Insulin Is Not Responsible For Weight Gain
"Don't eat fruit! The sugar in it will make you gain weight," proclaims the low carb zealot.
Lately, carbohydrates have gotten a bad reputation. Since moving to Los Angeles, I have encountered numerous clients who thought fruit was a fattening food group. They neglected fruit because it had too many carbohydrates and too much sugar. The belief is based on the idea that when carbohydrates are consumed, the hormone insulin is secreted causing the body to store fat instead of burning it. However, research has shown the number of calories you take in versus the calories you burn will determine weight gain, not a single hormone or macro-nutrient (1,2).
What is Insulin?
Every system in the body relies on energy to function. Breathing, moving, and eating requires power. This power comes in the form of macronutrients called carbohydrates, protein, and fat. But the body cannot use these nutrients for energy until they are broken down into smaller structures.
For example, let’s say you had steak and potatoes for lunch.
The carbohydrates from the potatoes will be broken down into usable energy called glucose. The body needs and relies on glucose to function, especially the brain and muscles.
Protein from the steak will be broken down into amino acids. These amino acids build and repair muscle tissue. They are essential and needed to increase one's muscle mass.
The fat from the steak will be broken down into fatty acids, which will be used to repair and create new cells, including certain hormones like testosterone. Without enough fat in a diet, the body's hormonal system would stop working.
When you are finished with the steak and potatoes, the pancreas will secrete the hormone insulin to lower blood glucose levels. Adequate blood sugar is required for the body to function, but too much can be toxic and cause issues. Insulin saves the day through a lock and key mechanism. Insulin acts as a key and unlocks the door to the cell. Once the door is open, the cell can take in the glucose and nutrients needed for cellular growth and repair.
During this process, fat metabolism reduces and glucose becomes the primary energy used. But this doesn't mean insulin increases body fat. As the day goes on, the body will switch back to a mixture of glucose and fat for energy.
The fuel you use to function doesn't matter much in the overall weight loss picture. Weight loss is dependent upon a calorie deficit i.e. the daily calories consumed versus the amount of energy used. So yes, you can still gain weight from a low carbohydrate diet if you eat too much.
Insulin isn't good or bad, nor is it solely responsible for weight gain. Just like you, insulin has a job to perform. Both carbohydrates and protein will secrete a significant insulin response, while fat will have a minimal response. Thus, we can't blame carbohydrates or insulin for weight gain since protein also secretes insulin.
At the end of the day, the number of calories you take in versus the calories you burn will determine weight gain, not a single hormone or macro-nutrient (1,2). Each macronutrient, including insulin, plays a crucial role in keeping the body alive. The ratio of these macronutrients in a diet will be dependent on one's body composition, activity level, food preference, and goals.