A Good Example of Failing to Control for Calorie Intake

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Calorie restriction has a such a large impact on health that you almost have to disregard any study of health and longevity in laboratory animals that fails to control for it. Even mild differences in levels of calorie intake can swamp out the effects actually being studied. In humans calorie restriction doesn't have the same dramatic effect on longevity as it does in mice - we'd have noticed by now - but it does produce a dramatic improvement in measures of health. So it is probably past time that we look with suspicion on any study that fails to account for levels of calorie intake.

This work seems like a good example of the type, as the researchers examined dietary habits that most likely correlate strongly with overall calorie intake, but did not control for calorie intake in the analysis:

Study participants were adults aged 35 years or over within the Health Survey for England (HSE). Since 2001, HSE participants have been asked about fruit and vegetable consumption on the previous day. Cox regression was used to estimate hazard ratios for an association between fruit and vegetable consumption and all-cause, cancer and cardiovascular mortality, adjusting for age, sex, social class, education, body mass index, alcohol consumption and physical activity.
We found a strong inverse relationship between fruit and vegetable consumption and all-cause mortality which was stronger when deaths within a year of baseline were excluded and when fully adjusting for physical activity. Seriously ill individuals may eat less due to illness-induced anorexia, or perhaps those with chronic illness receive more health advice and may therefore consume more fruit and vegetables. By excluding deaths within a year of baseline, we attempted to address reverse causality.

Fruit and vegetable consumption was significantly associated with reductions in cancer and cardiovascular disease mortality, with increasing benefits being seen with up to more than seven portions of fruit and vegetables daily for the latter. Consumption of vegetables appeared to be significantly better than similar quantities of fruit. When different types of fruit and vegetable were examined separately, increased consumption of portions of vegetables, salad, fresh and dried fruit showed significant associations with lower mortality. However, frozen/canned fruit consumption was apparently associated with a higher risk of mortality.

Link: http://jech.bmj.com/content/early/2014/03/03/jech-2013-203500.full