No proof coffee causes obesity and diabetes

'Is your caffeine fix making you fat?' is the compelling question posed by the Mail Online website, which goes on to report that a study 'shows five cups of coffee a day could cause obesity.' But the study in question involved mice, not people, and a chemical found in coffee, not coffee itself.

The research explored the effects of a substance called chlorogenic acid (CGA), a component of coffee. The rodents were given CGA to see how it affected their fatness and glucose regulation, which is related to the risk of developing type 2 diabetes.

Mice given a high-fat diet supplemented with CGA appeared to display signs of worse glucose regulation than those given the high-fat diet only, which suggests that consuming high levels of CGA may not be good for you.

Despite this finding, the headline 'five cups of coffee a day could cause obesity' was way off the mark. The study only demonstrated that a high-fat diet made the mice fatter, but did not prove that CGA – and by association coffee – makes you fatter, as the headline implies.

This study alone does not support the notion that coffee causes obesity. Still, drinking five or more cups of caffeinated coffee a day can lead to symptoms such as irritability and insomnia.

Where did the story come from?

The study was carried out by researchers from universities in Australia and Malaysia and was funded by the Australian Research Council.

It was published in the peer-reviewed Journal of Agricultural and Food Chemistry.

Both the Mail Online and The Daily Telegraph's reporting of the study seems pretty wide of the mark. The Telegraph's headline, 'Regular coffee drinkers 'at increased risk of weight gain',' is a major extrapolation of the actual study results.

In mitigation, it appears that their reporting was influenced by an overenthusiastic discussion of the potential implications of the research by the researchers themselves.  

What kind of research was this?

This was a laboratory-based animal study using male mice. The researchers suggest that the increasing prevalence of what is known in the medical profession as "metabolic syndrome" demands new treatments and prevention strategies. 

Metabolic syndrome is the medical term for a combination of diabeteshigh blood pressure and obesity. It puts you at greater risk of heart diseasestroke and other conditions affecting blood vessels (cardiovascular diseases)

The researchers point out that human observational studies have consistently linked higher coffee consumption with a lower risk of type 2 diabetes. It is thought that certain dietary polyphenols (organic chemical molecules found in food and drink) can have beneficial effects on several features of metabolic syndrome, such as reducing blood pressure.

Chlorogenic acid (CGA) is one of the most commonly consumed polyphenols in our diet and is a major component of coffee. CGA is also found in fruit such as plums, apples and berries. The researchers wanted to better understand how diet and CGA intake might interact to reduce the risk of certain components of metabolic syndrome, namely obesity, glucose intolerance and insulin resistance.

Insulin is the hormone that controls blood glucose. It is produced when blood glucose levels are high, causing the body's cells to take up glucose and use it for energy. When a person is described as having "glucose intolerance" and "insulin resistance" it means their body's cells are less sensitive to the action of insulin, so they can't regulate their blood sugar as well. This means they are at risk of developing – or may already have – type 2 diabetes.

Animal studies are a useful place to begin to develop a better understanding of the biological underpinnings of diseases. However, mice and men are not identical, so we can't assume positive findings in mice will lead to positive findings in people – this needs to be tested directly in studies involving humans.

What did the research involve?

The researchers designed a study that involved giving male mice a controlled diet over a 12-week period. Mice were either given:

  • a normal diet
  • a high-fat diet, or 
  • a high-fat diet and CGA

The researchers say that they used CGA at a dose that would be realistically attainable through the diet (1g per kg of food), rather than an exceptionally high experimental amount. In their article, they describe how people can obtain up to 1g of CGA from the daily consumption of coffee, but do not specify how many cups or the strength of the coffee.

The researchers tested the effects of the three diets on the following measures of metabolic syndrome:

  • high-fat diet-induced obesity – how fat the mice got due to their high-fat diet
  • glucose intolerance – an umbrella term for blood glucose levels that are higher than normal
  • insulin resistance – when the body fails to respond to the normal actions of the hormone insulin, which is essential for maintaining blood glucose levels within a normal range
  • fatty acid oxidation – the process where fats are broken down for energy in a cell
  • insulin signalling – essential for maintaining blood glucose levels within a normal range

Their analysis was appropriate and looked at whether the above measures of metabolic syndrome were influenced by the three different diets.

What were the basic results?

The study found that all the mice gained weight in the 12 weeks, with those on the normal diet gaining the least. Compared with mice on the normal diet, both groups on the high-fat diet gained significantly more weight, both with and without CGA.

However, mice given the high-fat diet plus CGA were no slimmer than mice fed a high-fat diet only. This is interesting, as you can buy CGA extracts as a slimming aid, suggesting that it may potentially be less effective than advertised.

Mice given a high-fat diet plus CGA had increased insulin resistance (a bad sign) compared with mice fed a high-fat diet only, which theoretically suggests a higher risk of type 2 diabetes. Likewise, the livers of mice fed a high-fat diet supplemented with CGA appeared to have a poorer fatty acid oxidation process than those given just the high-fat diet.

How did the researchers interpret the results?

Recognising their research was different to what had been found in some previous studies, the researchers concluded that, "Our results do not support the hypothesis that CGA can prevent development of features of the metabolic syndrome."


This rodent research suggests that mice given a high-fat diet supplemented with the polyphenol chlorogenic acid (CGA) fared worse than those given a high-fat diet. But this was only in terms of measures of glucose regulation that are related to the risk of developing type 2 diabetes.

The researchers point out that other studies have found that giving CGA to mice improved measures of their glucose regulation. When there are contradictory results from different studies like this, it can indicate that the biological processes involved are not fully understood. Because of this, these results are not likely to be reliable on their own. A better consensus of what is going on may come through further research into the area.

There are further limitations with this research to consider when thinking about the impact of this study:

  • The equation of the amount of CGA given to the mice to five cups of coffee in humans was relatively crude and may not be accurate. However, the researchers did make an effort to give the mice a dose of CGA that they thought might be roughly equivalent to the amount a person could get through drinking coffee, although it wasn't clear what type or strength of coffee this would be.
  • The researchers suggest that the results could have been influenced by the bacteria present in the guts of the mice. They suggest the gut bacteria can degrade CGA, which would alter its biological effect in the body. The precise combination of gut bacteria varies from mouse to mouse and person to person. This may account for some of the different results seen in this area of research, and would need to be measured in further studies.

This study represents preliminary research in mice and is a long way from being directly applicable to humans, as some of the media coverage implies. Further research is required to work out if coffee polyphenols are able to protect against metabolic syndrome and type 2 diabetes in humans or, alternatively, if they make people more vulnerable to these conditions.

Established ways of protecting against the harmful effects of metabolic syndrome include:

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