What is Cardiovascular Disease?
Cardiovascular diseases (CVDs) are diseases of the heart and circulation. They are the main cause of death in the UK, accounting for over 200000 deaths a year, and over 60000 of these a premature deaths. The main forms of cardiovascular diseases are coronary heart disease (CHS), and stroke.
Why have a heart and circulation?
The heart and circulation have one primary purpose - to move substances around the body. In very small organisms, such as unicellular creatures, substances such as oxygen, carbon dioxide and digestive products are moved around the organism by diffusion. Diffusion is the movement of molecules or ions from a region of their high concentration to a region of their lower concentration by relatively slow random movement of molecules.
Most complext multicellular organism, however, are too large for diffusion to move substances around thier bodies quickly enough. These animals usually have blood to carry vital substances around their bodies and a heart to pump it instead of relying on diffusion. In other words, they have a circulatory system. Some animals have more than one heart, e.g. the earth worm has 5.
Open Circulatory System
In insects and some other animal groups, blood circulates in large open spaces. A simple heart pumps blood out into cavities surrounding the animal's organs. Substances can diffuse between the blood and cells. When the heart muscles relaxes, blood is drawn from the cavity back into the heart, through small valved openings along its length.
Closed Circulatory Systems
Many animals have a closed circulatory system in which the blood is enclosed within tubes. The generates higher blood pressures as the blood is forced along fairly narrow channels instead of flowing into large cavities. This means the blood travels faster and so the blood system is more efficient at delivering substances around the body:
- The blood leaves the heart under pressure and flows along arteries and then arteioles (small arteries) to capillaries.
- There are extremely large numbers of capillaries. These come into close contact with most of the cells in the body, where substances are exchanged between blood and cells.
- After passing along the capillaries, the blood returns to the heart by means of venules (small veins) and then veins.
- Valves ensure that blood flows only in one direction.
Animals with closed circulatory systems are generally larger in size, and ofter more active than those with open systems.
Close - Single Circulatory System
Animals with a closed circulatory system have either single circulation or double circulation. Single circulation is found, for example, in fish.
- The heart pumps deoxygenated blood to the gills.
- Here the gaseous exchange takes place; there is diffusion of carbon dioxide from the blood into the water that surrounds the gills, and diffusion of oxygen from this water into the blood.
- The blood leaving the gills then flows round the rest of the body before eventually returning to the heart.
Note that the blood flows through the heart once for each complete circuit of the body.
Closed - Double Circulatory System
Birds and mammals, though, have double circulation:
- The right ventricle of the heart pumps deoxygenated blood to the lungs where it receives oxygen.
- The oxygenated blood then returns to the heart to be pumped a second time (by the left ventricle) out to the rest of the body.
This means that the blood flows through the heart twice for each complete circuit of the body. The heart gives the blood retunring from the lungs an extra "boost", which reduces the time it takes for the blood to circulate around the whole body. This allows birds and mammals to have a high metabolic rate, because oxygen and food substances required for metabolic processes can be delivered more rapidly to cells.
Properties of Water - Ideal Transport Medium
Water is unusual among small molecules. It is a liquid at normal remperature; at room remperature most other small molecules, such as CO2 and O2, are gases. Water is a polat molecule; it has an unevenly distributed electrical charge. The two hydrogens are pushed towards each other forming a V shaped molecule; the hydrogen end of the molecule is slightly positive and the oxygen end is slightly negative because the electrons are more concentrated at that end. It is this polarity that accounts for many of this biologically important properties.
The positively charged end of a water molecule is attracter to the negative ends of surrounding molecules. This hydrogen bonding holds the water molecules together and results in many of the properties of water being liquid at room temperature.
The Heart & Blood Vessels
Key differences between ARTERIES & VEINS & CAPILLA
Narrow lumen, thicker walls, more collagen, elastic fibres and smooth muscle & no valves
Wide lumen, thinner walls, less collagen, elastic fibres and smooth muscle & valves
The capillaries that join the small arteries (arterioles) and small veins (venules) are very narrow, about 10um in diameter, with walls only 1 cell thick.
How does blood move through the vessels?
Everytime the heart contracts (systole), blood is forced into arteris and their elastic walls strech to accommodate the blood. During diastole (relaxation of the heart) the elasticity of the artery walls causes them to recoil behind the blood, helping to push the blood forward. The blood moves along the length of the artery as each section in series streches and recoils in this way. The pulsing flow of blood though the arteris can be felt anywhere an artery passes over a bone close to the skin.
