Correlation and Causation–Nuts and Chocolate

Nuts are a big part of my diet.  It’s my habit to eat handfuls of nuts through the day, and a few times a week to incorporate almonds or cashews or peanuts into a main course.  Perhaps I should be cheering that a headline in ScienceDaily last week told us, “Nuts and peanuts (but not peanut butter) linked to lower mortality rates, study finds”.  So, is the study good news for me?  I can answer with assurance: “Probably.”  

The headline referred to this study, just published in the Journal of Epidemiology.  The researchers in Maastricht looked back at data from a Dutch survey on diet and mortality conducted from 1986-1997.  They found mortality rates of nut-eaters were 23% lower than people who reported eating no nuts.  23% lower mortality corresponds to 2½  years of life extension [how to calculate].  Threshold for the benefit was quite low at a few ounces per week, and more was not better.  (Past studies indicated that perhaps more is better.  I eat about 2 pounds of nuts in a week, perhaps over the top, because I like them and because I’m on a low-carb vegetarian diet.  There is no data in that range.)  Peanuts were found to be just as good as “tree nuts” (almonds, cashews, walnuts, Brazils, etc.) but peanut butter had no benefit whatever.

By itself, a finding like this is hard to translate into a dietary recommendation.  There are qualitative problems with methodology.  People are different, and a diet that is right for one person may be all wrong for another.  And if we eat more nuts, are we adding more calories?  Or are we eating less of something else?

There is also the quantitative problem of cross-correlations–correlation does not necessarily imply causation.  People who eat nuts are likely to be richer and better educated and more careful about their diets, likely to be eating less unhealthy snacks, less meat, less carbohydrates.  Any of these things could produce an incidental statistical association between nut consumption and longevity, with no indication that eating nuts confers a benefit.

Nut consumers were on average somewhat younger, leaner (in women), drank more alcohol, ate more vegetables and fruits, were less often hypertensive or never smokers (women), but were higher educated and more often used supplements, or postmenopausal hormone replacement therapy (HRT). Women with the highest nut consumption less often reported diabetes. [ref]


Cross-checking and corroboration

It’s common to correct with multivariate analysis, but multivariate analysis doesn’t work very well if there are more than a few variables, and it’s hard to know in advance which are the relevant ones.  Statistics ends up being an art as much as a science.

So the study gains credibility when previous studies, with different methodologies in different populations, come to the same conclusion.  There are several, the biggest and best of which are this one and this one.  With write-up of the new study, the authors include a “meta-analysis” of these past studies.  This is another layer of statistics which combines previous results to come up with a conclusion stronger than any one study could draw.  Meta-analysis is a pursuit that can keep a data geek happy and productive for weeks on end.  Happiness and productivity are both positively correlated with life expectancy ☺.

Though a self-identified stat geek, I confess to being unfamiliar with Cochran, Begg and Orsini; nevertheless a paragraph like this adds to the credibility of a study in my eyes:

In these analyses, the HR estimate for each study was weighted by the inverse of the variance of the log HR to calculate the summary HR and its 95% confidence interval (CI). Heterogeneity between studies was estimated using the Cochran’s Q test and I2 (the proportion of variation in HRs attributable to heterogeneity).  Publication bias was assessed by the Begg test.  In addition, we performed dose-response meta-analyses using generalized least squares regression described by Orsini et al. with restricted cubic splines (four knots, at 5th, 35th, 65th and 95th percentiles) to investigate potential nonlinearity in the dose-response relationship.

The multi-study meta-analysis results closely paralleled the present study.  All the diseases of old age were lower in people who ate nuts; cancer was marginally lower, and cardiovascular disease much lower.  Some of the older studies found less benefit for peanuts than for tree nuts, and beyond a few ounces a week, there was ambiguity about whether more nuts offered more benefit.



There is comparable statistical evidence for the benefit of eating chocolate!  For years, I have refused to take these studies seriously, figuring that they are funded by a consumer industry that is eager to rehabilitate its junk-food image.

And in fact, just last month there was a spoof done by a science journalist, intending to remind us how easy it is to lie with statistics.  The headline was “Slim by Chocolate”.  Here is  John Bohannon’s account of what he did, and what morals we should draw.  His main point is that we all should use our common sense and be skeptical of sensational health claims.  Who can argue with that?

But the topic he chose incidentally illustrated other points as well.  There are legitimate claims for health benefits from chocolate.  People are complicated, and no two bodies are alike.  Not only do foods affect our metabolisms differently, but even more various are the psychological effects of foods.  There are people who find a little bit of chocolate uniquely satisfying, and it helps them to eat a leaner, healthier diet in many other ways.  There are other people who find chocolate addictive, and the more chocolate they eat, the more they want.

I find it completely plausible that some people are better able to lose weight with chocolate than without.


