As humans, we take the ability to transform food into energy for granted. Eating a chocolate bar while hiking to keep up stamina is seemingly a straightforward and mindless process. However, it relies on a series of very complex biochemical reactions, all interlocking into each other as gears churning away in an engine blowing at full speed.
Most cells in the body can start from common sugar – glucose -and turn it into pyruvate, a small molecule that functions as an intermediary in the cell. The reaction that transforms glucose into pyruvate is known as glycolysis and constitutes one of the most essential building blocks of cellular biochemistry. Once pyruvate is synthesized, it enters a strict series of reactions known as the Citric Acid Cycle (also known as the Krebs Cycle), which uses up pyruvate and oxygen to make carbon dioxide (CO2) and chemical energy (which is released in the form of the small molecule ATP). The Krebs Cycle is the reason why we breathe in oxygen and breathe out carbon dioxide and it takes place within a special compartment in the cell known as a mitochondrion. Mitochondria are often known as the “power plant of the cell” due to their role as hosts of the Krebs Cycle and producers of all chemical energy in the cell. This whole process is known as respiration and is essentially the biochemical groundwork for the mundane business of breathing.
When oxygen is not available, for example in the muscle tissue of an athlete that is straining themselves to the limit, normal respiration is not possible. However, animals have developed an alternative mechanism to keep producing energy even when the oxygen supply in on the low. This is known as anaerobic respiration and it ends up producing chemical energy (always in the form of the small molecule ATP) and lactate. Anyone who has ever exercised knows the feeling of lactate (also known as lactic acid) accumulating into the muscles. The problem with anaerobic respiration is not only the accumulation of lactate, but also how inefficient it is. Regular respiration can produce 32 molecules of ATP out of a single molecule of pyruvate. However, anaerobic respiration only produces 2 ATP molecules per pyruvate molecule, making the process 16 times less efficient.
Bizarrely, cancer cells have been shown to re-direct their metabolism towards anaerobic respiration, even though it is dramatically less efficient than regular respiration. They do so by radically altering the genetic blueprint that makes up the “assembly instructions” of the cell. This means cancer cells need to consume much more glucose in order to produce the same amount of energy than regular cells. This is the reason behind the popular-science factoid that “cancer eats sugar”. It’s important, however, to understand that even though cancer cells are “addicted to sugar” a low-sugar diet is not in any way a cure for cancer. Cancer cells often thrive in low-oxygen conditions (known as hypoxia), where anaerobic respiration is essential to survive. Reducing diet sugar intake does not starve cancer cells individually, but address the whole organism (including the immune system cells that are supposed to be fighting the cancer). However, huge progress has been made in recent years in developing drugs that target the re-directed metabolism of cancer cells.