By the time the blood reaches the smaller arteris an capillaries there is a steady flow of blood. In the capillaries this allows exchange between the blood and the surrounding cells through the one cell thick capillary walls. The network of capillaries that lies close to every cell ensures that there is rapid diffusion between the blood and surrounding cells.
The heart has a less direct effect on the flow of blood through the veins. In the veins blood flow is assisted bt the contraction of skeletal muscles during movement of limbs and breathing. Low pressure developed in the thorax (chest cavity) when breathing in also helps draw blood back into the heart from the veins. Backflow is prevented by valves within the veins. The steady flow without pulses of blood means that the blood is under low pressure in the veins.
How the Heart works:
The chambers of the heart alternately vontraact (systole) and relax (diastloe) in a rhythmic cycle. One complete sequence of filling and pumping blood is acalled a cardiac cycle, or a heart beat. During systole, cardiac muscle contracts and the heart pumps blood out through the aorta and pulmonary arteries. During diastole, cardiac muscles relaxes and the heart fills with blood.
Phase 1: Atrial Systole
Blood returns to the heart due to the actiom of skeletal and gaseous exchange (breathing) muscles as you move and breathe. Blood under low pressure flows into the left and right atria from the pulmonary veins and vena cava. As the atria fill, the pressure of blood against the atrioventricular valves pushes them open and the blood begins to leak into the ventricles. The atria walls contract, forcing more blood into the ventricles. This is known as atrial systole.
Phase 2: Ventricular Systole
Atrial systole is immediately followed by ventricular systole. The ventricles contract from the base of the heart upwards, increasing the pressure in the ventricles. This pushes blood up and out through the arteris. The pressure of blood against the atrioventricular valves closes them and prevents blood flowing backwards into the atria.
Phase 3: Diastole
The atria and ventricles then relax during diastole. Elastic recoil of the relaxing heart walls lowers pressure in the atria and ventricles. Blood under higher pressure in the arteries is drawn back towards the ventricles, closing the semilunar valves and preventing further backflow. The coronary arteries fill during diastole. Low pressure in the atria helps to draw blood into the heart from the veins.
Closing of the atrioventricular valves and then the semilunar valves creates the characteristic sounds of the heart.
What is Atheroscleriosis?
Artherosclerosis is the disease process that leads to coronary heart disease and strokes. In atherosclerosis fatty deposits can either block an artery directly, or increase its chance of being blocked by a blood clot (thrombosis). The blood supply can be blocked completely. If this happens for long, the affected cells are permanently damaged. In the arteries supplying the hear this results in a heart attack, in the arteries supplying the brain it results in a stroke. The supply of blood to the brain is restricted or blocked, causing damage or death to cells in the brain.
What happens in atherosclerosis?
1.) The endothelium, a delicate layer of cells that lines the inside of an artery, separating the blood that flows along the artery from the muscular wall, becomes damaged for some reason. For instance, this endothelial damage can result from high blood pressure, which puts an extra strain on the layer of cells, or it might result from some of the toxins from cigarette smoke in the bloodstream.
2.) Once the inner lining of the artert is breached, there is an inflammatory response. White blood cells leave the blood vessel and move into the artery wall. These cells accumulate chemicals from the blood, particularly cholesterol. A deposit builds up, called an atheroma.
3.) Calcium salts and fibrous tissues also build up at the sire, resulting in a hard swelling called a plaque on the inner wall of the artery. The build up of fibrous tissue means that the artery wall loses some of its elasticity; in other words, it hardens.
4.) Palques cause the artery to become narrower. This makes it more difficult for the heart to pump blood around the body and cna lead to a rise in blood pressure. Now there is a dangerous positive feedback building up. Plaques lead to raised blood pressure and raised blood pressure makes it more likely that further plaques will form.
Why does the blood clot in arteris?
When blood vessel walls are damaged or blood flows very slowly, a blood clot is much more likely to form. When platelets, a type of blood cell without a nucleus, come into contact with the damaged vessel wall they change from flattened discs to spheres with long thing projections. Their cell surfaces change, causing them to stick to the exposed collagen in the wall and to each other to form a temporary platelet plug. They also release substances that activate more platelets.