The psychology of eating is the most individual thing about diets, and it plays an essential role.

Time after time, human psychological studies have demonstrated that for the great majority of people, will power is worse than useless in trying to control weight.  (Present company, of course, is excluded.  You and I both have perfect control over what we eat, regardless of what the statistics may say.)  People who set out to lose weight by adhering to a set of rules generally succeed for awhile, then well over 90% bounce back to a weight higher than they started.  Habits in themselves are hard enough to change; but in addition we have powerful and persistent homeostatic impulses whispering in our ears.  The body gravitates to a “set point” in weight and percentage of fat.  The most successful diets all manipulate those signals of craving and satiety with alterations to the body’s biochemistry.

We joke about pregnant women having aversions to some foods and cravings for others.

Methionine is an essential amino acid, and it is normally part of all protein that we eat (though some sources have more than others.)  There have been a lot of studies of methionine restriction, in which mice are fed an artificial diet of re-constituted proteins in which this one amino acid is missing.  Despite the fact that methionine has no distinctive flavor or smell, mice know at some level that they are missing methionine.  Some researchers report that the mice refuse to eat unless there is methionine in their food.

The moral of the story is that our bodies know what they want, and will nag at us until they get it.

For some people, phytochemicals in chocolate can play a positive role in regulating gut biota and controlling anxiety that can lead to nervous eating and other destructive behaviors. This new study comes from University of Aberdeen in Scotland, and was published last week in the British Medical Journal.  All-cause mortality was not compiled, but the study claims that eating more chocolate is associated with less cardiovascular disease, and the group with highest chocolate consumption enjoyed 23% less heart disease.  Here is a meta-analysis of studies in the past that have been less clear and consistent than for nuts.  The average is that people who ate the most chocolate had 25% fewer cardiovascular events compared to people who ate the least.  No studies have been done about cancer, or all-cause mortality.

Interestingly, the non–chocolate-eating group had the highest mean body-mass index, the highest percentage of participants with diabetes, and the highest levels of inactivity. On the other hand, “higher chocolate intake was associated with a higher energy intake, with lower contributions from protein and alcohol sources and higher contributions from fat and carbohydrates.” [ref]

Translation: chocolate eaters weigh less despite eating more.  Did Mr Bohannon stumble onto something that none of us expected?  Probably not.  Here are two studies [one, two] that find just what we would expect, that eating chocolate is associated with weight gain.


The bottom line

People who eat nuts and chocolate have lower rates of cardiovascular disease and live longer than people with comparable amounts of body fat who don’t.  If you can adjust your diet to add chocolate and nuts without gaining weight, you will probably benefit.  Remember always that diets are individual and the response of your own body is not the average response.

This is the era of big data.  We are awash in data.  What fun for people like me, who love to extract meaning from numbers!  Still, answers to basic questions remain elusive.  Finding correlations between single foods and particular diseases is a start.  But researchers might remember that our goal is to design diets and life styles that are healthy and adapted for each individual.  We have a long way to go.  More creative and ambitious study designs for the future might help.  I’ll have two examples for you next week.

15 thoughts on “Correlation and Causation–Nuts and Chocolate

  1. What do you make of the lack of positive effect for peanut butter? It doesn’t make a lot of sense, given that grinding a nut shouldn’t change its properties much. Unless people in the study were mainly eating very sugary or salty PB? But nuts can be very salty, too.

    • Hi Matt, An apple will keep a long time; but when it is cut in two, it begins to turn brown and oxidize within the hour. Nuts are the same way. As long as they remain whole, their skin protects them. When peanuts are ground up, they oxidize; this is not good. In general processed foods are not as healthy as whole foods.
      As for chocolate? “Dear Lord, please give me the strength to open an extra large chocolate bar with nuts, and not eat the whole thing at once.” Cheers

  2. Here is one thing I really never understood about all the studies and urban myths about how good chocolate is. Why no one talks about the key ingredient in chocolate, the beans of Cacao tree (Theobroma cacao). Chocolate is just a mix that other than the powder obtained from those beans contains a lot of crap like tons of sugar and vegetable fats, most of them transfats. It s just hilarious to see educated ppl falling for this chocolate benefits saying: Oh I need to eat more chocolate because it is healthy when it is obvious that it s only about one ingredient that may be healthy if those studies are right. Is the craving for chocolate so high that can blind even the most iluminated minds? Shouldn t the health recomandations sound like: Eat more cacao insted and not: Eat more cacao mixed with lots of suger and vegetable fats?
    Maybe I am missing something here so please correct me if I am wrong. And don t get me wrong, I love chocolate, I would eat only chocolate every day and nothing else.
    About nuts, I know that Brasilian nuts have a high level of methionine and since than I avoided nuts considering that most nuts have high amounts of methionine. I should check again the nutritional tables to see the facts for the other kinds of nuts.

