Nutrition (8 page)
NB Based on UK dietary reference values for energy, updated in 2011
* = Mixed breast/bottle feeding or unknown
A person who is maintaining their weight will have an energy intake that balances the amount of energy they expend as a result of their basal metabolic rate plus their physical activity. In general, someone who is slowly cutting back on energy intake will lose weight, while someone who consumes more food energy than they need will gain weight. If you cut back on food intake too drastically, however, your body switches to a ‘survival mode’ designed to improve your chances of survival during lean times. Your body’s use of energy becomes more efficient and less is wasted as heat, so weight loss slows.
The energy in food
Food contains different amounts of energy depending on its chemical structure. Molecules store potential energy in the bonds holding its atoms together. Your cells harness the energy stored in these chemical bonds by breaking them down and releasing energy as one molecule is converted into another.
The three food groups known as macronutrients are the main energy sources in food:
| Carbohydrate provides | 4 kcal (16.8 kJ) energy per gram |
| Protein provides | 4 kcal (16.8 kJ) energy per gram |
| Fat provides | 9 kcal (37.8 kJ) energy per gram |
Alcohol is also an important energy source for some people, providing 7 kcal (29.4 kJ) per gram – more than protein and carbohydrate, but less than fat.
How energy is released from food in the body
After carbohydrates, proteins and fats have been digested and absorbed, they are processed in a slow, complex, multi-step process that generates amazing quantities of energy sources: fatty acids, amino acids and glucose. These cell fuels are then ‘burned’ in tiny structures, found in almost every cell in the body, called mitochondria (single – mitochondrion). Mitochondria are the cellular equivalent of rechargeable batteries, and are found in all body cells except mature red blood cells, which have none.
CELL BATTERIES
Your mitochondria have their own double membrane and their own separate, genetic material that allows them to make the special enzymes needed to release energy from glucose and fatty acids. These enzymes act as triggers to encourage chemical reactions that would otherwise not occur, or would happen extremely slowly. Mitochondria also have their own protein-production units (ribosomes), and ‘reproduce’ by splitting in half (binary fission) just like bacteria. In fact, mitochondria are thought to have evolved from ancient bacteria that formed a symbiotic relationship with single-celled organisms in the primordial soup, soon after life first began on earth. The single-celled organisms benefited from gaining their own equivalent of fuel-injection engines, while the bacteria gained protection from the hostile, primordial environment outside the cell.
Mitochondria use oxygen, fatty acids and glucose to generate energy-rich storage molecules known as ATP (adenosine triphosphate). These packets of energy are then used to drive other metabolic reactions. The released energy can be converted into electrical energy (nerve conduction), into other chemical bonds or into power (contraction of muscle cells, movement of protein pores).
Most cells in your body can burn either fatty acids or glucose to generate ATP, with the exception of brain cells, which can only use glucose. Some cells such as red blood cells, kidney cells and sperm cells prefer to obtain most of their energy from the oxidation of glucose. Others, such as liver cells and exercising muscle cells, prefer to obtain most of their energy from the oxidation of free fatty acids when given the choice, as this is more energy-efficient. As soon as you start exercising, however, your muscle cells switch to using glucose, or their own stores of a starchy substance known as glycogen.
Because muscle cells need so much energy, they contain the highest concentration of mitochondria, and regular exercise can both multiply the number of mitochondria found in muscle cells and increase their size. This helps to give trained athletes increased strength and stamina as their energy reserves last longer.
Producing energy from glucose
Your cells have evolved a way to liberate the energy from glucose in a controlled way so that they do not burst into flames or explode during the process. Thankfully, stories of spontaneous combustion remain a myth as, instead, your cells break down each glucose molecule using a series of over twenty different chemical reactions, the rate of which is carefully controlled by metabolic enzymes. Many of these enzymes need help in the form of vitamins, minerals and coenzymes to work properly, which is why these micronutrients are so important for health.
The combination of glucose with oxygen (oxidation) releases energy plus two waste substances: carbon dioxide and water. Most energy is used to form energy-storage molecules (ATP) but some energy is given out as heat. The overall equation for the process, known as cell respiration, is:
Glucose + oxygen
carbon dioxide + water + energy
carbon dioxide + water + energyC
6
H
12
O
6
+ 6 O
2
6 CO
2
+ 6 H
2
O + energy
6
H
12
O
6
+ 6 O
2
6 CO2
+ 6 H
2
O + energy
The breakdown of glucose to release its energy involves three different, but closely linked, metabolic pathways: glycolysis, the citric acid (Krebs) cycle and the electron transport chain.
