The principles of the ketogenic diet: fat metabolism and potential risks.

Special Focus 5: What is the principle behind the ketogenic diet?

The Atkins Diet, the Stone Age Diet, the first phase of the South Beach Diet, the first phase of the Maggie Diet, the fish-only diet, the egg-only diet, the high-fat diet... Among the various diets, as long as they involve low carbohydrate intake, they all have some degree of influence from the ketogenic principle.

The ketogenic diet has been around in the West for decades and has made a comeback in various forms. Recently, it has become fashionable again, and many middle-aged men who feel they have status are rushing to join the ranks of those who follow the ketogenic diet, thinking that this way of eating is particularly magical and high-class.

So, what exactly is ketosis? This question is difficult to understand for those who haven't studied biochemistry. It's best to borrow a biochemistry textbook and look at the section on energy metabolism, as ketosis involves carbohydrate metabolism, fat metabolism, and protein metabolism.

The chemical composition of fat is triglycerides, which are glycerol molecules in which three hydroxyl groups are bonded to three fatty acids via ester bonds. To break down fat for energy, both glycerol and the three fatty acids must be broken down and completely oxidized into carbon dioxide and water. Like glucose, glycerol is easily used by the body for energy and produces no waste, directly converting into carbon dioxide and water.

Unlike glycerol, fatty acids are difficult to break down completely without the help of oxaloacetate, which comes from carbohydrate metabolism. They get stuck halfway through (through the β-oxidation pathway, they become acetyl-CoA, but because they are not bound to oxaloacetate, they cannot enter the tricarboxylic acid cycle for complete oxidation and decomposition), and then become toxic intermediate products (ketone bodies).

Normally, fatty acids can produce a lot of energy when they are broken down into carbon dioxide and water. For example, the complete oxidation of one molecule of palmitic acid with 16 carbons can generate 106 molecules of ATP (the direct energy source that the body can use, which can be thought of as "currency" used by the body's metabolism).

If the oxidation of fatty acids stops at ketone bodies, only a small amount of energy can be produced. For example, one molecule of palmitic acid, after being converted into eight acetyl-CoA molecules and then into eight ketone body molecules (acetoacetic acid), only generates the energy equivalent to 26 molecules of ATP. The energy of the 80 molecules of ATP contained in the eight acetyl-CoA molecules is completely wasted. In other words, the oxidation of one molecule of fat would normally produce about 9 kilocalories of energy for the body to use, but if it is all converted into ketone bodies, only about 1.5 kilocalories of energy can be produced.

Ketone bodies become toxic when they accumulate to a certain level. Without the help of carbohydrates, excessive accumulation of ketone bodies can make the blood acidic, causing acidosis and even death.

Conversely, this wasteful activity becomes a loophole that can be exploited for weight loss.

It's often said that breaking down 1 kilogram of pure fat requires 9,000 kilocalories of energy, which seems incredibly strenuous. Theoretically, even if you didn't eat anything all day, you'd only burn about 2,000 kilocalories, resulting in a weight loss of just 220 grams. For someone eager to lose weight, this slow pace is truly disheartening.

If you strictly limit your carbohydrate intake, preventing fat from being broken down into carbon dioxide and water, thus preventing the production and excretion of ketones and the waste of energy, then fat will be burned much faster. This is one of the principles of the ketogenic diet.

However, while dieters may be willing to expend energy, their bodies are not. Seeing ketones accumulating excessively in the blood, the body becomes very anxious and will try to save itself through three pathways.

◆ Gluconeogenesis breaks down proteins, utilizing the glucogenic amino acids within them to replace glucose. This involves the waste of dietary protein and the breakdown of body protein. Simultaneously, removing excessive amino acids increases the burden on the liver and kidneys, as they must be converted into urea in the liver and excreted through the kidneys. For healthy individuals, this isn't a significant issue, but for those with impaired liver and kidney function, especially patients with diabetic nephropathy and obese individuals with already compromised liver function, it represents a substantial burden. Incidentally, compared to directly ingesting carbohydrates to obtain glucose, gluconeogenesis requires additional energy, and the synthesis of urea after deamination also consumes energy. Furthermore, gluconeogenesis increases the consumption of B vitamins.

◆ Strive to eliminate ketone bodies (some people call this "detoxification," which isn't wrong, as they are indeed toxic). Some ketone bodies are excreted through urine, and a small amount is also exhaled. Thus, ketone bodies that haven't been completely broken down and still contain a lot of energy are excreted. This is a significant waste of energy. In other words, although the energy intake seems high, a large portion is actually wasted through excretion. For people with declining kidney function, this also greatly increases the burden on the kidneys.

◆To maintain the blood's pH balance, the body consumes calcium from bones. When the buffering system alone is insufficient, the body mobilizes calcium from the bones to balance the acid load caused by ketone body buildup. This leads to decreased bone density and calcium loss. Over time, this increases the risk of osteoporosis.

The first pathway, "breaking down protein," can be utilized by dieters. Once the body breaks down protein, weight loss becomes significant. Breaking down 1 kg of pure fat requires 9000 kcal, while breaking down 1 kg of pure protein only requires 4000 kcal. Furthermore, body protein is bound to water; losing protein also reduces body water content. Muscles contain 70% water; losing 1 kg of muscle protein can result in a weight loss of at least 2 kg.

Even with a high intake of fat, body fat still breaks down rapidly; although a high intake of protein is consumed, the body cannot fully utilize it, and muscle continues to slowly break down and decrease: this is the magic of the ketogenic diet.

Most Chinese people are accustomed to a high-carbohydrate diet for many years. When adopting a ketogenic diet, the body is not adapted to digesting so much fat and protein at each meal. Besides, without white rice and white steamed buns to accompany them, what is so appealing about hearty dishes like braised pork or braised fish? The ketogenic diet makes people feel that although they can eat high-fat, high-protein fish, meat, and eggs as much as they want, their appetite is not good, they feel full very easily, and their food intake is naturally limited.

Therefore, for most Chinese people, the so-called very low carbohydrate diet, including high protein, low carbohydrate diet and high fat, low carbohydrate diet, not only has a ketogenic effect, but also a dieting effect.

The wasted energy, coupled with the loss of protein and water, makes the short-term weight loss effect of a low-carbohydrate diet truly remarkable. No special exercise is needed to lose weight; it's an irresistible temptation for those eager to lose weight.

However, keeping the body in a state of stress for a prolonged period, constantly struggling to eliminate waste, can place a very heavy metabolic burden. Due to differences in genetics and physical constitution, everyone's ability to adapt to the ketogenic diet varies, and some people who are more sensitive to it may experience significant adverse reactions.