LCHF is Low Carb, High Fat. This is a way of eating that helps reduce the risk of diseases caused and worsened by insulin resistance (more on that later). We have had significant increase in these diseases in the past 40 years. Prior to the 1950s we ate real food that contained real fat. In the 1950s it was postulated that cholesterol might be the cause of heart attacks, and that eating fat likely caused an increase in cholesterol. This was never proven, but it was successfully pushed by one researcher named Ancel Keys (see The Big Fat Surprise in book links) until it was finally accepted as science by leading health officials and nutrition experts in the US. In the late 70s the food pyramid resulted and "low fat" became the new way of life for most of Americans. We did very well at changing to low fat. Problem is that there are 3 macronutrients: protein, fat, and carbohydrates, and reducing one means increasing one of the others. Carbohydrate (especially processed) and sugar intake went up significantly. Add to that the advent of easily accessible highly processed and high sugar foods, vending machines, packaged foods in multiple flavors and the new science of "creating flavors", food has become too easy, unreal (food-like substance) and very addictive.
With the food in the 70s came soaring obesity rates followed almost identically by diabetes rates about 10 years later. Other diseases, all linked to insulin resistance, such as cancers, Alzheimer's disease, fatty liver disease, sleep apnea and many more increased as well. Most ironically, heart disease, which low fat was meant to prevent, has really not decreased. Primary medical treament for these diseases relies on multiple medical interventions, and we became a country of pill poppers to treat all of these diseases.
Simplified, the cause is insulin. We all need insulin - it is a growth hormone that is used by the body to get glucose (a sugar that is the end product of most simple carbohydrates) into the cells to be used as fuel. Another job of insulin is to take extra fuel and put it in storage as fat. When eat too much sugar and simple/processed carbohydrates our bodies start to not recognize our insulin very well, whether due to constant high insulin or constant high blood sugars. Our bodies then put out more insulin. This excess insulin then stores the extra blood sugar that can't get into cells as fat, and we again feel hungry and crave more carbohydrates. This turns into a vicious cycle where we eat high carb diets, we feel tired and gain weight, we eat more carbs, again and again. Eventually this leads to diabetes in addition to many other metabolic problems.
Fat metabolism works completely differently and fat has been vilified for too long! Not only does fat NOT cause heart disease (there is not scietific proof that even saturated fat does), it does not cause the release of insulin either. Fat makes us feel satisfied, causing us not to eat or have cravings. Fat also helps us metabolize vitamins, and can be turned into a different, excellent kind of fuel called ketones. If we eat a high fat, moderate protein (more on this in a later post) and low carb diet most of the time, we will avoid metabolic problems, feel more full, maintain a better weight, and in general have more energy by avoiding the ups and downs of blood sugar changes caused by carbs. This is the reasoning behind LCHF.
There are many diets that can be considered low carb, high fat - LCHF, Paleo (to a point depending on dried fruit and potato intake), Banting, and Primal to name a few. My general recommendation is to find a way to eat real food (not food-like substance) that is doable for you, and then do it every day for the rest of your life.