Carbohydrates Won't Make You Fat
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. Also, the brain can use another form of energy called ketones, but we will discuss that another time. Protein from the steak will be broken down into amino acids. These amino acids build and repair muscle tissue. Without adequate amino acid intake (i.e., protein) your body would fall apart. 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. Each macronutrient plays a crucial role in keeping the body alive.
When you are finished with the steak and potatoes, the glucose, amino, and fatty acids will fill your blood triggering a release of insulin from the pancreas. Insulin is an anabolic hormone. It is responsible for cellular growth, including muscle and adipose (fat) tissue. Insulin releases from the pancreas in response to the amount of glucose in the blood. When blood glucose levels become too high, it damages cells and puts you at risk for type II diabetes. Thus, a higher blood glucose level requires more insulin, while lower levels require less insulin. To be healthy, you want to keep your blood sugar at moderate levels, not too high or not too low.
Lately, insulin has gotten a bad reputation due to the low carbohydrate community blaming it for the obesity epidemic. But insulin isn't good or bad, nor is it responsible for making you fat. Rather, insulin keeps us alive and allows us to function. It acts as a key and opens the door of cells. Once this door is open, the cell can take in the nutrients (glucose, amino and fatty acids) needed for cellular growth and repair, metabolic energy, fat, and glucose storage. The graph below does an excellent job of explaining this.
During this process, the body shuts off its ability to burn fat for energy and uses glucose instead. However, this process doesn’t shut down the body’s ability to burn fat for the whole day. Remember, the body rarely ever uses a single fuel source. It uses a mixture of glucose (carbs & protein) and fat. As blood glucose increases the body uses more glucose for energy. When blood glucose decreases, the body switches back to fat for energy. Put simply, the foods you eat will determine the fuel your body uses for energy. If you have a high carbohydrate diet the body will prefer glucose over fat. And if you have a high-fat diet the body will burn more fat versus carbohydrates.
But at the end of the day, the most important factor is calories consumed versus calories burned (i.e. calorie deficit versus calorie surplus). A caloric deficit occurs when you burn more calories than you consume. The reduction of calories causes the body to burn stored glucose and fat for energy. During a calorie surplus, the body is saturated with energy. The saturation of energy signals fat storage versus fat burning. The excessive amount of calories forces the body to increase the size and the number of fat cells to store the energy. So if your calorie limit is 2000 calories and you eat 2500 calories, regardless if it is in the form of fat or carbohydrates, you will gain weight. Now, I don’t want you to freak out and think you will gain body fat everytime you overeat. It is perfectly fine to be a calorie surplus, you just have to be in a caloric deficit on the other days. The problem lies in the weekly and monthly calorie surplus.
All of the diets in the world—including, intermittent fasting, low carb, paleo, and vegetarian—all operate on the principle of calorie restriction and calorie deficit. Put simply; these diets help you consume fewer calories than your body needs. Remember, these eating behaviors aren't magical and don't speed up your metabolism (1). The real "magic" lies in the hunger reduction you experience when following them. The diets mentioned above call for nutrient-dense foods that are higher in protein and fiber which are known to reduce appetite. However, you can still gain weight if you don't manage your portions and overeat. I know, I sound like a broken record. But just in case you don’t believe me, go ahead and eat a jar of almond butter every night. Even though it is paleo, vegan and vegetarian, you will still gain weight if you consume this amount of calories.
Now, I am not here to bash any diet and say one is better than another. They all work well, but only if you consume fewer calories than your body needs. But don’t expect weight loss to happen overnight. Remember, you didn’t gain weight quickly nor will you lose it that quickly. You should strive to lose one to two pounds a week. To do this, I recommend fasting 12 to 16 hours a day (7 pm to 7 am) to reduce your eating window and calorie intake. This eating window should include real, whole foods that are nutrient dense. A nutrient dense food will give the body the vitamins and micronutrients it needs to function while minimizing your hunger and calorie total. Does all this make sense? If not, here's what you need to remember. The number of calories you take in versus the calories you burn will determine weight gain, not a single macronutrient (2).
Johnston CS, Tjonn SL, Swan PD, White A, Hutchins H, Sears B. Ketogenic low-carbohydrate diets have no metabolic advantage over nonketogenic low-carbohydrate diets. Am J Clin Nutr. 2006 May;83(5):1055-61. PubMed PMID: 16685046.
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.