Insulin Is Not Responsible For Weight Gain
Lately, carbohydrates and insulin have gotten a bad reputation due to the belief that they cause fat storage and weight gain. The belief is based on the idea that when carbohydrates are consumed, the hormone insulin is secreted causing the body to store more fat. So then all we have to do is reduce carbohydrates to reduce insulin levels, which will decrease fat storage….right? It makes sense as I type this, but the body is hardly ever this simple. To make sense of the subject, I decided to write a general description of digestion and insulin’s role inside the body.
Every system in the body relies on energy to function. Breathing, moving, and eating requires power. Just like your car relies on a specific type and amount of fuel to run, so do you. 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 correctly, 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 glucose, amino, and fatty acids will fill your blood. If these blood glucose levels go unchecked, the body is at risk for disease due to the toxicity of high blood sugar. To prevent this from happening, the pancreas will secrete insulin to lower blood glucose levels. Insulin saves the day by acting as a key and unlocking the door to our cells. Once this door is open, the cell can take in the nutrients needed for cellular growth and repair, metabolic energy, fat, and glucose storage.
During this process, glucose becomes the primary energy used, and fatty acids become secondary. As the day goes on, the body switches back to an even supply of glucose and fat for energy. It is important to understand that the body uses a mixture of glucose and fat to function. Some people might use 60 percent glucose, while others use 40 percent. But this doesn’t matter in the overall weight loss picture. Remember, weight loss is dependent upon a calorie deficit and that deficit is not based on one meal, it is based on the daily calories consumed versus the amount of energy used.
So you see, 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.
Gardner CD, Trepanowski JF, Del Gobbo LC, Hauser ME, Rigdon J, Ioannidis JPA, Desai M, King AC. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA. 2018 Feb 20;319(7):667-679. doi: 10.1001/jama.2018.0245. Erratum in: JAMA. 2018 Apr 3;319(13):1386. JAMA. 2018 Apr 24;319(16):1728. PubMed PMID: 29466592; PubMed Central PMCID: PMC5839290.
Acheson KJ, Schutz Y, Bessard T, Anantharaman K, Flatt JP, Jéquier E. Glycogen storage capacity and de novo lipogenesis during massive carbohydrate overfeeding in man. Am J Clin Nutr. 1988 Aug;48(2):240-7. PubMed PMID: 3165600.