Reviewing the Reviews

I’m Dr. Michael Greger and this is Nutrition Facts.

There’s one thing we’ve been thinking about a lot lately, and that’s how to stay healthy in the middle of a global pandemic. Especially since we’ve learned that those with underlying health problems like obesity, hypertension, diabetes, and heart disease are more likely to have serious complications if they contract COVID-19. So what do we do? We try to stay healthy with evidence-based nutrition.

Since Nutrition Facts is a service that focuses on the best available balance of evidence. we read a lot of reviews.  So today we take a moment to review the reviews. In our first story, the health effects of tea, coffee, milk, wine, and soda.

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.

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 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 wine numbers 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. 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.

Finally today, a review of reviews on the health effects of animal versus plant foods.

Instead of just looking at individual studies, or individual reviews of studies, what if you looked at a review of reviews? What did the data show for food groups?

The first thing the authors did was split it up into plant-based foods and animal-based foods. For the broadest takeaway, we can look at the totals. The vast majority of studies on whole plant foods show either protective or, at the very least, neutral effects, whereas most reviews of animal-based foods identified deleterious health effects or, at best, neutral effects.

Let’s break these down, though. The plant foods consistently rate uniformly well, reflecting the total, but the animal foods vary considerably. If it wasn’t for dairy and fish, the animal foods total would swing almost entirely neutral or negative.

So, the protective effects may be relative, arising not necessarily from what they’re consuming, but rather from what they’re avoiding. This may best explain the fish findings. After all, the prototypical choice is between chicken and fish, not chicken and chickpeas.

And, not a single review found a single protective effect of poultry consumption. Even the soda industry could come up with a 14% protective effects, but despite all the funding from the National Chicken Council, and the American Egg Board, chicken and eggs got big fat goose eggs.

Also, like the calcium in dairy, there are healthful components of fish: the long-chain omega-3 fatty acids. Not for heart health. In the most extensive systematic assessment of effects of omega-3 fats on cardiovascular health to date, increasing the fish oil fats had little or no effect on cardiovascular health. In fact, if anything, it was the plant-based omega-3s found in flaxseeds and walnuts that was protective. But the long-chain omega-3s are important for brain health. Thankfully, just like there are best-of-both-worlds non-dairy sources of calcium, there are pollutant-free sources of the long chain omega-3s, EPA and DHA, as well.

The bottom line is that when it comes to diet-related diseases such as obesity, type 2 diabetes, mental health, bone health, cardiovascular disease, and cancers, even if you lump all the animal foods together, and ignore any industry funding effects, and just take the existing body of evidence at face value, nine out of ten study compilations show that whole plant foods are, in the very least, not bad, whereas about eight out of ten of the reviews on animal products show them to be not good.

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To see any graphs, charts, graphics, images, or studies mentioned here, please go to the Nutrition Facts Podcast landing page. There you’ll find all the detailed information you need – plus links to all of the sources we cite for each of these topics.

For recipes, check out my How Not to Die Cookbook. It’s beautifully designed, with more than 100 recipes for delicious and nutritious meals. Speaking of new books, I have a new book just out – How to Survive a Pandemic – now out in audiobook, read by me, and e-book with physical copies out in August. Pre-order the physical copy now or download the e-book and audiobook now as well.

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Thanks for listening to Nutrition Facts. I’m your host, Dr. Michael Greger.