Glycolysis
Glycolysis occurs inside the cell fluid (cytoplasm) rather than in the mitochondria. It consists of a series of nine steps that produce two molecules of an end substance called pyruvate from every molecule of glucose (see
Diagram 3
). Pyruvate is a useful intermediary as it can be converted back into glucose, used to make fatty acids (for storage) and used to make an amino acid (alanine). More usually, however, it is converted on to acetyl-coenzyme A then fed into the next stage of the energy-making process known as the citric acid cycle.
Diagram 3
). Pyruvate is a useful intermediary as it can be converted back into glucose, used to make fatty acids (for storage) and used to make an amino acid (alanine). More usually, however, it is converted on to acetyl-coenzyme A then fed into the next stage of the energy-making process known as the citric acid cycle.
As you can see, glycolysis initially requires an investment of two molecules of ATP for each molecule of glucose to form an intermediary sugar (dihydroxyacetone phosphate). Once this sugar splits in half, however, each of the two halves generates two molecules of ATP as it is converted on to pyruvate, for a net gain of two ATP molecules. In addition, each half generates a hydrogen atom, which is absorbed by a special carrier molecule, NAD+ (nicotinamide adenine dinucleotide, derived from vitamin B3). This carrier feeds the hydrogen atom into the electron transport chain – to be discussed shortly – where it is processed to release another three molecules of ATP. In this way, one glucose molecule generates a total of eight packets of energy (ATP) during the initial glycolysis pathway.
PACKETS OF ENERGY
Each molecule of ATP contains two phosphate bonds which, when ‘broken’ or hydrolyzed, release energy for use in the body. ATP (adenosine triphosphate) readily releases its potential energy, in a controllable amount, when a phosphate molecule splits off to leave ADP (adenosine diphosphate). ADP is then usually converted straight back into ATP again, for the next round of energy-producing reactions. It is this regeneration of ATP for which fatty acids and glucose are essential. The reactions involved in energy storage and production are collectively known as cell respiration, and result in the production of carbon dioxide plus water.
Meanwhile, back in the glycolysis pathway, the next step depends on whether oxygen is readily available (for example, when walking gently), or in short supply (during vigorous exercise).
Anaerobic glycolysis
If oxygen is in short supply, pyruvate is processed to yield its remaining energy via a rapid process known as anaerobic glycolysis, or fermentation. This also occurs in the cell cytoplasm, and involves the simple conversion of pyruvate to lactic acid (also known as lactate). This quickly generates two packets of energy (ATP) from each molecule of glucose released from a muscle’s emergency stores of glycogen.
Although anaerobic fermentation provides energy at a fast rate, it is inefficient and produces much less energy than when oxygen is available. Muscle cells therefore only use anaerobic glycolysis when they absolutely have to, for example during vigorous exercise.
Interestingly, this process of anaerobic fermentation is thought to be one of the most ancient pathways in our metabolism, as life originally evolved in an atmosphere that lacked oxygen.
A COMMON CAUSE OF CRAMP
Lactic acid formed during this anaerobic fermentation enters the bloodstream and travels to the liver, where it is recycled back to glucose using oxygen. The need for extra oxygen to regenerate glucose in this way – known as the oxygen debt – is what makes you out of breath and gasping for air after brisk exercise. A buildup of lactic acid in exercising muscles can also trigger cramps. A ‘stitch’, for example, may involve cramping of muscle in the diaphragm, although this is controversial – a more recent idea is that a stitch is due to irritation of the peritoneal membrane lining the abdominal cavity.
When oxygen becomes plentiful again (for example, during your recovery period from exercise) muscle cells revert to the more efficient process of burning glucose with oxygen. To do this, a carrier takes pyruvate molecules from the cell fluid into the cells’ energy production factories, the mitochondria. Here, pyruvate is converted into another key substance, acetyl-coenzyme A. Remember these names – pyruvate and acetyl-coenzyme A – they are key molecules formed from the breakdown of fatty acids and protein, as well as from the breakdown of glucose.
Citric acid cycle
The citric acid cycle is arguably the most important series of metabolic reactions in your body. It is sometimes referred to as the tricarboxylic acid cycle, or the Krebs cycle, after the biochemist Sir Hans Adolf Krebs, who, in 1953, shared a Nobel Prize for its elucidation. This series of reactions takes place within the mitochondria of nearly all of your body cells.
Essentially, this cycle of chemical reactions takes acetylcoenzyme A (formed from the breakdown of carbohydrates, fat and protein) and releases the energy contained within its chemical bonds by oxidizing it completely to form carbon dioxide and water. All the macronutrients you eat – fats, proteins and carbohydrates – can be used to make energy via this route.
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