One is made of fruit. The other is caffeinated. So a smoothie is a healthier option than a coffee, right? Don't be so sure, says Michael Mosley, as he weighs the evidence.

Which is healthier - coffee or smoothies?

It seems obvious that the answer must be a smoothie. After all, drinking coffee is a necessary evil while having a smoothie, made from fruit, is part of your five-a-day. But when you look into the scientific studies they reveal something much more surprising. Let's start with coffee.

There have been numerous claims down the years that drinking coffee will increase your risk of a whole range of terrible things from heart disease to cancer.

These claims have been largely based on case control studies, where you take a group of people who drink coffee and compare them with another matched group who don't.

The problem with this approach is that coffee drinkers are more likely than non-coffee drinkers to have other "bad" habits, like drinking alcohol or smoking, so it is hard to tease apart what is really doing the harm.

A more reliable way to get at the truth is to do what is called a prospective cohort study. You take a group of disease-free individuals, collect data about them, then follow them for a large number of years to see what happens.

When scientists collected data on the coffee drinking habits of 130,000 men and women and then followed them for over 20 years they found that coffee is rather a good thing (The Relationship of Coffee Consumption with Mortality, Annals of Internal Medicine, June 2008).

They crunched the numbers and concluded that "regular coffee consumption was not associated with an increased mortality rate in either men or women".

In fact, data from this study suggests that moderate coffee consumption is mildly protective, leading to slightly lower all-cause mortality in coffee drinkers than non-coffee drinkers. Based on this and other studies the most effective "dose" is two to five cups a day. More than that and any benefits drop off. There are hundreds of different substances in coffee, including many different flavonoids (compounds widely found in plants that have antioxidant effects). Which of these ingredients is beneficial, we simply don't know.

When it comes to the brain, however, the answer seems to be "caffeine". In research recently published in the World Journal of Biological Psychiatry (July 2013) they found that people who drank two to four cups of caffeinated coffee a day were half as likely to commit suicide as those who either drank decaff or fewer than two cups a day. This research pulled together data from three studies that had followed more than 200,000 people for more than 14 years, so it's pretty reliable. It is also supported by a number of other studies, which makes this claim even more plausible.

One reason why caffeine may be a mild anti-depressant is that as well as making you more alert, it increases levels of neurotransmitters in the brain, like dopamine and serotonin, that are known to improve mood.

The researchers don't recommend going overboard, noting that "there is little further benefit for consumption above two to three cups".

One note of caution is that these trials began many years ago so the sort of coffee consumption being tested is almost certainly good, old-fashioned coffee.

A simple mug of coffee delivers somewhere between zero and 60 calories, depending on whether it is black, white or white with one sugar. Cappuccinos, lattes and mochas contain coffee but they also contain a lot of calories - anything between 100 and 600 - so when it comes to fancy coffees I limit myself to the occasional tall, skinny cappuccino (70 calories).

Media playback is unsupported on your device Media caption Michael Mosley tells Newsnight which foods are good and bad for us

But what about fruit smoothies?

Cappuccinos, lattes and mochas contain a lot of calories - between 100 and 600

They may consist of pure fruit but by the time you've got rid of the peel and mashed the fibre then you have already lost many of the potential health benefits. What you are mainly left with is a sugary drink. In a survey published in early 2013, researchers found that out of 52 commercial smoothies, 41 had more sugar than Coca-Cola (a 12oz can contains the equivalent of about nine teaspoons), and all had more calories.

Fruit smoothies are acidic and the bits cling to your teeth, so dentists are not enthusiastic. An apple a day may keep the doctor away, but not when it's been peeled, blended, mashed and packaged.

In a study published in August 2013 in the British Medical Journal (Fruit Consumption and risk of type 2 diabetes) they found that while eating fruit cuts your risk of developing diabetes, drinking it appears to increase the risk.

This was another big study involving lots of people followed for many years. An interesting finding was that different fruits gave different levels of benefit. Three servings of blueberries, for example, cut the risk of diabetes by 26%, while eating apples, pears, bananas and grapefruits also had a positive, albeit much smaller, effect.

Overall those who ate fruit cut their risk of developing diabetes by 2%, while those who drank it (more than three glasses of fruit juice a week) increased their risk by 8%.

More bad news for fruit juice drinkers comes from a case-controlled study done in Western Australia that examined the daily diets of more than 2,000 people. They found that eating some types of fruit and vegetables (cabbage, broccoli, cauliflower and apples) cuts your risk of colorectal cancer, while drinking fruit juice was associated with an increased risk of rectal cancer. Sugary drinks lead to raised levels of the hormone insulin and persistently high levels of insulin are associated with increased risk of some cancers. The researchers point out that many things that protect against bowel cancer, such as antioxidants and fibre, are lost or diminished during the juicing process.

Image caption A smoothie may count towards your five-a-day, depending on how it's made

None of these studies specifically looked at the health benefits or otherwise of fruit smoothies, which are a relatively recent phenomenon, nor did they look at the impact of different types of juice - for instance, whether it was freshly squeezed or from concentrate, home-made or shop-bought. I would assume, for example, that drinking a home-made vegetable smoothie is going to be a lot better for you than a commercial fruit smoothie.

And I very much doubt that the occasional fruit juice or fruit smoothie is going to do any harm. Nonetheless, I've personally given up buying them and we rarely have them in the house. I eat whole fruit and when it comes to soft drinks I'm sticking to water, tea and, of course, coffee.

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Michael Mosley answered readers' questions on Twitter using #AskMichael.

Q: The article seems to conflate smoothies and juices re lack of fibre. Does fibre in a handmade smoothie blunt insulin?

A: Commercial juices and smoothies are lower in fibre than eating fruit and veg. Handmade ones may be better but will still be lower than fruit and veg. Eating apples will keep you feeling full for longer than drinking same amount as apple juice. I met someone who takes it to the extreme. He peels the apple, eats the peel, throw rest away.

Q: Why do you persist in offering fad diets when we have the examples of sportsmen who just eat right and exercise?

A: The simple message, "eat less, do more exercise" simply doesn't work with most people. It's time to acknowledge that.

Q: Is it ok to eat 4 fruits each day, eg: banana, grapes, apple, orange? Is this too much sugar?

A: Banana and grapes have lots sugar. Apples, pears and berries have less sugar. Vegetables generally have lower GI and calories. Commercial smoothies are often banana based, so they have lots of sugar and calories.

Q: What is worse? High fat, low carbohydrate or high carbohydrate low fat diets?

A: I think the important thing is the type of carbohydrates and type of fat. Highly processed food is bad.

Q: It seems what we are told is either good or bad for us changes like the wind. How do we know what to believe?

A: The problem is that many health beliefs are based on poor studies. The best option is a prospective cohort study. In a prospective cohort study you follow people over time, not at a moment in time. When this was done for coffee drinking it was shown to be mildly beneficial.

Q: I only ever have proper butter, never spreads or margarine. Do I need to only have tiny amounts though?

A: I eat butter rather than margarine, as do many cardiologists I know. But it's rich in calories, so I am restrained.

Q: Any good news on the horizon for chronic migraine?

A: On Trust Me I'm a Doctor we looked at Botox which seems to work quite well for chronic migraine.

Q: Drinking enough water is important, but how important is the pH value in tap water?

A: The idea we should drink 2 litres a day is a myth. Drink when you're thirsty. I don't know about the effect of pH in water.

Q: Is one banana a day too many?

A: No. Eating fruit is generally good.

Q: Yes or no, should we drink pure fruit smoothies?

A: I don't. If it tastes sweet it has lots of sugar. Some have more than 10 teaspoons of sugar.