The dawn of nutrition: From Santorio's weight scale to Lavoisier's combustion theory

In the 17th century, the Italian medical professor Santoro believed that "matter is indestructible." At that time, the human body was considered a mysterious entity, beyond scientific comprehension. Santoro designed a large balance scale to weigh himself, recording his weight changes, as well as the weight of his food and excrement. He discovered that his actual weight was lower than calculated. He wondered if this was due to the water content in his sweat and exhaled breath. However, at that time, it was impossible to create a specialized device to measure the water excreted by the body.

In the 18th century, British chemist Christopher Cavendish (1731–1810) first discovered hydrogen (H). Subsequently, nitrogen (N) and oxygen (O) were discovered. At that time, it was also known that carbon dioxide (CO₂) was present in the air exhaled by humans. French chemist Antoine Lavoisier (1743–1794) was the first to propose the theory of combustion, that is, combustion is the result of a substance combining with oxygen. Lavoisier also made significant contributions to nutrition. It can be said that modern nutrition science began with him.

Lavoisier proved that carbon and hydrogen in food, through the action of oxygen in the human body, produce carbon dioxide and water, generating heat. He placed a groundhog in a box and measured the amount of carbon dioxide produced over 10 hours, finding that this amount was the same as the amount produced by burning 3.33 grams of carbon in a sealed box. Lavoisier concluded that the carbon dioxide exhaled by the groundhog over 10 hours was the result of the combustion of 3.33 grams of carbon within its body. Just as Lavoisier was conducting the same experiments on humans, the French Revolution broke out, and Lavoisier was unfortunately sent to the guillotine by the radicals. The law of conservation of matter proposed by Saint-Doriovese was proven to apply to living organisms almost 200 years later.

In 1816, French physiologist Jean-Jacques de Margent discovered that dogs fed only sugar and oil would soon die, but would survive if fed nitrogenous food. In 1838, Dutch chemist and physician Hermann Moirde studied the components of nitrogenous foods. He named it "Protein," using the Greek word "Proteinos," meaning the most important thing. Since then, the term "protein" has been used worldwide.

In 1834, the British physician Prawood, in his book *Digestive Functions*, elucidated the three basic elements of food: sweetness, oiliness, and protein. At that time, only three major nutrients were identified: carbohydrates, proteins, and lipids. The world-renowned German chemist Liebig (1803-1873) conducted extensive research, including in nutrition. He devised methods of organic chemical analysis to analyze various plant and animal bodies. Based on this analysis, he concluded that carbohydrates, proteins, lipids, and minerals were essential for humans.

This view quickly became the famous Liebig's theory of the four essential nutrients. Subsequently, nutritional research flourished worldwide, centered in Germany. The German scientist Werner, by adjusting the amounts of various nutrients in his animal feed, discovered that excessive nutrients did not necessarily maintain health. Therefore, through experiments, he calculated the necessary amounts of various nutrients for humans, concluding that an adult needs 137 grams of protein, 117 grams of lipids, and 352 grams of carbohydrates daily.

Osborn and Mendel of Yale University discovered that feeding rats a protein isolated from corn caused their growth to stop, and they soon died. However, adding tryptophan prevented growth, but the rats did not die. Adding lysine resulted in normal growth. This demonstrates that proteins come in various forms, with different amino acid compositions leading to different nutritional values. It also indicates that plant proteins generally have lower nutritional value than animal proteins.

Around 1935, W.C. Ross in the United States synthesized the amino acids that make up natural proteins. He fed these synthesized amino acids to animals in various combinations and observed their development. The results showed that there are 20 amino acids that make up natural proteins, of which 10 are essential for the development of rats and 8 are essential for human development. These essential amino acids for animals and humans are known as essential amino acids. At this time, the nutritional significance of proteins and essential amino acids was only recognized.