Dispelling nutrition myths, ranting, and occasionally, raving

Low-fat vs low-carb



There’s a lot to unpack from this Daily Mail article a friend alerted me to. The premise: a couple of identical twin doctors decide they need to lose weight. In order to determine which fad diet is better, one goes low-carb, the other goes low-fat. The fact that they are genetically identical and maintained similar levels of physical activity means that they assumed any differences observed would be due to diet. While there is merit to using identical twins in research studies it’s important to note that this was not a scientific experiment. One subject in each group doesn’t lead to robust findings. Other issues: obviously both men knew which diet they were on and there was no control group. So, this wasn’t good science but it’s still possible that we could learn something interesting from their little “experiment”.

Before we look at what they “learned” from the experiment there are a few things in the article that I want to touch on. Firstly, the headline states that “one twin gave up sugar”. Well, yes, but this twin also restricted all forms of carbohydrate, not just sugar. The twin who went on the low-carb diet thought that it would be effective because of the insulin hypothesis of weight gain.

Because these carbohydrates are highly refined, they tend to raise blood sugars and blood insulin levels quickly.  This will tend to cause weight gain and obesity.  This is known as the Carbohydrate-Insulin Hypothesis (CIH), and is the basis of the Atkins diet and many other low carbohydrate (Dukan) and very-low-carbohydrate diets (ketogenic diet). (1)

While it sounds convincing, we know this hypothesis to be incorrect. For one thing, there are many people/populations who consume high-carbohydrate diets and never develop obesity. For another, insulin on its own does not cause obesity (2). Minor quibble, really, but as doctors discussing weight loss they should have their facts straight. Obesity is complicated and there’s no smoking gun out there.

An important point that they make is:


These topics fall between the cracks at medical school. Yes, we understood biochemistry and food metabolism, and knew a lot about the consequences of being overweight. But which diets work, why we eat too much and why losing weight is so hard don’t sit within any medical speciality.

Pity that they didn’t take the opportunity at this stage to point out that this means that assuming your family doctor will tell you if your weight is a health concern or that they are a good resource for weight loss are dangerous assumptions. If you are concerned about your weight being a health issue you need to speak up and voice those concerns. Ask for a referral to a dietitian who specialises in weight management or to a reputable weight management clinic. I really wish they would have mentioned the great resource that we dietitians can be for all things diet and nutrition.

As you may have guessed, both doctors lost weight during the course of their experiment. I hope that they were the only ones who were surprised by this outcome. Naturally they lost weight; they were both on highly restrictive diets, they were both active males, and this was done over a very short period of time (one month). Imagine trying to sustain a diet with the barest minimum of fat or no carbohydrates for the rest of your life!

Fortunately, the doctors reached a conclusion that I actually agree with! That: “For any diet to work you have to be able to keep it up for the rest of your life.”  As I’ve said many times before: to see sustainable weight loss you need to make sustainable changes.

Author: Diana


10 thoughts on “Low-fat vs low-carb

  1. But which twin lost the mist? I’d guess it was the low carb one, rather than the low fat?


    • I think that it was actually the low-fat one. Not completely sure though.


      • Just having rewatched the results part- The high sugar diet lost 1kg in mass, 0.5kg of muscle mass and 0.5kg of fat, with the high fat diet losing 3.5kg, with 2kg being muscle and the rest being fat. Hope it helps :)


      • Thanks! I’m a little confused though… Neither diet was really high-sugar nor high-fat. One was low-carb/sugar and the other was low-fat.


      • That’s the way they referred to the two different diets on the show- it got pretty confusing! I guess it’s more sensational to say high-sugar rather than low fat. Was an interesting watch though.


  2. How can they tell what percentage of the loss was muscle or fat?


  3. Oh Diana you are so brilliant! Great research once again!

    You must be so good at your job! Of course you know the low-carb dieter would lose because carbs retain water. That’s why, when they go off that diet, going back to carbs, gaining water weight, they blame the carbs once again! I hope they drank a lot of water to wash away the ketones running around in their blood stream on a high fat diet. Without good fat, one’s brain takes a hit especially our myelin sheath. How long do we have to preach BALANCE? :)

    Liked by 1 person

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