The direct contact of blood with collagen within the damaged artery wall also triggers a complex series of chemical changes in the blood. A cascade of changes results in the solubel plasma protein called prothrombin being converted into thrombin. Thrombin is an enzyme that catalyses the conversion of anothe soluble plasma protein, fibrinogen, into long insoluble strands of the protein fibrin.These fibrin strands form a tangled mesh that traps blood cells to form a clot.
Why do only arteries get atherosclerosis?
The fast flowing blood in arteries is under high pressure so there is a significant chance of damage to the walls. The low pressure in the veins means that there is less risk of damage to the walls.
The Consequences of Atherosclerosis
Coronary heart disease: Narrowing of the coronary arteris limits the amount of oxygen rich blood reaching the heart muscle. The result may be a chest pain caing angina. Angina is usually experienced during exertion. Because the heart muscle lacks oxygen, it is forced to respire anaerobically. It is thought that this results in chemical changes which trigger pain but the detailed mechanism is still not known.
It a fatty plaque in the coronary arteris ruptures, cholesterol is released which leads to rapid clot formation. The blood supply to the heart may be clocked completely. The heart muscle supplied by these arteris does not receive any blood, so it is said to be ischaemic (without blood). If the affected muscle cells are starved of oxygen for long they will be permanently damaged. This is what we call a heart attack or myocardial infarction.
Stroke: If the supply of blood to the brain is only briely interrupted then a mini-stroke may occr. A mini stroke has all the symptoms of a full stroke but the effects last only for a short period, and full recovery can happen quite quickly. However, a mini stroke is a warning of problems with blood supply to the brain that could result in a full stroke in the future.
1.2 - Who is at risk of cardiovascular disease?
Probability and Risk:
What do we mean by risk?
Risk is defined as "the probability of occurrence of some unwanted event or outcome". It is usually in the context of hazards, that is, anything that can potentially cause harm. Probability has a precise mathematical meaning and can be calculated to give a numerical value for the size of the risk.
You can calculate the chance that you will have an accident or succumb o a disease by simply rolling a dice. You will not necessarily suffer the accident or illness, but by looking at past circumstances of people who have taken the same risk, you can estimate the chance that you will suffer the same fate to a reasonable degree of accuracy.
Working Out Probabilities:
There are 6 faces on a die. Only one face has all 6 dots, so the chance of throwing a six is 1 in 6. Scientists tend to express 1 in 6 as a decimal 0.16 (recurring). In other words, each time you throw a standard die, you have about a 0.17 chance of throwing a one. When measuring risk you must always quote a time period for the risk.
Estamating risks to health:
In 2005, 19429 people in the UK died due to injuries or poisoning. The total UK population at the time was 60209408, so we can calculate the average risk in a year of someone in the UK dying from injuries or poisioning as:
19429 in 60209408 or 1 in 60209408/19429
= 1 in 3099 = 0.00032
Perception of Risk:
People frequently underestimate or over estimate risk. People may greatly overestimate some risks (such as the chances of contracting vCJD from blood transfusions) while underestimate others (such as the dangers of driving slightly faster than the speed limit or playing on a frozen lake).
There is a tendency to overestimate the risks of sudden imposed dangers where the consequences are severe, and to underestimate a risk if it has an effect in the long term future, even if that effect is severe, for example, the health risks associated with smoking or poor diet.
A useful distinction is sometimes made between risk and uncertainty. When we lack the data to estimate a risk precisely, we are uincertain about the risk. For example, we are uncertain about the environmental consequences of many chemicals.
Different types of risk factor:
In the UK the estimated risk of any one of us having fatal heart disease in any one year is about 1 in 600 compared to 1 in 1050 for a fatal stroke. However, these probabilities use figures for the whole population, giving averages which make the simplistic assumption that everyone has the same chance of having cardiovascular disease. This is obviously not the case.
The averages take no account of any risk factors - things that increase the chance of the harmful outcome. When assesing an individual's risk of bad health, all the contributing risk factors need to be established.
There are many different factors that contribute to health risks, for example:
Heredity, Physical environment, Social environemtn & Lifesyle and Behaviour choices.
Risk Factors for Cardiovascular Disease:
Identifying risk factors for CVD:
Two commonly used designs for this type of study are:
- Cohort studies - a group of people are followed over time to see who develops the disease.
