Why is it so difficult for scientists to study the effect of soft drink consumption on weight? People who drink lots of sugary soda or fruit punch may have other less-healthful habits that affect their weight, such as eating fast food or watching more television, so it can be difficult to tease out the effects of soft drink consumption alone.
Another challenge for researchers is that people often change their habits when they look in the mirror and see that they are overweight. If a study only measures soft drink habits at one point in time (what scientists call a cross-sectional study), it can't tease out cause and effect. That is, it might find that obese people are less likely to drink sugary soda, and miss the fact that drinking sugary soda led them to gain excess weight in the first place. This is almost certainly why nearly all cross-sectional studies show that obese people drink more diet soda.
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