The carbohydrate insulin hypothesis is the claim that obesity is caused by high insulin levels, that the insulin increases fat storage while starving the body. And that this leads to increased hunger and decreased energy expenditure. So therefore eating carbohydrates leads to fat storage, prevents fat loss, leads to overeating, and lowers your metabolism, thereby resulting in obesity.
However, this theory has seemingly been debunked. Obesity researcher, Dr. Kevin Hall conducted a landmark study to end the debate over the low-carb diet hypothesis. And this landmark study showed that low-carb diets are actually worse for fat loss than higher-carb diets.
The study
Hall’s team started with a group of 19 male and female obese volunteers without overt disease. Volunteers were randomly assigned to one of two groups:
- Controlled baseline diet for 5 days, followed by 6 days of 30% calorie restriction. Calorie intake was reduced by reducing carbohydrate intake ONLY.
- Controlled baseline diet for 5 days, followed by 6 days of 30% calorie restriction. Calorie intake was reduced by reducing fat intake ONLY.
All diet phases were strictly controlled under metabolic ward conditions. The baseline phase was used to determine each volunteer’s calorie needs. After completing one diet phase (e.g., low-carbohydrate), each volunteer went home for 2-4 weeks, then completed the other diet phase (e.g., low-fat). This is called a crossover study design, and it’s extremely robust because each person is compared to himself, which reduces measurement variability. In other words, this study compares Bob eating a low-carbohydrate diet to Bob eating a low-fat diet.
At several points during the experiment, Hall’s team took a wide variety of metabolic measurements, including metabolic rate and fat oxidation. They also measured body weight, and directly measured body composition using DXA, a gold-standard method.
They also measured blood insulin levels, urine C-peptide levels, and several other hormones. C-peptide is a protein that’s co-released with insulin but has a longer half-life in circulation than insulin. It ends up in the urine, and it’s commonly used as a marker of overall insulin secretion over the course of the day.
Results
As expected, the low-carbohydrate diet led to a 22 percent reduction in overall insulin secretion, as measured by urine C-peptide. In contrast, the low-fat diet had no impact on insulin levels.
Also as expected, the low-carbohydrate diet caused greater weight loss than the low-fat diet. The low-carbohydrate diet also led to a higher rate of fat burning. Yet this did not quite make up for the fact that the low-carbohydrate diet contained more fat than the low-fat diet.
Metabolic measures indicated that the low-carbohydrate diet caused a 245 gram (0.5 lbs) loss of body fat over the 6-day diet period, while the low-fat diet led to a 463 gram (1 lb) loss of body fat over the same period. Total energy expenditure decreased significantly in the low-carbohydrate group, but not in the low-fat group (-98 vs. -50 kcal/d). DXA analysis showed that both groups lost fat, but the changes were too small and the variability too large to detect a significant difference between groups.
This study doesn’t mean that high levels of insulin aren’t problematic. Not all carbohydrates are equal when it comes to insulin response. And this is why it is better to focus primarily on fruit, and fruit-like sugars, for carbohydrates. Fruits and sugars that contain fructose don’t stimulate insulin secretion the way that starches, breads, and pastas do. Because of this, they don’t result in insulin surges and blood sugar crashes. So, they can be used safely to help regulate blood sugar, even for diabetics. (Note: Struggle with hypothyroidism and diabetes? Discover why sugar isn’t the problem and what you can do about in this article on “Hypothyroidism and Diabetes: How to Reverse It”.)
It’s easy to become consumed in the search for some miracle weight loss diet that doesn’t exist. And we now know that low-carb diets are not the answer for weight loss. However, we also know that your thyroid regulates your metabolism. And anything that improves your thyroid function WILL help give you a metabolic advantage. And this can be accomplished through your diet.
