the US twin was making some of his energy from protein, not just fat. And that undermines the whole experiment.

If you care to single out one single element of the programme that shows how utterly confused people are about the whole sugar vs carbs debate then please choose this one. Even with eminent nutritionists designing the diet they can’t manage to design a trial that genuinely pits carbs against fats.

Fat chance

So, although the Fat Twin lost the most weight (he lost one and a half kilos of fat in 4 weeks compared with the Carb Twin’s half kilo) the results were damned because he also lost 2kg of muscle. 

This doesn’t show us that a high fat diet endangers muscle mass, it shows us that a poorly designed diet does. The protein levels in his diet were so high that his body chose to use protein – either dietary or ‘self’ – to fill the energy gap in preference to fat. We also know that the twins were not exercising during this time so that would also explain some muscle loss.

So let’s look at the final weight loss results.

The Carb twin had lost one kilo, half of it was fat and half muscle. So 50% of his paltry weight loss was, apparently, muscle. The Fat Twin had lost three and a half kilos, noticeably round his middle (although the positive impact of that wasn’t mentioned) but two kilos of that was reported to be muscle – more than 50%. Quite rightly, they highlighted that losing muscle is not a good outcome. We don’t want to lose muscle mass when we diet and good diets avoid that.

But, although they calculated that less than half his weight loss was fat, the didn’t adequately prove the balance was muscle. It’s likely that some of that weight loss was  glycogen, which is the body’s way of storing carbs. Glycogen is very heavy as it’s stored in combination with water and we store up to 3 kilos in the liver and muscles. Low carb diets lead to glycogen depletion which may account for some if not all of the balance.

Even if the fat twin did lose muscle during the trial it’s more likely the result of a high protein diet stalling ketosis and forcing the body to consume itself. No points for  analysis here guys.

Then the cholesterol levels were measured. No change. Which completely refutes the high fat/cholesterol hypothesis that underlies the widespread prescription of statins, and most of the UK’s dietary policy. No discussion of this either, except to say it was surprising. Really?

Finally, they tested the post-diet insulin response. The Carb Twin was apparently handling sugars better than ever by producing more insulin, although the doctor had the grace to admit that this result ‘might’ lead to long term problems. The Fat Twin was responding less well to insulin with higher blood sugar levels than when he started. The doctor told him that he was practically pre-diabetic.

Hold on though. What does this result really show? It simply tells us that if you completely starve yourself of sugar and then consume a huge amount your body doesn’t handle it very well. Not surprising.

It doesn’t tell us what his blood sugar levels were earlier that morning, before the glucose test (although we know from other tests during the four weeks that they were admirably low). They weren’t measuring the effects of his diet on the blood glucose, they were measuring the effects of breaking his diet. It’s just not properly thought through. It would have been much more interesting to measure the blood sugar levels both before and after the insulin test.

So, by sleight of hand, intentional or otherwise, they seem to show us that the ‘high fat’ diet is bad for us, when in reality all they’ve shown us is that, even with the resources of the BBC behind them, the combined brain power of the nutritional and medical establishment in this country can’t design a small-scale diet trial.

It’s getting addictive

Just as I was about to switch off, things started to get interesting. They were interviewing Professor Kenny who conducts research into obesity and addiction. He has concluded that it’s not the fat, nor the carbs, it’s the combination of the two that overrides our reward circuits and sends us into addictive behaviour. I like his logic, and his research is exciting. So exciting in fact that it allowed the programme to ditch the whole fat vs carbs hypothesis and conclude that we didn’t have to worry about one or the other so long as we didn’t eat them together.

Yup. Right at the end of the debate they switched from the physiology to the psychology of eating, something they didn’t consider along the way. For example, when Carb Twin couldn’t stop eating at lunchtime it was put down to the lack of protein, rather than the fact that eating carbs alone can also short-circuit our reward mechanisms – while eating fat alone didn’t seem to. And when we watched the Carb Twin telling us that his fat devoid diet was totally joyless why was no one asking if that was a psychological reaction to not having his reward circuits tickled.

This convenient hook led them to renounce processed food as the culprit behind the obesity problem and pronounce all ‘extreme’ diets as nonsense.

Yet, as we have seen, they didn’t test an extreme fat diet. To my knowledge no one has ever run a properly conducted test on an diet that contains very high fat levels, no carbs and ‘just enough’ protein.

There are no significant conclusions to be drawn here.

We were left in mid air.


If this programme was submitted for peer review then it would be criticised on all fronts. Very few of the assumptions stand up to scrutiny, never mind the results, and some of the logic is illogical.

It’s lazy pop science.

The best you can say is. ‘more research needed’.

But what did you expect?

It’s TV.

If you’ve had the tenacity to read this far I congratulate you. I’d guess it also means you’d like to gain a proper understanding of how your body uses carbs and fats and proteins to create energy and vitality.

I’d love to help.

Book a Critical Wellness Workshop and let’s find out.

23 thoughts on “Fats vs Carbs: is truth really on the Horizon?

