Epidemiology is the study of disease in populations. In this regard, epidemiology deals with associations. Associations are connections between two variables that may not necessarily be cause and effect. For example, fire engines are found at the scenes of accidents, and there is therefore an association between a fire engine and an accident. However, it would not right to say that fire engines cause accidents (although they may well do in certain limited circumstances). Associations are useful in epidemiology because they can be easily performed on large populations to help understand disease rates and the factors associated with them. In isolation such associations are not that useful, but taken with evidence from other types of studies such as clinical trials on humans, animal studies and cell culture studies, they can help form a picture of the nature of diseases within populations. Taking the evidence in such a holistic way makes epidemiological evidence useful in nutritional science.
Researchers have investigated the association between soft drink consumption and obesity many times, Generally the findings consistently show that increased soft drink consumption is a risk factor for weight gain and obesity. In other words, there is an association between drinking soft drinks and gaining weight. In particular, drinking soft drinks may increase the risk of developing abdominal obesity, characterised by weight gain around the internal organs of the viscera. This obesity is typified by the apple body shape, and is indicative of serious insulin resistance. For example, in one study, researchers investigated the association between soft drink consumption and abdominal obesity in Spanish men aged 25-74 years between 2000 and 20091. The results of the study showed that a 100 kcal increase in soft drink consumption was associated with a 1.1 cm increase in waist circumference. Therefore there was a significant dose response association between soft drink consumption and waist circumference in these Spanish men.
However, substitution of 100 kcal of soft drink consumption with 100 kcal of whole milk or 100 kcal of juice was associated with a 1.3 and a 1.1 cm reduction in waist circumference, respectively. Taking these result in isolation makes them difficult to interpret. However in combination with the many other human, animal and cell culture studies, they clarifies the already apparent picture that soft drinks are a causative factor in weight gain and obesity. That they should increase the abdominal fat mass, as measured by an increase in waist circumference, suggests that soft drinks are able to cause insulin resistance, which in turn increases the accumulation of fatty acids in the tissues around the central viscera. The reason for this is likely the high concentration of fructose or high fructose corn syrup in such drinks. Fructose is able to cause insulin resistance because it acts as a metabolic toxin in the liver, increasing fatty acid production in a similar way to alcohol.
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