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2.1 Dietary scenarios
In many cases, a risk-benefit question explicitly involves comparison of two scenarios – a reference scenario (e.g. the current diet) and an alternative scenario in which the diet is changed, for example by introduction of a new policy, new product, or new advice to consumers. In other cases, the risk-benefit question may relate only to a single scenario, e.g. what is the balance of risks and benefits for a particular dietary scenario (often, the current diet). In practice, assessing a single scenario is equivalent to a two-scenario question comparing the diet in question to an implicit reference diet in which all of the risks and benefits are reduced to zero (even though such a diet may not be realistically achievable or desirable). This point is discussed further in the BRAFO approach (Hoekstra et al., submitted). The Qalibra framework requires specification of two scenarios, but is equally applicable to one-scenario questions (by setting all intakes to zero for the reference scenario).
In assessments of new policies, products or consumer advice, it is essential to be clear what assumptions will be made regarding the effectiveness or uptake of these, and this must be taken into account in defining the dietary scenarios.
It is also essential to be clear whether the scenarios take dietary substitution into account. For example, when someone eats more fish it usually means they will eat less meat or vegetables. The substitution may affect intake of contaminants or nutrients other than those present in the food of primary interest (e.g. fish), and may also cause changes in calorie intake. All of these things may affect health and ideally should be taken into account. However, often this is problematic and broadens the scope of the assessment dramatically, to include modelling foods and health effects other than those which are the primary focus of the risk-benefit question. If these are not included, their potential relevance and importance should be considered as part of the uncertainty analysis at the end of the assessment (see later).
Note that generally, it is not sufficient to consider only the change in intakes between the two assessment scenarios. Because dose-response functions are often non-linear, total exposure is important. Thus exposure through other sources than those directly described in the scenarios cannot be ignored. Therefore, it is often necessary to estimate background exposure and combine this with the sources of primary interest.
Note that changing dietary scenarios may have many other types of consequence in addition to health impacts, e.g. economic, cultural or legal consequences, and it is legitimate and appropriate to consider these in decision-making. The Qalibra framework addresses only the assessment of health impacts.