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Home » Coffee health effects – what does the best science say

Coffee health effects – what does the best science say

Coffee is one of the most consumed beverages worldwide, with tea being number one. And as I have mentioned previously, I am an unrepentant coffee lover. Over the years, there have been a number of claims about coffee health effects, both positive and negative, many without any solid scientific evidence in support.

Claims about coffee health effects goes back centuries. These claims were often confusing and contradictory. How many “studies” have we read about that said drinking it was good for your heart. Or bad for your heart. Or it prevented cancer. Or it increased your risk of cancer.

Part of the confusion is that the popular press, with its strange dependence on false equivalence, often presents two contradictory scientific studies as equivalent, even if they aren’t. Well, we’re going to look at a powerful new study that examined health outcomes that can be related to coffee. Let’s see what they say.

The coffee health effects study

When you have a lot of different studies about coffee health effects, the best way to quickly sort through them is to look at a meta-analysis or systematic review, considered to be the pinnacle of the hierarchy of scientific research. Basically, these type of studies remove bias, poorly designed studies, and other low quality data points, then roll up the best data to answer questions. Personally, I try to use systematic reviews, as opposed to cherry picked primary research studies, to support any scientific statement I make.

Well, now we have a meta-analysis of coffee health effects, published in BMJ and written by Robin Poole. If you’re going to make a claim about coffee, then using a high quality meta-analysis published in one of the leading medical journals in the world is a good start.

This study is an “umbrella review” of over 200 primary studies and other systematic reviews. It’s like the meta-analysis of meta-analyses. Yeah, it is a thing, and it is a very powerful methodology to find evidence for coffee health effects, both negative and positive.

The results

There are pages of data in this study, most of which shows no harm or benefit from coffee.  However, the data does seem to show that drinking about 3-4 cups of coffee a day is more likely to benefit our health than it is to harm it.

This same research group that published the new study in BMJ has previously published studies about coffee drinking and liver disease, which were included in this new systematic review. They found that there was a lower risk of cirrhosis and cancer of the liver in individuals who drank more coffee.

Before we start drinking coffee by the liter, the meta-analysis could not establish causality. For example, it’s possible that coffee drinkers consume less alcohol or other things that may impact the liver. On the other hand, maybe coffee has some unknown protective effect. These studies just don’t give us this information.

Moreover, most of the data in the analysis came from observational studies, which are not as powerful as clinical trials. As a result, the research team plans to “to conduct a randomised controlled trial in patients with non-alcoholic fatty liver disease to see if coffee works as a treatment to reduce the risk of the disease progressing.”

Seriously, that will be a very important study if it confirms the epidemiological data that seems to show that drinking coffee has some benefit to liver disease.

But if you’re going to make a claim of a benefit, it’s important to determine whether there are potential harms. This is one of the important pieces of information to determine if there is clinical importance to claims of beneficial coffee health effects.

Here are three critical graphs that show much of the critical data from this study.

Fig 2 High versus low coffee consumption and associations with multiple health outcomes. Estimates are relative risks and effect models are random unless noted otherwise. For type 2 diabetes, P value was significant for non-linearity. No of events/total for leukaemia could not be split from other outcomes. All estimates were from our own reanalysis apart from preterm birth in first and third trimester and leukaemia.

Fig 3 Any versus no coffee consumption and associations with multiple health outcomes. Estimates are relative risks and effect models are random unless noted otherwise. All estimates were from our own reanalysis apart from acute leukaemia, urinary tract cancer, and colorectal cancer.

Fig 4 Consumption of one extra cup of coffee a day and associations with multiple health outcomes. Estimates are relative risks and effect models are random unless noted otherwise. No dose response analyses were re-analysed.

These three graphs show the top 10 most harmful and 10 most beneficial effects of various levels of coffee consumption, including none at all. As you can see, almost all of the harmful and beneficial effects are barely different than unity, meaning no effect whatsoever.

But there is information that we can glean from the data that is interesting. First of all, from data not presented in these graphics, studies showed about a 10-12% decrease in all-cause mortality as a result of drinking 3-4 cups of coffee per day. Now, that’s not a huge difference, and we don’t know if coffee is the causal factor, but it is extremely interesting. It’s much better than reading that there is a 10-12% increase in mortality risk. I’d stop drinking coffee at that point.

More interestingly, coffee consumption appeared to be consistently associated with a lower risk of mortality from all causes of cardiovascular disease, coronary heart disease, and stroke – the largest reduction in relative risk was at drinking three cups of coffee per day. The researchers found that the risk was reduced by 19% for mortality from cardiovascular disease, 16% for coronary heart disease, and an impressive 30% for mortality from stroke.

Given that cardiovascular disease is the leading cause of death in the USA, I would suggest that additional clinical research into coffee health effects would be extremely beneficial in this area. And, I’m feeling much better about my 4 cups of coffee this morning.

Interestingly, my experience with cardiologists (which is significant, since that’s where most of my research was done) is that they generally recommend avoiding coffee. Well, I’m going to be carrying this study with me and point them to the data.

But the data shows some more important health benefits than harms. Drinking coffee was associated (remember, correlation, not necessarily causation) with a lower risk of developing type 2 diabetes, metabolic syndrome, gallstones, renal stones, and gout. The researchers also found less statistically significant association between coffee and some types of cancer, Parkinson’s disease, depression, and Alzheimer’s disease.

But the most powerful data showed the greatest benefit of coffee was in lowering the risk of liver diseases.

On the other hand, there weren’t a lot of harms that were shown by drinking coffee, except in one area. Drinking coffee during pregnancy continued to be  linked to low birth weight, premature birth (in the first six months of pregnancy) and miscarriage.

This is not exactly new news. There are guidelines for limiting caffeine intake in pregnancy. And it’s good to see that we should continue the recommendation based on this data.

The coffee health effects summary

There is a lot of good news here for those who consume java, unless you’re pregnant. You shouldn’t touch the drink if you are.

There seems to be a link to lower risks of all-mortality causes. There are pretty strong links to reduced mortality from cardiovascular disease and lowered risk of cirrhosis and liver cancer. And there seems to be less powerful, but still significant, evidence that the risk of a boatload of other conditions may be reduced.

But I want to remind everyone, before you all cite this article as “proof” that coffee is healthy, there are limitations to the study that I described above. But, it is compelling, if not absolutely convincing, evidence that coffee has few harms, and a lot of critical health benefits.

At the very least, it appears that coffee, even drinking heavy amounts, is not going to harm you, again, as long as you’re not pregnant. And it is clear that my cardiologist needs to reexamine his views on coffee and cardiovascular health.

So, drink coffee. And for those of you who don’t, I’m concerned about you.

Conflict of interest statement

I am a full-time coffee drinker. I love my coffee, morning, noon and night. I am an addict of Starbucks (yeah, I know, the snobs look down on me), Peets (a West Coast thing), and Death Wish Coffee (if you desire flavor plus all the caffeine your heart can handle). I prefer my coffee to be a darker, robust roast, such as Italian or French roast brewed in a French press. Yes, I’m snobby about my coffee.

Otherwise, I have no relevant interests in coffee, and receive no funding from Big Coffee. So you can trust the science in this article.



Michael Simpson

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