Tag: Mammalian metabolism

While excess carbohydrates are easily stored as fat in the human body, the reverse is a far more complex metabolic question that largely depends on the organism. For decades, it was considered a biochemical certainty that mammals could not convert fatty acids into carbohydrates, with only minor exceptions. A deeper look reveals the critical differences in metabolic pathways between animals, plants, and microorganisms.

In mammals, carbohydrates are stored primarily in the form of a complex, branched polysaccharide called glycogen. While plants store carbohydrates as starch, this form of energy is stored in the liver and muscles of mammals to serve as a readily available energy reserve. This system is critical for maintaining blood glucose levels and providing fuel for physical activity.

Over a million human deaths worldwide are attributed to poor nutrition annually, highlighting the critical importance of dietary components, including essential fatty acids like linoleic acid. Mammals are unable to synthesize this vital nutrient themselves, requiring it to be obtained exclusively from their diet, a phenomenon rooted in both specific enzyme deficiencies and long-term evolutionary adaptations.

According to genetic studies, the inability of certain animals to produce their own vitamin C is due to a mutation in the GULO gene. This gene encodes the final enzyme in the ascorbic acid biosynthesis pathway, rendering affected species dependent on dietary intake.

Over 90% of overall gluconeogenesis in humans comes from substrates like lactate, glycerol, and specific amino acids. However, not all organisms possess the metabolic pathway needed to synthesize glucose from fatty acid-derived acetyl-coA. This article explores which of the following organisms cannot convert acetyl-coA derived from fatty acids into glucose and why, focusing on the key differences in metabolic cycles.