Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

One phrase you’ll hear repeatedly in my videos and books is “best available balance of evidence.” What does that mean? When making decisions as life-or-death important as to what to best feed ourselves and our families, it matters less what a single study says, but rather what the totality of peer-reviewed science has to say.

Individual studies can lead to headlines like this: “Study Finds No Link Between Secondhand Smoke and Cancer.”

To know if there’s really a link between secondhand smoke and lung cancer, it would be better to look at a review or meta-analysis that compiles multiple studies together. The problem is that some reviews say one thing––breathing other people’s tobacco smoke is a cause of lung cancer—and some reviews say another, saying the effects of secondhand smoke are insignificant, and further, such talk may foster “irrational” fears. And, hey, while we’re at it, you can even directly smoke four or five cigarettes a day and not really worry about it; so, light up.

Why do review articles on the health effects reach such different conclusions? Well, as you can imagine, about 90% of reviews written by tobacco industry-affiliated researchers said it was not harmful, whereas you get the opposite number with independent reviews. Reviews written by tobacco researchers had 88 times the odds of concluding secondhand smoke was harmless. It was all part of a deliberate corporate strategy to discredit the science––to, in their words, develop and widely publicize evidence that secondhand smoke is harmless.

Okay, well, can’t you just stick to the independent reviews? The problem is that industry-funded researchers have all sorts of sneaky ways to get out of declaring conflicts of interest. So, it’s hard to follow the money. But, even without knowing who funded what, the majority of reviews still concluded secondhand smoke was harmful. So, just like a single study may not be as helpful as looking at a compilation of studies on a topic, a single review may not be as useful as a compilation of reviews. So, looking at a review of reviews, like this one, can give you a better sense of where the best available balance of evidence may lie. In this case, it’s probably best not to inhale.

Wouldn’t it be cool if there were reviews-of-reviews for different foods? Voilà! An exhaustive review of meta-analyses and systematic reviews on the associations between food and beverage groups and major diet-related chronic diseases. Let’s start with the beverages. The findings were classified into three categories: protective, neutral, or deleterious. First up: tea versus coffee. In both cases, most reviews, for whichever condition they were studying, found both beverages to be protective. But you can see how this supports my recommendation for tea over coffee. Every cup of coffee is a lost opportunity to drink something even healthier: a cup of green tea.

No surprise, soda sinks to the bottom. But still, 14% of reviews mentioned protective effects of drinking soda!? Well, most were references to papers like this: a cross-sectional study that found that 8th grade girls who drank more soda were skinnier than girls who drank less. Okay, but this was just a snapshot in time. What do you think is more likely, that the fatter girls were heavier because they drank less soda, or that they drank less sugary soda because they were heavier? Soda abstention may therefore be a consequence of obesity, rather than a cause, yet it gets marked down as protective; there’s a protective association.

Study design flaws may also account for these wine numbers. This review of reviews was published back in 2014, before the revolution in our understanding of the evaporating health benefits of alcohol, suggesting that the presumed health benefits from “moderate” alcohol may have finally collapsed, thanks, in part, to a systematic error of misclassifying former drinkers as if they were lifelong abstainers, as I revealed in a deep dive in my latest video series on the subject. Sometimes there are unexplainable associations, though. For example, one of the soft drink studies found that increased soda consumption was associated with lower risk of certain types of esophageal cancers.

Don’t tell me—the review was funded by, Coca-Cola? The review was funded by, Coca-Cola! Does that help explain these positive milk studies? Were they all just funded by the dairy council? Even more conflicts of interest have been found among milk studies than soda studies, with industry-funded studies of all such beverages approximately “four to eight times more likely to be favorable to the financial interests of the [study] sponsor….”

Funding bias aside, though, there could be legitimate reasons for the protective effects associated with milk consumption. After all, those who drink more milk as a beverage may drink less soda, which is even worse; so, they come out ahead. But it may be more than just relative benefits. The soda-cancer link seems a little tenuous, not just because of the coke connection, but it’s hard to imagine a biologically plausible mechanism, whereas even something as universally condemned as tobacco isn’t universally bad. As I’ve explored before, more than 50 studies have consistently found a protective association with Parkinson’s, thanks to nicotine. Even secondhand smoke may be protective. Of course, you’d still want to avoid it. It may decrease the risk of Parkinson’s, but increases the risk of an even deadlier brain disease: stroke, not to mention lung cancer and heart disease, which has killed off millions of Americans since the first Surgeon General’s report was released.

Thankfully, by eating certain vegetables, we may be able to get some of the benefits without the risks, and the same may be true of dairy. As I’ve described before, the consumption of milk is associated with increased risk of prostate cancer, leading to recommendations suggesting men may want to cut down or minimize their intake. But milk consumption is associated with decreased colorectal cancer risk. This appears to be a calcium effect. Thankfully, we may be able to get the best of both worlds by eating high-calcium plant foods, such as greens and beans.

What does our review-of-reviews study conclude about such plant-based foods, in comparison to animal-based foods? We’ll find out, next.

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