    • I eat 100% cacao/cocoa 3 spoons daily in my special shakes(xylitol, frozen blueberry, frozen strawberries, collagen-high glycine low methionine-, and organic vegetable protein).

      The longest lived human ever by far ate about 2 pounds of chocolate a week, that might or might not have had anything to do with it. But it is a quantity of consumption very rarely achieved by humans, and this is a rare specimen she died way later than most other supercentenarians, and from what I hear, if I’m not mistaken, the cause of death was choking not natural death but accidental one.

      As for nuts, I eat and would recommend macadamias, they’re very low in protein and high in monounsaturated fat and have omega-7 fats.

  3. That’s a good point, Dan. I wonder if any of these studies took the trouble to ask people what brand chocolate they ate so they could separate the effects of trans fats.

    Hershey bars don’t have trans fat
    Mars chocolate (M&Ms, Snickers, etc) don’t have trans fat.
    Nestles chocolate doesn’t have trans fat

    I don’t doubt that many of the cheaper brands have trans fats.

    – Josh

    • Unfortunately, Hersheys and Nestle began adding Polyglycerol polyricinoleate (PGPR) to save money and use less cocoa butter in their chocolate. There are still purer brands out there.

  4. Peanut butter may be less healthy than whole nuts due to oxidation at least during grinding if not storage (it is so thick, I wonder if inner layers still oxidize once it is butter?). Also there is frequently other dreck added to a lot of commercial brands – here’s what showed up on google search for “Jiffy Peanut Butter.”

    Creamy Peanut Butter – Jif Peanut Butter

    (This and the next Google item did not link to an active webpage on the Jif site so I copied the Google listing for its information.)

    I think most nuts are eaten roasted, and I wonder how roasting (and the oils used to roast) affects their health benefits…. So many questions, it’s hard to get at the truth.

    • Hi Karen,
      Speaking of ‘dreck’, I eat some every day! 🙂 Here in the Netherlands many brands and types of PB are for sale, but only one of them is felt to be standard and this brand is probably the one consumed by the vast majority of the Rotterdam study participants. I have a pot sitting here on my kitchen table. Label says: 85% peanuts, 13% vegetable fats: rapeseed oil, soy oil and hardened palm oil, 2 % salt.
      The fats are not further specified, though 7% saturated fat is mentioned elsewhere on the label. I never paid attention to this…I need to start a hunt for a dreckless peanut butter!
      Josh, I wonder how you read the table on resveratrol in PB on the Linus Pauling Institute website. For me, as an absolute layman, this reads as if PB has a lot less resveratrol than raw peanuts:

      • Peter –

        Table 2. Total Resveratrol Content of Selected Foods

        Peanuts (raw) 1 cup (146 g) 0.01-0.26mg
        Peanuts (boiled) 1 cup (180 g) 0.32-1.28mg
        Peanut butter 1 cup (258 g) 0.04-0.13mg
        Red grapes 1 cup (160 g) 0.24-1.25mg

        It’s hard to know how to interpret this. The mg of resveratrol varies widely (from .01 to .26 mg) even within the samples of raw peanuts. Why should boiled peanuts have so much more resveratrol than raw peanuts when the boiled peanuts are half water? Peanut butter seems to have a narrow range of values – higher minimum and lower maximum.

        My guess is that peanuts, like grapes, produce resveratrol when they are attacked by fungus. This would explain why some sources of peanuts, grown in dry areas, have very little resveratrol. A fringe benefit of high resveratrol may be that peanuts high in resveratrol are resistant to the infection that produces aflatoxin, a carcinogen and neurotoxin.

        All these quantities of resveratrol are tiny compared to the common supplement dosage of 100mg or more. There is puzzling and contradictory data on dosage of resveratrol, and we cannot assume that more is better.

        – Josh

      • Hallo Peter-

        I looked up health-food type stores – is Marqt close to you? (I love that the world is so much smaller nowadays!). May be a bit spendier than your normal grocery. Here in the USA I go to Whole Foods. Another option if you are brave and have a blender or food processor: buy the peanuts and make it yourself. I think making it is pretty easy, but cleanup may be a trick! ;^) That would likely give you the least “dreakful” peanut butter possible. (There are, of course, all sorts of tips about this on the Internet.)


  5. Danish health authorities are rasinig concerns about the possible contamination of cacao with cadmium. Eating more than 100g of dark chocolate a day will exceed the daily allowance of cadmium intake. Cadmium accumulates in the liver and kidneys and cause irreversible damages. I don’t wan to start fear mongering, but everything with moderation, also chocolate. As a side note: African chocolate apparently contains less cadmium than South American chocolate (mainly due to vulcanic activity)

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