- Case- Control studies - a group of people who have the disease are compared with a group with do not have the disease.
Cohort studies follow a group of people over time to see who develops the disease and who does not. During the study, people's exposure to suspected risk factos is recorded so any correlations between the risk factors and disease development can be identified. It maty take a long time for the condition to develop so these studies can take years and be very expensive.
Case Control Studies:
In a case control study a group of people with a disease (cases) are compared with a control group of individuals who do not have the disease. Information is collected about the risk factors that they have been exposed to, allowing factors that may have contributed to development of the disease to be identified.
The control group should be representative of the population from which the case group was drawn. Sometimes controls are individually matched to cases; known disease risk factors, such as age and sex, are similar in each case and control pair. This allows scientists to investigate the potential role of unknown risk factors. It should be noted that factors used to match the cases and controls cannot be investigated within the study, so it is important not to match on variables which could potentially turn out to be risk factors.
Features of a good study:
To identify correlations between risk factors and disease, studies need to be carefully designed.
Clear Aim: A well designed study should include a clearly stated hypothesis or aim. The design of the study must be appropriate to the stated hypothesis or aim and produce results that are valid and reliable.
Representative sample: A representative sample must be selected from the wider population that the study's conclusions will be applied to. Seleciton bias occurs when those who participate in a study are not representative of the target population.
Valid & Reliable results: Any methods used must produce valid data, from measuremenrs that provide information on what the study set out to measure.
Risk factors of CVD:
Your chances of having coronary heart disease or a stroke are increased by several inter-related risk factors, the majority of which are common in both conditions. These include:
- High Blood Pressure,
- Blood Cholesterol and other dietary factors,
- Genetic Inheritance,
Age & Gender make a difference:
The risk of cardiovascular disease is higher for men than women in the UK. CVD also increases with age; this may be due to the effects of aging on the arteris - they tend to become less elastic and may be more easily damaged. With increasing age the risks associated with other factors may increase, causing a rise in the number of cases of CVD.
High Blood Pressure:
Elevated blood pressure, known as hypertension, is considered to be one of the most common factors in the development of CVD. High blood pressure increases the liklihood of atherosclerosis occuring.
Blood pressure is a measure of the hydrostatic force of the blood against the walls of a blood vessel. You should remember that blood pressure is higher in the arteris and capillaries than in veins. The pressure in an artery is highest during the phase of the cardiac cycle when the ventricules have contracted and forced blood into the arteries. This is the systolic pressure. Pressure is at its lowest in the artery when the ventricles are relaxed. This is the diastolic pressure.
Measuring blood pressure:
A sphygmomanometer is a traditional device used to measure blood pressure. It consists of an inflatable cuff that is wrapped around the upper arm, and a manometer or gauge that measures pressure. When the cuff is inflated the blodd flow through the artery in the upper arm is stopped. As the pressure in the cuff is released the blood starts to flow through the artery. This flow of blood can be heard using a stethoscope positioned on the artery below the cuff. A pressure reading is taken when the blood first starts to spurt through the artery that has been closed. The is the systolic pressure. A second reading is taken when the pressure falls to the point where no sound can be heard in the artery. This is the diastolic pressure.
The SI units for pressure are kilopascals, but in medical practice it is traditional to use millimetres of mercury, mmHg.
Blood pressure is recorded as two numbers, one over the other. The systolic pressure is above the diastolic pressure. You would expect an average healthy person to have a systolic pressure between 100 and 140mmHg and a diastolic pressure of between 60 and 90mmHg.
What determines your blood pressure?
Contact between blood and the walls of the blood vessels cause friction, and this impedes the flow of blood. This is called peripheral resistance. The arterioles and capillaries offer a greater total surface area, restricting flow more, slowing the blood down and causing the blood pressure to fall. The fluctuations in pressure in the arteries are caused by contraction and relaxation of the heart. As blood is expelled from the hearm pressure is higher. During diastole, elastic recoil of the blood vessels maintains the pressure and keeps the blood flowing.
If the smooth muscles in the walls of an artery or an arteriole contract, the vessels constrict, increasing resistance, In turn, your blood pressure is raised. If the smooth muscles relax, the lumen is dilared, so peripheral resistance is reduced and blood pressure falls. Any factor that causes arteries or arterioles to constrict can lead to elevated blood pressure. Such factors included natural loss of elasticity with age, release of hormones such as adrenaline, or a high salt diet. In turn, blood pressure can lead to atherosclerosis.
