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Diet, energy and how to read a health claim questions
Energy balance, basal metabolic rate and what sets it; the energy value of carbohydrate, protein, fat and alcohol; what a balanced diet must supply, including essential amino acids, essential fatty acids, vitamins, minerals and fibre; BMI and its limits; cohort, case-control and randomised designs and what each can show; confounding, reverse causation and measurement error in dietary studies; relative against absolute risk; and a way of reading a health headline that survives contact with the study behind it.
5 original questions · 19 marks · the diet, energy and how to read a health claim notes · Nutrition, digestion and health evidence
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A cross-sectional survey of 40 000 adults reports that those taking a vitamin supplement have a 12 per cent lower rate of heart disease than those who do not. A newspaper concludes that the supplement protects the heart. Evaluate that conclusion.
Mark scheme
- B1 in support: the sample is large, so random error is small and the association is unlikely to have arisen by chance alone
- B1 in support: an association of this kind is a reasonable hypothesis to generate, and it can be tested directly by a design that settles the question
- B1 against: the design is cross-sectional, measuring a population at one moment, so it cannot say which came first. People already diagnosed may have started or stopped taking supplements because of the diagnosis
- B1 against: nobody allocated the supplement, so the two groups differ in many ways at once; people who take supplements tend to smoke less, exercise more and earn more, and a relative risk of 0.88 is close enough to 1 to be produced by residual confounding alone
- B1 judgement: the conclusion is not supported by this study, which reports an association as though it were a cause; what would settle it is a randomised controlled trial of the supplement against a placebo, since randomisation balances the confounders nobody measured
In a population, 4 people in every 100 develop a particular cancer during their lifetime. A cohort study reports a relative risk of 1.25 for that cancer in people with a high intake of one food. Calculate the absolute lifetime risk for those people, calculate the increase in absolute risk in percentage points, and calculate how many people would have to eat that way for one extra case to arise.
Mark scheme
- M1 the absolute risk in the exposed group is the baseline absolute risk multiplied by the relative risk
- A1 4.0 × 1.25 = 5.0 per cent, so about 5 people in every 100 rather than 4
- A1 the increase in absolute risk is 5.0 − 4.0 = 1.0 percentage point
- M1 the number needed for one extra case is 1 ÷ 0.010 = 100 people
Compare a cohort study with a case-control study, referring to who is recruited, what measure each produces, and the main weakness of each.
Mark scheme
- B1 a cohort recruits people who are free of the disease and follows them forward in time, whereas a case-control starts with people who already have the disease and compares them with matched people who do not
- B1 a cohort records exposure before the disease appears, so the time order is right by construction, whereas a case-control asks both groups about past exposure and cannot establish which came first
- B1 a cohort gives incidence and a relative risk, whereas a case-control gives an odds ratio, because the investigator chose how many cases and controls to recruit and so cannot know the risk in either group
- B1 a cohort needs very large numbers and many years and is still open to confounding, whereas a case-control is quick and efficient for a rare disease but is open to recall bias and to unfair choice of controls
A cohort study finds that adults who reported eating the least fruit were later diagnosed with cancer more often than those who reported eating the most. Suggest two explanations for this association other than a protective effect of fruit, and suggest for each what the investigators could do to test it.
State the conditions under which basal metabolic rate is measured, and state what a person's basal metabolic rate mostly depends on.
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