Cholesterol, diet and energy budgetsEdexcel A-Level Biology A: Revision notes

Cholesterol is a lipid made by the liver and obtained from the diet. It is insoluble in blood plasma, so it is carried in lipoproteins, which are particles with a lipid core and a protein coat.

• Low-density lipoprotein (LDL) carries cholesterol from the liver to the body tissues, including the artery walls. High LDL means more cholesterol is deposited, increasing the risk of atheroma and so of cardiovascular disease (CVD).
• High-density lipoprotein (HDL) carries cholesterol from the tissues back to the liver for breakdown or excretion, so it is protective.

When analysing data, look at total cholesterol (a higher value usually means more risk) and the ratio of HDL to LDL (a higher proportion of HDL means lower risk). Blood cholesterol is measured in mmol dm⁻³.

The evidence for a causal relationship between blood cholesterol and CVD comes from several sources:

• Correlation: populations and individuals with higher total and LDL cholesterol have higher rates of CVD.
• Genetic evidence: people with familial hypercholesterolaemia have very high LDL and develop CHD early.
• Intervention trials: drugs that lower LDL (statins) reduce heart attacks and strokes.
• Mechanism: atheroma plaques contain cholesterol, and LDL is taken up into the artery wall.

A correlation alone does not prove causation, because other factors (smoking, blood pressure, age) also vary. Some people with high cholesterol never develop CVD and some with low cholesterol do, so the evidence is strong but not simple.

People use scientific knowledge to lower their risk of coronary heart disease (CHD).

• Diet: less saturated fat lowers LDL; less salt lowers blood pressure; more fibre and unsaturated fats help.
• Exercise: raises HDL, lowers blood pressure and helps maintain energy balance and healthy mass.
• Not smoking: carbon monoxide reduces oxygen carried by haemoglobin, nicotine raises heart rate and blood pressure, and smoking damages artery linings.

Obesity indicators:

• Body mass index (BMI) = mass (kg) ÷ height² (m²). Healthy is 18.5 to 24.9; 25 to 29.9 is overweight; 30 or above is obese. BMI cannot separate fat from muscle.
• Waist-to-hip ratio (WHR) = waist circumference ÷ hip circumference. A high ratio means fat is stored around the abdomen, which carries higher CHD risk (thresholds are usually given, about 0.85 for women and 0.90 for men).

An energy budget compares energy intake (from food, in kJ) with energy expenditure (basal metabolic rate plus activity and heat from digesting food).

• Intake = expenditure: body mass stays steady.
• Intake > expenditure: the surplus is stored as fat, so weight gain and eventually obesity.
• Intake < expenditure: stored fat is used for respiration, so weight loss.

Worked example: intake 11 500 kJ and expenditure 9 800 kJ per day gives a surplus of 1 700 kJ. If 1 kg of fat stores 37 000 kJ, the surplus over 30 days is 51 000 kJ, so about 1.4 kg of fat is gained.

Obesity raises the risk of CHD, high blood pressure and type 2 diabetes. Energy balance is restored by eating less energy or exercising more.

Vitamin C (ascorbic acid) is a reducing agent. The blue dye DCPIP is decolourised when it is reduced by vitamin C, so the volume of food extract needed to remove the blue colour shows how much vitamin C it contains.

1. Measure 1.0 cm³ of DCPIP into a test tube.
2. Add the juice from a burette or syringe, drop by drop, mixing after each drop.
3. The end point is when the blue colour just disappears.
4. Repeat to get concordant results and calculate the mean.
5. Run the same titration with ascorbic acid of known concentration to calibrate.

Concentration of the juice = (volume of standard ÷ volume of juice) × concentration of the standard. Control the DCPIP volume, temperature and freshness of the sample; vitamin C is lost by heating, light and storage. Coloured juices make the end point harder to judge.