Last night a BBC documentary aired called ‘Sugar v Fat’. If you’re in the UK, you can watch it here. The show charted the experiences of identical twin doctors in their (I think) mid-30s, who were put on very different diets for a period of 30 days. One ate a very fat-rich diet, while the other ate on rich in carbs (sugar and starch). From now on, I will refer to them as ‘High-fat’ and ‘High-carb’ for ease.
A basic premise behind the show is that identical twins have identical DNA. So, having twins eating different diets is akin to testing two diets in the same person at the same time.
The twins underwent experiments over the course of the month. At one point, for instance, the twins’ mental powers were put to the test through a simulated stock trading exercise. ‘High-fat’, apparently, fared much worse than ‘High-carb’ here.
The men were fed breakfasts consistent with their new diets, and equal in calories. 3 hours later ‘High-carb’ seemed hungrier than his sibling, and ate more at a free buffet too.
The men were exercised on bikes and then made to race up a hill. ‘High-fat’ was given a pat of butter before the race, while ‘High-carb’ got a carbohydrate gel. ‘High-carb’ won the race by a country mile.
At the end of the experiment, ‘High-fat’ had lost about 4 kg, about 1.5 kg of which was fat. ‘High-carb’ lost 1 kg in weight, about half of which was fat and half lean tissue (muscle).
Finally, the men underwent a ‘glucose tolerance test’. From what I could make out, ‘High-carb had maintained blood sugar control as a result of increased insulin secretion. ‘High-fat’ was told his glucose tolerance was worse, and that he was not far from being ‘pre-diabetic’. Much was made of this.
In the end, the conclusion was (basically), that it’s not either fat nor sugar that causes obesity, it’s overeating. We were told that foods with a combination of fat and sugar that are highly palatable and rewarding that drive people to overeat.
The fundamental problem with this programme is that it essentially dressed up anecdote as science. It’s not a good idea to judge the impact of different diets by testing them on a limited number of people in this way. It’s not much different from someone could claiming that ‘smoking never did me any harm’. And they might be right, but I’d hesitate before using that observation as the basis for advice I might give about the health effects of smoking.
The programme did point out that we cannot necessarily extrapolate the twins’ experience to the general population. That’s right, but unfortunately, I expect that is what literally millions of people may do as a result of viewing this programme.
Even the idea that testing different diets in identical twins is especially insightful is flawed. That’s because other factors beyond the twins’ DNA may have impacted on their physiological functioning. For example, ‘High-fat’ underwent mental testing in New York, while ‘High-carb’ was in London. A ton of potential factors that have nothing to do with ‘macronutrient ratios’ in the diet may have accounted for any differences in performance, including jet-lag, hydration status and the type and amount of light in the room.
‘High-carb’ stated with absolute certainty that it was his carb-fuelled breakfast that gave him the edge over his fat-munching brother. In reality, though, there is absolutely no way he could know that at all.
After the glucose tolerance test (about 38 minutes into the show) ‘High-carb’ is told he he’s making more insulin and handling sugar better. When the person officiating over the tests (Dr Richard Mackenzie from the University of Westminster in London) is pressed (by ‘High-fat’) as to whether this is a good thing, he conceded that in the long term, this could lead to problems. Of course it could, because producing more insulin to regulate blood sugar sounds likes ‘insulin resistance’ is here or on its way. Insulin resistance, by the way, is usually the key underlying factor in type 2 diabetes.
Yet, the boys end up almost rejoicing in this result, somehow. And Dr Mackenzie gives ‘High-carb’ a clean bill of health.
He had stern words for ‘High-fat’, though. He is told he is dire straights and close to being ‘pre-diabetic’. Why, because his fasting blood glucose had gone from 5.1 mmol/l to 5.9 mmol/l. But is this difference statistically significant? And could anything else have caused the rise? Stress? Running down the street on the way to the appointment? Being a bit sleep deprived?
But none of this could stop Dr Mackenzie telling ‘High-fat’ that he must stop his diet and that if he continues his pancreas is going to stop producing insulin. Curiously, ‘High-fat’s glucose tolerance test results were not described at all. If he seemed to be in such bad shape, how come we only got to hear about his fasting test result?
I have to say, I got more of a sense of anti-fat bias through this show. The glucose tolerance testing episodes is a case in point. Also, when nutritionist Amanda Ursell was talking the boys through the diets at the start, we heard all about the bad breath and constipation ‘High-fat’ would inevitably suffer. In contrast, we heard not a peep about the potential adverse effects of a diet rammed full of sugar and fast sugar-releasing starches (such as blood sugar dysregulation and its tendency to induce mood instability, fatigue, sweet cravings and to disrupt sleep).
If the twins and the producers and Dr Mackenzie wished to genuinely gauge the impact different diets have on health, they could have looked to an abundance of published studies. This review of 23 relevant studies show quite clearly that, overall, low-carb diets produce better results regarding weight loss and disease markers (including those for type 2 diabetes).
This programme did not draw on this research, and was essentially a piece of theatre. While I found the twins very funny at times, I reckon the show was actually a tragedy.