Needless to say, when reviewing the quality of the evidence, emphasis should be placed on well-structured and well-executed studies. The soundness of epidemiological studies has been strengthened when supported with measurements of the individuals' salt intakes. This has been done by deriving estimates of salt intake from measurements of daily sodium excretion in urine. Using this approach, cross-sectional epidemiological studies have been useful in establishing relationships between morbidity and dietary salt intake. This was done, for example, when comparing people in different nations with varying salt intakes. People in Japan have high salt intakes compared with Europeans, and studies which have included Japan have helped produce some important quantitative relationships between salt intake and disease risk. Longitudinal studies have been used where there has been a change in salt consumption within a country or community.
For example, salt intake in Finland has declined by one-third since the 1970s, and this has been associated with changes in the profile of cardiovascular parameters and disease (Karppanen and Mervaala, 2006).
To summarize, four types of epidemiological evidence have been used when setting standards for the upper limit of safe dietary salt intakes, as well as the relationship between salt intake and disease risk. They are assessments of clinical outcome by retrospective, prospective, cross-sectional or intervention approaches. In retrospective studies, the data relate to past events, whereas in a prospective study the data are collected from a given starting date and over a follow-up period. In cross-sectional studies the data refer to one point in time. Retrospective, prospective and cross sectional studies are observational (meaning that there is no manipulation from the researcher). In intervention (or experimental) studies there is manipulation of the situation and the consequences are tested using data obtained either before the intervention or from a contemporary group that was not manipulated.
List the relative advantages and disadvantages of these four types of epidemiological study in the determination of the upper safe limit for dietary salt intake:
You will now consider the findings of some epidemiological studies that have been carried out concerning salt consumption and a range of health conditions.