linguistic:translation, stylistic:style_transfer
Plant and animal physiology

Liebig's work on applying chemistry to plant and animal physiology was especially influential. By 1842, he had published Chimie organique appliquée à la physiologie animale et à la pathologie, published in English as Animal Chemistry, or, Organic Chemistry in its Applications to Physiology and Pathology, presenting a chemical theory of metabolism. The experimental techniques used by Liebig and others often involved controlling and measuring diet, and monitoring and analyzing the products of animal metabolism, as indicators of internal metabolic processes. Liebig saw similarities between plant and animal metabolism, and suggested that nitrogenous animal matter was similar to, and derived from, plant matter. He categorized foodstuffs into two groups, nitrogenous materials which he believed were used to build animal tissue, and non-nitrogenous materials which he believed were involved in separate processes of respiration and generation of heat.

French researchers such as Jean-Baptiste Dumas and Jean-Baptiste Boussingault believed that animals assimilated sugars, proteins, and fats from plant materials and that animals could not synthesize complex molecules. Liebig's work suggested a common ability of plants and animals to synthesize complex molecules from simpler ones. His experiments on fat metabolism convinced him that animals must be able to synthesize fats from sugars and starches. Other researchers built upon his work, confirming the abilities of animals to synthesize sugar and build fat.

Liebig also studied respiration, at one point measuring the "ingesta and excreta" of 855 soldiers, a bodyguard of the Grand Duke of Hessen-Darmstadt, for an entire month. He outlined an extremely speculative model of equations in which he attempted to explain how protein degradation might balance within a healthy body and result in pathological imbalances in cases of illness or inappropriate nutrition. This proposed model was justifiably criticized. Berzelius stingingly stated that "this facile kind of physiological chemistry is created at the writing table". Some of the ideas that Liebig had enthusiastically incorporated were not supported by further research. The third and last edition of Animal Chemistry (1846) was substantially revised and did not include the equations.

The third area discussed in Animal Chemistry was fermentation and putrefaction. Liebig proposed chemical explanations for processes such as eremacausis (organic decomposition), describing the rearrangement of atoms as a result of unstable "affinities" reacting to external causes such as air or already decaying substances. Liebig identified the blood as the site of the body's "chemical factory", where he believed processes of synthesis and degradation took place. He presented a view of disease in terms of chemical process, in which healthy blood could be attacked by external contagia; secreting organs sought to transform and excrete such substances; and failure to do so could lead to their elimination through the skin, lungs, and other organs, potentially spreading contagion. Again, although the world was much more complicated than his theory, and many of his individual ideas were later proved wrong, Liebig managed to synthesize existing knowledge in a way that had significant implications for doctors, sanitarians, and social reformers. The English medical journal The Lancet reviewed Liebig's work and translated his chemical lectures as part of its mission to establish a new era of medicine. Liebig's ideas stimulated significant medical research, led to the development of better techniques for testing experimental models of metabolism, and pointed to chemistry as fundamental to the understanding of health and disease.

In 1850, Liebig investigated spontaneous human combustion, dismissing the simplistic explanations based on ethanol due to alcoholism.