Controversial conclusions from meta-analyses in nutrition are of tremendous interest to the public and can influence policies on diet and health. When the results of meta-analyses are the product of faulty methods, they can be misleading and can also be exploited by economic interests seeking to counteract unflattering scientific findings about commercial products.

The term meta-analysis was coined by Glass in the mid-1970s for a set of techniques designed to characterize and combine the findings of prior studies in order to increase statistical power, provide quantitative summary estimates, and identify data gaps and biases. When applied to studies conducted with similar populations and methods, meta-analyses can be useful. However, many published meta-analyses have combined the findings of studies that differ in important ways, prompting Eysenck to complain that they have mixed apples and oranges—and sometimes “apples, lice, and killer whales”—yielding meaningless conclusions.1