Signs of High Blood Pressure:
One sign of high blood pressure is oedema, fluid building yp in the tissue and causing swelling. Oedema may also be associated with kidney or liver disease, or with restricted body movement.
At the arterial end of a capillary, blood is under pressure. This forces fluid and small molecules normally found in plasma out through the capillary walls into the intercellular spaces, forming the tissue fluid. The capillary walls prevent blood cells and larger plasma proteins from passing through, so these stat inside the capillaries.
If blood pressure rises above normal, more fluid may be forced out of the capillaries. In such circumstances, fluid accumulates within the tissues causing oedema.
Production of tissue fluid in a capillary bed:
Our choices in food, in particular the type and quality of high energy food, can either increase or decrease our risk of developing certain diseases, including cardiovascular diseases.
Carbohydrates, lipids and proteins are constituents of our food which store energy. Alcohol can also preovide energy.
Carbohydrates: The term means "hydrated carbon". If you look at each carbon in a carbohydrate molecule, you should be able to work out why, bearing in mind that hydration mean adding water.
Most people are familiar with sugar and starch being classified as carbohydrates, but the term covers a large group of compounds with the general foruma Cx(H20)n.
Sugars are either monosaccharides, simple sugar units, or disaccharides, in which two simple sugar unita hve combined in a condensation raction. Long straight or branched chains of sugar units for polysaccharides.
Monosaccharides are single sugar units with the general forumla (CH2O)n, where n is the number of carbon atoms in the molecule. Monosaccharides have between three and seven carbon atoms in the molecule,but the most common number is six. For example, the monosaccharids glucose,galactose and fructose all contain six carbon atoms and are known as hexose sugars.
A hexose sugar moleculae has a ring structure formed by five carbon atoms and an oxygen atom; the sixth carbon atom projects above or below the ring. The carbon atoms in the molecule are numbered, starting with 1 on the extreme right of the molecule. The side branches project above or below the ring, and their position determines the type of sugar molecule and its properties.
Monosaccharides provide a rapid source of energy. They are readily absorbed and require little or, in the case of glucose, no change before being used in cellular respiration. Glucose and fructose are found naturally in fruit, vegetables and honey; they are both used extensively in cakes, biscuits and other prepated foods.
Monosaccharide - Glucose
Glucose is important as the main sugar used by all cells in respiration. Starch and glycogen are polymers made up of glucose subunits joined together. When starch or glycogen is digested, glucose is produced. This can be absorbed and transported in the blood stream to cells.
Monosaccharide - Galactose
Galactose occurs in our diest mainly as part of the disaccharide sugar lactose, which is found in milk. Notice that the -OH groups on carbon 1 and carbon 4 lie on the opposite side of the ring compared with their position in glucose.
Monosaccharide - Fructose
Fructose is a sugar which occurs naturally in fruit, honey and some vegtables. Its sweetness attracts animals to eat the fruits and so help with seed dispersal.
Two singel sugar units can join together and form a disaccharide (double sugar) in a condensation reaction. A condensation reaction is so called because a water molecule is released as the two sugar molecules combine in the reaction. Condensation reactions are common in the formation of complex molecules. The bond that forms between two glucoses is known as a glycosidic bond or link.
Common disaccharids found in food are sucrose, maltose and lactose.
The glycosidic link between two sugar units in a disaccharids can be split by hydrolysis. This is the reverse of condensation: water is added to the bond and the molecule splits into two. Hydrolysis of carbohydrates takes place when the molecules splits into two. Hydrolysis of carbohydrates takes place when carbohydrates are digested in the gut, and when carbohydrate stores in a cells are broken down to release sugars.
Disaccharide - Sucrose
Sucrose, formed from glucose and fructose, is the usual form in which sugar is transported around the plant.
Disaccharide - Maltose
Maltose, formed from two glucose molecules, is the disaccharide produced when amylase breaks down starch. It is found it germinating seeds such as barley as they break down their starch stores to use for food.
Disaccharide - Lactose
Galactose and glucose make up lactose, the sugar found in milk.
The glycosidic bond between the two monosaccharides in Sucrose can be split by hydrolysis. In this reaction water is added.
