Carbohydrate insulin model of obesity
Low carb advocates will often be heard saying “insulin is a fat storage hormone.” This statement is foundational to the low carber’s belief that reducing carbs will reduce how much fat is in your body. By starving the body of carbs, less insulin will be secreted, and therefore, less fat will be stored. Additionally, your body switches to using fat as a fuel source and not glucose on a low carb diet. So, the low carbers contend, you are both burning more fat for fuel, and storing less fat at the same time.
Essentially, what this model of obesity is saying is that a calorie from a carb has a compounding effect on body fat composition than a calorie from fat or protein.
But what happens when you put this theory to the test? Now we know. Gary Taubes, one of the most prominent ketogenic advocates, raised $40M and formed the Nutrition Science Initiative (NuSI) to study and advocate for low carb diets. NuSI funded a study to prove that on a keto diet (KD), you could lose a significant amount of body fat.
The NuSI Study
17 obese men were put in a metabolic ward (where 100% control of food and environment is achievable) where the men consumed a standard high card baseline diet (BD) (50% card, 35% fat, 15% protein) for 4 weeks, and then switched to a low card keto diet (KD) (5% carb, 80% fat, 15% protein) for 4 more weeks. Critical to the study is that the overall calories were identical for each 4 week cycle. The carb-insulin model, in this experiment, predicts that the BD should accelerate fat storage, while the KD should accelerate fat loss (because there is less insulin to store fat in KD).
The results:
Subjects showed a more rapid weight loss on the KD (3.5 lbs the first week compared with less than 0.5 lbs per week on the BD). But this isn’t the whole story. What about the body’s fat composition? Exactly the opposite of what NuSI predicted. Fat loss slowed down when the subjects switched to the KD. In fact, the overall fat levels increased on KD.
What Happened
There are only three macronutrients: Carbohydrates, Fat, and Protein. If you cut carbs, you have to make up the calories with either fat or protein. If you look at the diet of any low-carber, it will be fat dominated. This increase in dietary fat is simply overwhelms your body’s ability to burn it, as it is not the preferred fuel source. Even when you are burning fat for fuel, and have little insulin in circulating to store fat, low carbers are eating so much fat, that their fat stores still increase.
What about the weight loss? How can it be that there was more weight loss, but an increase in body fat? Without access to carbs, the body does start to burn fat, but it also burns protein as fuel (gluconeogensis). When switching to KD, the subjects started to burn lean mass for fuel, and this, combined with water weight loss, explains the more rapid overall weight loss. This burning of lean mass has been observed in other studies as well. Crossfit athletes that have gone on a KD, have been observed to lose leg muscle mass by as much as 8 percent.
Conclusion
In one of the first actual controlled test of the low carb key theory of carb-insulin model of obesity, the theory “failed experimental interrogation.” In other words, the model has been falsified in its first experiment. Keep in mind, this study was devised and funded by Nutrition Science Initiative whose aim is to study and advance low carb and ketogenic initiatives. Having the central tenet of low carb mantra disproven hasn’t bode well for the NuSI either. “the once-flush organization is nearly broke and all but gone. It’s been three years since it last tweeted, two years since it’s had a real office; today NuSI consists of two part-time employees and an unpaid volunteer hanging around.”
Sources
The NuSI Study: https://www.ncbi.nlm.nih.gov/pubmed/27385608
NutritionFacts.org: https://nutritionfacts.org/video/keto-diet-theory-put-to-the-test/
Loss of muscle on KD: https://www.ncbi.nlm.nih.gov/pubmed/29910305
Collapse of NuSI: https://www.wired.com/story/how-a-dollar40-million-nutrition-science-crusade-fell-apart/