The answer requires a little biological knowledge. There is nothing really complicated about why a low carbohydrate diet works. To put it simply, the human body has two methods of turning food matter into energy (i.e. metabolism).
One method involves burning sugars to create energy. Many sports drinks are full of sugar, which gives competitors a quick boost before a competition / game. The other type of metabolism is fat-burning, and that is both the burning of dietary fat, as well as body-fat. The process of burning fatty tissue is called lipolysis.
Why does a low carbohydrate diet lead to weight loss?
Well, due to the extremely high sugar consumption that most people in a western world partake in, most people are burning sugar for energy, and fat is just digested and what is not used, is expelled. This means that excess sugars (which also cause an insulin response, which as mentioned above, stops fat-burning) lead to increased fat reserves.
But, by restricting the consumption of carbohydrates, the human metabolism is forced to switch to fat-burning. carbohydrates only last for two days in the human body, which means if you reduce your intake to 20g (or less) per day, then within two days you will start burning fatty tissue for fuel – i.e. lipolysis commences. It is impossible (in almost all people) to not burn fat, as it is the only fuel source available.
Once the body starts burning fat, it does not discriminate between body-fat and dietary fat – all fat is burnt, meaning that you lose weight.
What Is Ketosis?
Many low-carb diets refer to ketosis / ketones – ketosis is the process of forming ketones, which are the by products of fat-burning. To put simply, if your burn fat, you expel ketones (in urine and breath), and these can be detected using ketone test strips.
Another way to explain ketosis, is that it is a state in metabolism occurring when the liver converts fatty tissue into fatty acids and ketone bodies which can be used by the body for energy. However, lipolysis is really the correct term for dieting and weight loss purposes.
Fat Requires More Energy To Digest
That is not all though. Fat requires more energy to digest. So 1000kcal of fat will use up more energy than 1000kcal of carbohydrates. This is really another reason why calories counting does not work. Red meat can take up to 3 days to be digested, whereas fresh fruits (a complex carbohydrate) only takes 12 hours.
Other than the fact that by calories counting you are depriving yourself of much needed sustenance, by eating a reduced fat diet, your body uses less energy to convert the food into fuel. So, 100kcal of carbohydrates will likely lead to weight gain, whereas 100kcal of fat and protein will lead to weight loss.
Blood Sugar Response
But that is still not all. A high carbohydrate diet results in blood sugar levels rising and falling throughout the day. This leads to food cravings, hunger pangs, and sometimes dizziness, lethargy and fatigue, as well as poor concentration. This makes you want to eat more. Even if you have already consumed an adequately large meal (in terms of total calories).
Whereas, a high protein diet / low carb diet stabilises the blood sugar levels, controls insulin, and means that you no longer have food cravings and hunger pangs. Atkins referred to the food cravings experienced as a result of falling blood sugar levels as “false hunger”.