Australasian Science: Australia's authority on science since 1938

How Healthy Is a Cup of Your Favourite Brew?

cuppa

Coffee offers protection from heart disease and Type 2 diabetes, and even some cancers. No more than six small cups of coffee per day are required for these effects, but excessive amounts should be avoided because of unwanted side-effects from caffeine.

By Margaret Allman Farinelli

Drinking tea and coffee may be better for you than you realised, but more is claimed than is substantiated.

Tea is the second-most consumed beverage in the world after water, having been consumed in Asia for centuries and attributed with medicinal properties. There are assertions it protects against heart disease and cancer and delays the ageing process due to the polyphenols and catechins in tea.

Most people drink black tea in Australia, but green tea has become popular because it has higher concentrations of catechins. Tea is often preferred to coffee as it is has less caffeine.

There is no convincing evidence as yet to suggest that either type of tea protects against cancer, although many claims are made that green tea protects against breast and endometrial cancer, oesophageal and stomach cancer. This does not mean the claims are entirely false; rather, the jury is still out with more research needed.

But there is some good news for tea, which appears to lower the risk of stroke, fasting blood glucose and LDL cholesterol levels, with green tea exhibiting stronger effects. The mechanism whereby tea prevents stroke is not yet fully understood but likely pathways are anti-inflammatory effects and lowering of LDL cholesterol. Benefits seem to show an increasing protection with greater tea consumption, with four or more cups per day optimal.

Japanese studies indicate that drinking green tea might prevent depression, but how this happens is unclear. As for tea being a youth elixir, a search of the scientific literature fails to provide support for this.

But there is a nutritional downside to drinking tea. When tea is consumed with meals, the absorption of dietary iron from cereals and vegetables is reduced, so the obvious solution is to enjoy your tea between meal times.

On balance, tea seems to offer some distinct pluses for health without harmful consequences while we await explanations for its mechanisms of action.

Our next favoured brew is coffee, which is available in many forms. Deciding whether drinking coffee is good or bad for your health creates more diverse opinions than tea. The latest evidence reveals that drinking coffee can lower your risk of Type 2 diabetes, protect against heart disease and perhaps even some cancers.

Yet it was only a decade or so ago that we were told to avoid coffee. I clearly remember the doctor’s instructions to my grandmother after a heart attack – :avoid all coffee, even instant” – so why are nutritionists doing a backflip on this one?

Research from Harvard University involving two large cohorts of people – the Nurses’ Health Study in women and the Health Professionals study in men – shows that drinking up to six small cups of coffee per day does not increase the chance of death from any cause, including heart disease or cancer.

But a caution before you reach for that extra latte is that these two studies need to be confirmed by others. Why might we have formed a different opinion previously?

There are some negative properties of coffee. First, coffee does contain chemicals that raise LDL cholesterol levels in the blood. As we have been warned repeatedly, elevated blood cholesterol increases the chance of a heart attack.

Kahweol and cafestol in coffee possess cholesterol-raising properties, but across the world people brew their coffee in different ways. Boiling, French press (plunger style), Italian espresso and filtering are common preparation methods.

What nutritional scientists have discovered is that the harmful substances are contained in the oily fraction of coffee. When you filter coffee, the kahweol and cafestol get trapped in the paper. So while we thought coffee must be bad for us it comes down to the way we take our favourite brew as well as the amount.

The second bad point about coffee is the caffeine content. One small cup (240 mL) contains about 100 mg of caffeine, which is a stimulant that can raise blood pressure and cause anxiety and sleeping problems. Caffeine is absorbed from the gut in less than an hour and distributed to all tissues in the body. It takes about 5–6 hours to metabolise half a dose in the liver.

The evidence about coffee itself and blood pressure is still a little confusing. It seems that boiled coffee is more likely than filtered coffee or instant coffee to raise blood pressure, while other studies have shown that for habitual coffee drinkers the increase in blood pressure may be non-existent or very small.

For individuals with blood pressure problems, anxiety or insomnia, the best way is to quit the brew and see if it makes a difference. Certainly if you feel totally stressed out from drinking coffee you should give it a miss. The general advice for pregnant women is to only drink a maximum of two coffees per day because caffeine metabolism may drop to half the rate in the non-pregnant state. Coffee drinking is not recommended for children because their smaller body size means a higher caffeine dose per kilogram, and might result in sleep disturbances.

One of the reasons scientists have not been able to work out the associations between coffee and disease is that it is just one of a whole host of dietary and lifestyle influences occurring simultaneously. People who drink coffee might also be smokers, or eat lots of cake with their coffee, or drink alcohol and coffee together, while those who belong to the “café set” may exercise less. It can be difficult to work out which factor is the main issue, but population research is very aware of these “confounders” and how to measure them, and uses powerful statistics to pinpoint the true effects to individual factors.

Coffee reduces the chance of Type 2 diabetes, but if you are told you are at an increased risk there is no need to race out and start drinking more. While people drinking four or more cups per day have a lower risk of developing diabetes, maintaining or reducing to a healthy weight and being physically active every day is better advice. So if you like coffee, don’t stop.

Coffee is a very complex mixture of chemicals, and just what confers the protection from diabetes needs to be further explained. Paradoxically, if you already have diabetes then coffee might worsen blood glucose control, most likely via the effect of caffeine. It is apparent we still have much to learn about all the chemicals in coffee and how they affect the human body.

So when you wake up tomorrow morning you can reach for that first cuppa or inhale the aroma of coffee brewing safe in the knowledge that you might actually be doing yourself some good.

Key Points

  • Tea and coffee are the most commonly consumed beverages in the world after water.
  • Green tea with a high catechin content has slightly more health attributes than black tea, but both are associated with lowering the risk of stroke, LDL cholesterol and blood sugar.
  • Coffee offers protection from heart disease and Type 2 diabetes, and even some cancers.
  • No more than six small cups of coffee per day are required for these effects, but excessive amounts should be avoided because of unwanted side-effects from caffeine.

Margaret Allman Farinelli is Associate Professor at the University of Sydney and a Fellow of the Dietitians Association of Australia.