Following is my understanding after reading
this book, pardon any mistakes.
Low-carb or no-carb diet works by limiting insulin secretion. Insulin is secreted from the pancreas in response to the glucose in the blood. It's job is to encourage the cells to accept glucose from the blood, there by reducing the blood sugar levels. Those destination cells could be muscle cells, liver or fat cells.
As graaja already mentioned, we eat 3 kinds of food:
- Carbs - These convert quickly into glucose and go directly into blood. That is why carbs satiate your hunger very fast.
- Proteins - These take long time to convert into glucose and need the liver to do that.
- Fat - Don't confuse edible fat (cheese/butter/oil) with physical fat. Edible fats are required to digest vitamins and protein. If you starve yourself of edible fat, your body can't break down the proteins and vitamins you are eating. Many foods containing these vitamins also come with the fat required to digest them. For example, the fat in egg yolks allows the body to access the vitamins A and D it’s so abundant in. So all the egg whites in the world won’t help you out if you’re not eating them with the fats in the yolk to access that protein properly. Read more here.
When you eat carbs, it quickly turns into blood sugar, and insulin is secreted to supply the glucose to the cells. If the person is doing physical labour or workout, the muscle cells will demand a big share of the glucose. The left over glucose will be supplied to the liver to store it as glycogen for future usage. However, the liver can only store up to around 5% of its mass as glycogen. So all the remaining glucose will be sent to the fat cells. If you eat further carbs, it results in more blood sugar, and further insulin will be secreted and it will all end up in fat cells. This storage in fat cells is what makes you fat. Inside the fat cells they form triglycerides.
Sugar and starch also cause insulin to secret much faster, because these too become blood sugar very fast.
When you eat proteins only, the glucose is generated at a low rate from the liver, so the rate of insulin secretion will also be low. Then even a low activity person's muscle cells and liver will accept most of the glucose and nothing much is left for the fat cells. This is why protein diet doesn't make you fat.
Coming back to carbs/sugar/starch, let us see what other harm it can cause.
1) The insulin secretion in high quantity will eventually make the cells resistant to insulin. Thus it requires more insulin to encourage those cells to accept the glucose from the blood stream. So the pancreas will produce even more insulin. And the cells will become even more resistant. This vicious circle will continue until the insulin production is no more able to meet the demand. That is when the person is diagnosed as having type 2 diabetes.
2) LDL is the protein that carries the cholesterol. LDL particles exist in different sizes. On one hand there are the large, fluffy, cotton-ball like molecules, and on the other hand the small dense molecules. Studies show that people whose LDL particles are predominantly small and dense, have a threefold greater risk of coronary heart disease. Furthermore, the large and fluffy type of LDL may be protective. There is an inverse correlation between blood levels of triglycerides and LDL particle size. Thus, the higher your triglycerides, the higher the number of small LDL particles. Conversely, the lower your triglycerides, the higher the number of large, fluffy LDL particles. Read more
here. Now remember triglycerides, they are formed in fat cells because muscles and liver cells couldn't deal with the deluge of glucose. In other words, carbs/sugar increase the chance of triglycerides creation, which increases small LDL particles, and hence higher chance of heart disease.
Thus, low-carb or no-carb diet decreases the chance of diabetes, heart disease and obesity.
The most dangerous drug most of us are addicted to is sugar, it is nothing but slow poison to our body. Even if you avoid using white sugar, it is also present in natural form in most fruit. I know we are conditioned to think that all fruits are good, but you need to remember that most fruits have abundant sugar. In fact, it is the result of thousands of years of selective breeding to maximize sugar. So replace fruits with vegetables, that will cut down sugar drastically.
Check
this site for details on the carb/sugar content of fruits and vegetables.
We humans in the current form have been around for 2.5 million years. But agriculture was invented only 10,000 years ago, and it took another 5000 years to spread to rest of the civilized world. Most tribal communities stuck with hunting and foraging until the 20th century. So what were humans eating in the first 99.6% of the time? Meat/Fish and wild berries/plants, whose carb content was less than 10%. They got meat and fat from hunting, and berries/plants from foraging. Our carb intake increased only with the invention of agriculture, that is in the last 0.4% of human history. Finally, processed sugar and flour became widely available only after the industrial revolution, that is the last 200 years. That is not enough time for humans evolve into carb/sugar tolerating species.
It is been shown through many studies that tribal communities in 20th century who switched from hunting/foraging to modern processed food started having instances of diabetes, BP, and heart disease, which was unknown in their communities until then. We survived as species at least 99.6% of the time eating meat and fat. Yet we blame these modern diseases on meat and fat. Real culprit is sugar, followed by starch and carbs.
Fat Loss
Now let's see how low-carb diet helps fat loss. Most of the following content is contributed by moderator ampere who has been following this diet since few years.
Body's energy production cycle is called the Tri Carboxylic Acid or TCA cycle. Whenever one does any activity, the first source of TCA is always Glucose. And the easiest source of glucose is carbs, getting it from fat is more complex. Therefore body will always preserve fat, whenever it sees carb coming into the system.
People who eat carbs and exercise don't lose fat, because the body uses the glucose from the carbs and leaves the accumulated fat alone. So if you eat and workout until hunger strikes you, and then eat carbs again, your fat will be left alone. There will be no weight loss.
In other words, if you eat proteins/fat instead of carbs, the glucose supply comes in a long trickle instead of a sudden flood. So if you workout or spend energy under these conditions, the body will desperately look for alternative source and finds the fat storage in fat cells and starts using that. That results in fat loss.
If you have heard the term runner's high, its basically glucose in your muscles getting consumed first and then body turning to fat for energy. That's why during the second session of workouts or run, one does not feel panting for breath that much. Its the fat mode of energy conversion kicking in.