The original study, published this week in the BMJ - British Medical Journal - outlines the entire protocol followed in the Swedish study that is quoted in the above article. Here's a key quote, by the researchers, on the limitations of their work (bold emphasis added):
Among the weaknesses of the study are concerns about misclassification of dietary exposures, particularly as diet was assessed at enrolment only and was self reported through a food frequency questionnaire, which, however, is a standard practice in large cohort studies. With respect to capturing seasonality of consumption, participants were enrolled in a time frame longer than one year, and we expect that seasonal variation was balanced and therefore largely accounted for. In any case, errors in the ascertainment of diet are generally not correlated with errors in the ascertainment of incidence of cardiovascular disease and, hence, are unlikely to generate substantial confounding. The long interval between exposure and outcome is a source of concern, because certain participants may change their dietary habits during the intervening period. However, this is more likely to generate non-differential misclassification and, thus, attenuate the evaluated association. In fact, we saw a tendency for the incidence rate ratios to decline with longer follow-up. Information on waist:hip ratio was missing for a large fraction of the cohort, and data on drugs for cardiovascular diseases were not available (the relevant registry was not operational during most of the follow-up period). Finally, we did not have data on blood cholesterol, an important risk factor for cardiovascular diseases, but even if such values were available, they would probably be, at least partly, intermediates in the association between diet and incidence of cardiovascular disease. As in all observational studies, residual confounding cannot be confidently excluded, but our control of potential confounders is as effective as that in other cohort studies with similar objectives.
They followed these women an average of 15 years, but only did a food survey ONCE, before the start of the study, in the early 90s. They did not have blood cholesterol data, nor did they track if any of the participants made any change to their dietary habits over the 15 year study.
That the media takes this study and essentially reports it as "fact" is just troubling.