Metabolism - How Your Cells Make Energy

One of the most common postpartum symptoms is a lack of energy. While some fatigue is certainly par for the course, debilitating fatigue – such that day after day you feel you cannot even get out of bed – is not. Some women say they are absolutely exhausted and yet cannot sleep at night, even while their babies are sleeping. We have found that many women who lack energy also complain about weakness in their muscles and a rapid heart rate. The good news is that many women who thought their fatigue was normal have been surprised at how much more energetic they can be with a few nutritional and lifestyle adjustments.

Let's go down to the cellular level and examine what happens there to drain your energy reserves. This may sound complicated at first, but hang in there and you will discover why it is so important to get the right nutrients to maintain your energy.

Where does your body get energy from? You probably know that the food you eat is metabolized, or burned," in your body to make energy. The foods you eat are broken down into their most basic components in your digestive system, or gastrointestinal tract. These basic nutrient components amino acids from protein foods, glucose (sugar) from carbohydrates, and fatty acids from fats are then absorbed into tiny blood and lymphatic vessels that line the intestines. The nutrients then either pass through the liver or circulate in the bloodstream until they are taken up by cells that need fuel. Vitamins, minerals, and other micronutrients (nutrients that take part in bodily processes but are not burned for energy) are absorbed and circulated in a similar way.

Important parts of the metabolic process then happen in microscopic power plants called mitochondria that exist in almost every cell in your body. About 2,500 mitochondria sit within each kind of cell in the body (except for red blood cells), and some cells can increase the numbers of mitochondria they have if the body perceives a need for more energy. For example, muscle cells create more mitochondria over time if you increase the energy demands on the muscles with an aerobic exercise program.

Energy is produced in the mitochondria by the breaking apart of the bonds that hold fuel molecules together. That energy is stored in the form of a molecule called adenosine triphosphate, or ATP, and is released as needed by the splitting apart of the ATP molecule into adenosine diphospate (ADP) and inorganic phosphorus. Think of ATP as the workhorse of the cell, supplying the energy for whatever cellular work needs to be done. For a muscle cell, this could be contraction; for an immune cell, it could be killing off bacterial invaders; for one of the cells that make up the intestinal lining, it could be bringing nutrients into and out of the bloodstream.This conversion of food to energy is driven by a series of chemical reactions driven along by enzymes ––complex molecules that regulate the rate of chemical reactions in the body. Micronutrients such as vitamins and minerals act as coenzymes and cofactors in the mitochondria, working alongside the enzymes to keep energy production going.

Figure 3.1 is a diagram that represents the metabolic processes that takes place in the mitochondria. It may look complicated at first, but as you read on, you will find that it is simpler than it looks.

Every fuel that goes into your body protein, fat, or carbohydrate is eventually transformed into a single substance, called acetyl coenzyme A (acetyl co–A) (5), before it is metabolized in the mitochondria. This allows these three different types of fuel to enter the same mitochondria1 energy making process. To become acetyl co–A, glucose (blood sugar) undergoes a process termed glycolysis (2), fats undergo beta–oxidation (3), and proteins undergo dearnination (1). The resulting acetyl co–A is a fuel that is transformed into ATP through the processes of the citric acid cycle and the electron transport chain. Once all of this has taken place, metabolic waste, in the form of carbon dioxide and water, is all that is left of the fuel that started out as (hopefully) a nutritious meal.

In glycolysis, molecules of glucose are transformed into substance called pyruvic acid, or pyruvate. This process does not require oxygen, and so is called anaerobic (without oxygen) metabolism. This transformation, which requires the presence of vitamins B1, (thiamin) and B3, (niacin), yields two units of ATP (physiological energy) and leaves behind two molecules of pyruvate for each available molecule of glucose. Glycolysis produces energy quickly but inefficiently. Anaerobic metabolism is like trying to keep a fire going with nothing but tiny twigs. The flames up quickly, but go out quickly. Glycolysis –– anaerobic energy production is often involved in the fight–or–flight reaction, a response to stress that mobilizes the body for a very fast expenditure of energy. The classic fight–or–flight example is that of a person who encounters a predator, such as a lion, in the wild –– and who must then immediately either run from or fight the danger. Anaerobic metabolism helps to provide the quick energy release that helps us to respond to emergency situations. After the emergency is over, the body should be able to return to aerobic metabolism for he majority of its energy.

Approximately 90 percent of the body's energy production should be aerobic, taking place within the mitochondria. Assuming the conditions are right, the end result of glycolysis –– pyruvate –– is first converted into acetyl co–A. (If the cells do not have enough oxygen available, however, they may convert pyruvate into another substance, lactic acid [lactate], which is usually involved in anaerobic energy production.) Acetyl co–A enters into the series of biochemical reactions known as the citric acid cycle, or Krebs cycle, and the electron transport chain (ETC). Here it is acted on by several enzymes and nutrient coenzymes to generate energy in the form of ATP. By the time the cycle has run its course, all that is left of the original molecule of glucose is carbon dioxide, water, and thirty–six units of ATP. This portion of the energy production process is aerobic –– in other words, it requires oxygen –– and it is obviously much more efficient than glycolysis, since it yields thirty–six ATP units for each original glucose molecule, while glycolysis yields only two. In addition to oxygen, the citric acid cycle requires the presence of adequate amounts of certain nutrients, among them vitamins B1, (thiamin), B2, (riboflavin), and B3, (niacin); lipoic acid; pantothenic acid; the minerals iron, magnesium, manganese, phosphorus, and sulfur; and the amino acids arginie, aspartic acid, cysteine, glutamic acid, glutamine, histadine, isoleucine, methionine, phenylalanine, proline, tyrosine, and valine. The electron transport chain, which is the other energy (ATP)–producing metabolic pathway within the mitochondria, helps to produce another three units of ATP. This process requires the presence of coenzyme Ql0, magnesium, zinc, and vitamins B2, B3, C, and K.

Another nutrient, called carnitine, serves as a sort of shuttle for fatty acid molecules, transporting them cross the membranes that surround the mitochondria so that they can be transformed into acetyl co–A and used for aerobic metabolism. Carnitine is made in the body from the amino acids lysine and methionine, with the help of iron and vitamins B2, B6, and C.