  1. Nigel Sproat says:

    This is far and away the most lucid and insightful analysis I have read of what was a frustrating documentary. Most of us could have predicted the outcome beforehand so no surprises there. It was the methodology that most infuriated. Incidentally the Doctor who presided over the glucose tolerance tests has contributed to the discussion on Dr Briffa’s blog. http://www.drbriffa.com/2014/01/30/my-take-on-the-horizon-documentary-sugar-v-fat/
    Personally, I think he really needs to read what is posted here (and so should the BBC producers). Thank you for your insight.

  2. PhilT says:

    The body composition discussion was incorrect to use the term “muscle” for the fat free mass loss determined by the BodPod density measurement. Much of this would be water, especially in the low carb case, and without something like DEXA, MRI or nitrogen balance there is no way of knowing the composition of the lost fat free matter.

    Fat boy wasn’t keto adapted, the highest ketone concentration on the flipchart during the cycle test was 0.5, about half or less what it should be. This would impair ketone supply to the brain as there is a peak ketone flux at higher concentrations.

    The carby woman dietitian should not have advised fat boy, they should have found an expert in low carb nutrition – Volek, Phinney, Noakes spring to mind. Two teams of experts in a shoot out would give the two subjects the best advice.

    A crossover design would have improved it – switch the boys round after a 2 week washout. As you correctly point out they weren’t the same at the outset, regardless of being twins.

  3. Martin Stillwell says:

    Excellent analysis Dawn, and I agree with Nigel that your post should be forwarded to the BBC team, the 2 doctors and the ‘experts’ used in the programme.

  4. Dawn Waldron says:

    Thank you Martin. I’ve forwarded it to Horizon in the vain hope that they might take notice. I’ll try to find the email for the doctors and experts and get it in front of them too. Encouragement is always gratefully received!

  5. Arleta (@Arleta_W) says:

    Well pointed and informative article Dawn. Calling it a pop-science is a spot on. Unfortunately there are and there will be more of this kind of TV products as it is about the audience marks more than anything else. It is a dangerous trend because one may interpret the so called research foundings presented in the show accordingly to one’s personal preferences and convenience, pick and chose the information and justify the bad habits. What’s more frightening is that the two doctors admitted having no clue about the basics of nutrition before they started the “research” (not that they knew much more after). That proves the changes in medical educational system are really needed. Proper nutrition is the core of our overall health and every medical treatment. If we can’t get from our doctor an accurate and comprehensive advise, where do we go? … TV?

  6. Henry Hyde (@Henry__Hyde) says:

    Thank you for your calm and sane critique of the programme, succinctly putting into words what I had been struggling to express. I knew that there was a trick being played on me somewhere, and felt that far from adding to the sum of human knowledge, this Horizon had undone much of the good work that has been done by others over the last 20 years. I felt, frankly, cheated and, as a type-2 diabetic looking for sensible answers, more confused than educated by their ‘results’.

  7. Dawn Waldron says:

    How nice to hear from you Henry. I’m so glad it came across as calm and sane, especially as I was seething a little inside. I hope you feel reassured that low carb is a great strategy for managing Type 2 diabetes. Thanks for visiting the blog.

  8. Alex says:

    Thanks for debunking a very poorly thought out programme. The problem is that there are those who are going to use the information as gospel and will never get as far as reading your very thorough critique.

  9. t1hba1c says:

    Hi Dawn,

    Great blog, thanks. as a Type1 Diabetic triathlete/ironman/endurance cyclist on a high fat, low carb diet (avg less than 50g carbs/day) I take exception to the appalling lack of clarity in that program on pretty much every aspect covered. The demonstrable lack of understanding of even the basics, known by any endo, was shocking. The whole “show” was a shambles!!

  10. Phil Bowler says:

    I would love to give the BBC my thoughts on this disgraceful ‘Horizon’, but the link above, from JanC, does not work…

  11. Rachel Price says:

    Hello Dawn

    I watched Horizon and then read your comments and just wanted to ask you a simple question which may be because I’m stupid!

    The one important thing that seemed to come over from the programme, which I have talked about to other people, is that the real danger to our health is eating the fatty sugary things – ie cakes, biscuits, puddings etc etc. Is it also right to think that if we eat say a meal with fatty meat, starchy vegetables, then a pudding with cream and then cheese and biscuits, this will have the exact response in our bodies as eating say a sugary doughnut because we are feeding our bodies fat and sugar together, even if the sugar is produced by the breaking down of the starches? Or even just having a snack of cheese and biscuits…..Could we eat sugar and fat on alternate days??

    I am not really sure anymore and yet I thought that I really knew what was what!

    Rachel x

    On Thu, Jan 30, 2014 at 3:22 PM, Dawn Waldr

  12. Steven Stallone says:

    I just watched this online. Everything about this documentary felt wrong. It was like a stone in my shoe but I couldn’t get at it and had to leave it alone. I felt like an idiot after watching it because I was questioning everything.

    When you mentioned the constipation and the bacteria in the gut dying then the lightbulb went on. I didn’t get the cycling either… yes, sugar does give you energy… and…?

    This was a spot on analysis and thankfully your experience has given you a more concrete point of view unlike me. For this article alone you’ve gained a reader.