Polysaccharids are polymers made up from simple sugar monomers joined by glycosidic links.
There are three main types of polysaccharide found in food: Starch and Cellulose in plants, and Glycogen in animals. Although all three are polymer of glucose molecules, they are sparingly soluble (they do not dissolve easily) and do not taste sweet.
Starh and glycogen act as energy storage molecules within cells. These polysaccharides are suitable for storage because they are compact molecules with low solubility in water. This means that they do not affect the concentration of water in the cytoplasm and so do not affect movement of water into or out or the cell by osmosis.
Strach, the storage carbohydrate found in plants, is made up of a mixture of two molecules, amy lose and amylopectin.
- Amylose is composed of a straight chain of between 200 and 5000 glucose molecules with 1,4 glycosidic links between adjacent glucose molecules. The position of the bonds causes the chain to coil into a spiral shapel
- Amylopectin is also a polymer of glucose but it has side branches. A 1,6 glycosidic link holds each side branch onto the main chain.
The compact spiral structure of starch and its insoluble nature makes it an excellent storage molecule. It does not diffuse across cell membranes and has very little osmotic effect within the cell.
Starch is a major source of energy in our diet, and is common in many foods. It occurs naturally in fruit, vegetables and cereals, often in large amounts. The sticky gel formed when starch is mixed with water makes it a good thickening agent and it is also added to many food products as a replacement for fat.
Bacteria, fungi and animals store glycogen instead of starch. Glycogen is another polymer composed of glucose molecules. Its numerous side branchs mean that is can be rapidly hydrolysed, giving easy access to stored energy. In humans gylcogen is stored in the liver and the muscles.
Cellulose in the diet is known as dietary fibre, and it is also referred to as a non- starch polysaccharide. Up to 10000 glucose molecules are joined to form a straight chain with no branches ( the glucose molecules have a slightly different structure to those found in starch). The structure of cellulose is considered in Topic 4.
Indigestible in the human gut, cellulose has an important function in the movement of material through the digestive tract. Dietary fibre is thought to be important in the prevention of "Western diseases" such as coronary heart disease, diabetes and bowel cancer.
Lipids enhance the flavour and palatability of food, making it feel smoother and creamier. They supply over twce the energy of carbohydrates, 37kj of energy per gram of food. This can be an advantage if large amounts of energy need to be consumed in a small mass of food. It also means a large amount of energy can be stored in a small mass, for example in seeds.
Lipids are organic molecules found in every type of cell; they are insoluble in water but soluble in organic solvents such as ethanol. The most common lipids that we eat are triglycerides, used as energy stores in plants and anim als. Triglycerides are made up of three fatty acids and one glycerol molecule linked by condensation reactions. The bond that forms between each fatty acid and the gycerol is known as an ester bond. Three ester bonds are formed in a triglyceride.
* A condensation reaction removes water from between the glycerol and fatty acids to form ester bonds.
If the fatty acid chains in a lipid contain the maximum number of hydrogen atoms, they are considered to be "saturated". In a saturated fatty acid, the hydrocarbon chain is long and straight.
There are no carbon to carbon double bonds and no more hydrogens can be added to it.
Straight, saturated hydrocarbonscan pack together closely. The strong intermolecular bonds between triglycerides made up of saturated fatty acids result in fats that are solid at room temperature.
Monounsaturated fats have one double bond between two of the carbon atoms in each fatty acid chain. Polyunsaturated fats have a larger number of double bonds. A double bond causes a kink in the hydrocarbon chain; these kinks prevent the unsaturated hydrocarbon chains packing closely together. The weaker intermolecular bonds between unsaturated triglycerides result in oils that are liquid at room temperature. Olive oil is particularly high in monounsaturated fats. Most other vegetable oils, nuts and fish are good sources of polyunsaturated fats.
Other types of lipid:
Cholesterol is a short lipid molecule. It is a vital component of cell membranes with roles in their organisation and functioning. The steroid sex hormones (such as progesterone and testosterone) and some growth hormones are produced from cholesterol. Bile salts, involved in lipid digestion and assimilation, are formed from cholesterol. For all these reasons, cholesterol is essential for good health. Cholesterol is made in the liver from saturated fats and also obrtained in our diet, found associated with saturated fats in foods such as eggs, meat and dairy products.