Carbs, insulin, and obesity: Is the carbohydrate-insulin model dead or alive?

Cute young woman measuring her waist isolated on pink

Is the carbohydrate-insulin model of obesity (CIM) dead? Or does CIM explain the secret to weight gain and weight loss? 

Or is it somewhere in the middle?

Researchers John Speakman and Kevin Hall published a perspective editorial in the journal Science, detailing what they conclude is the failure of the carbohydrate-insulin model of obesity.

Keep in mind, this is not a discussion about if low-carb diets are beneficial for weight loss and blood sugar control. This is a discussion of why low-carb diets work. For the countless individuals who have treated their diabetes and lose weight with low-carb diets, the answer may not matter.

As Drs. Speakman and Hall define it, the CIM postulates that weight gain, specifically body fat gain, results from the action of insulin. 

Eating carbs increases insulin more than any other macronutrient. In this model, the insulin rise that comes after eating carbohydrates stimulates fat cells to store energy and prevents the fat cells from releasing and burning fat for energy. The result is a continual accumulation of fat. 

The authors cite mouse studies, short-term human feeding studies, and a non-randomized four-week feeding study to support their claim.

However, they do not disregard insulin’s role completely. Instead, they acknowledge insulin’s vital role in weight gain or loss and its “pleiotropic action on multiple organs,” which means that it has a variety of effects throughout the body. 

So, even though the authors claim CIM is dead, they stress that insulin may still play a role in weight gain and weight loss.

However, as Dr. David Ludwig mentioned in his response, the opinion paper falls short of proving CIM is dead. 

Extrapolating animal data to human obesity has serious limitations. We have different physiology, we don’t eat mouse chow, and we have free choice of food in an uncontrolled food environment. We also have emotional and cultural relationships to food that lab mice don’t.

Also, any study of low-carb diets lasting less than four weeks is woefully inadequate — given the adaptation process to low carb recently demonstrated by Dr. Ludwig. 

What data support CIM? Dr. Ludwig and colleagues conducted a five-month trial demonstrating that those following a low-carb diet after a period of weight loss increased their energy expenditure by about 300 to 500 calories on average.

Dr. Ludwig’s meta-analysis also shows trials lasting longer than two weeks demonstrate the same, while trials lasting less than two weeks do not. This highlights the need to account for an adaptation period in any low-carb study.

But as Drs. Hall and Speakman point out, the data do not universally support the CIM as the primary driver of obesity. 

Diet Doctor contributor Dr. Ted Naiman tweeted Eating less refined carbs improves outcomes due to higher satiety per calorie — not the internal starvation of the ‘carbohydrate-Insulin’ model [CIM], which I declared dead years ago.”

When asked for a comment, Dr. Naiman clarified that carbs and insulin still play a role in obesity, but the concept of “internal starvation” from high insulin doesn’t make sense to him when blood levels of glucose and triglycerides are elevated.

Additionally, Dr. Naiman is clear that carbs and insulin can drive food choices, and that is an important concern for people wanting to lose weight. Focusing on nutrients and satiety per calorie, and worrying less about CIM, is likely the best path to healthy weight loss.

And as Dr. Ethan Weiss tweeted, Is it necessary to believe that increased carbohydrate consumption has caused the obesity epidemic to also believe that reduced carbohydrate consumption can lead to weight loss? No.”

Part of the problem of competing theories may be trying to simplify things too much and assuming there is only one main cause of obesity.  

If there is one main cause, it is likely eating in a way that does not reduce hunger, does not provide adequate nutrition, and stimulates the consumption of too many calories. 

Another part of that problem may be the focus on temporary postprandial rises in insulin as opposed to chronic hyperinsulinemia that accompanies obesity and insulin resistance. 

Generally speaking, eating carbs causes insulin to go up. But in many metabolically healthy people, the insulin level comes back down to baseline relatively quickly. 

In those who have insulin resistance or metabolic syndrome, the insulin level may not return to a baseline but may remain elevated for an extended period or even continually. This is what we refer to as chronic hyperinsulinemia

This difference in baseline insulin and insulin response could determine who can and can’t safely eat high-carb diets. 

Another question is, why does it matter if CIM is accurate as a cause of obesity? If it was the main driver of obesity, the solution would be simple — reduce carbs, lower insulin, and lose weight.

But we already know that reducing carbs and lowering insulin works for weight loss in most people anyway. There are multiple studies demonstrating improved weight loss with low-carb diets. Maybe asking why it works is less important than knowing that it works. 

We also have to acknowledge low-carb eating isn’t the only approach for weight loss. Intermittent fasting, high-protein diets (both high- and low-carb versions), and even high-carb, very-low-fat diets can also help people lose weight.

Again, it comes down to eating in a way that provides adequate nutrition, addresses hunger, and helps you lose mostly fat mass. Most interventions that fit these criteria also reduce insulin. 

Reducing carbs may have a greater effect on insulin and that may be part of the reason for their success. But it isn’t the only reason.

So what can we make of all the controversy?

  1. Insulin plays a role in weight gain and weight loss, but it may not be the same role in both situations.
  2. Chronic hyperinsulinemia likely plays a larger role in obesity than short-term post-meal insulin rises.
  3. CIM does not describe the only manner in which we gain and lose weight and is not the only mechanism by which low-carb diets work.
  4. In addition to lowering insulin, low-carb diets help with healthy weight loss by: 
    1. Reducing highly processed and high- sugar foods that may cause cravings and stimulate overeating 
    2. Naturally reducing caloric intake through improved satiety 
    3. Increasing protein intake
    4. Allowing only high-quality, high-fiber carbs
  5. Other dietary patterns that achieve these same goals will likely also result in sustainable weight loss.

Is CIM dead or alive? 

Does it matter? 

What matters is that you find a lifestyle approach that helps you lose weight in a healthy manner. 

Thanks for reading, 

Bret Scher MD FACC