Apples are one of the UK’s favourite fruits, with each of us eating on average 2 or 3 of them every week. Their appeal is celebrated annually on Apple Day, October 21st, and traditions like apple wassailing show our passion for apples traces back for hundreds of years.
Apples also have a long-standing association with health, and nowadays are often seen as a convenient and healthy snack. But what does the science have to say about whether an apple a day can really keep the doctor away?
Many studies have shown that people whose diets are rich in fruits and vegetables tend to have a reduced risk of developing chronic conditions like cancer, cardiovascular disease and diabetes. Exactly why is difficult to pin down, as there are many other social and economic factors that contribute to the development of disease, and fruits and vegetables contain lots of different components that could be keeping us healthy. Each of us might also digest and metabolise dietary compounds differently, based on our genetics or microbiome, further adding to the complexity.
Fuji apple cells
One group of compounds of particular interest are the polyphenols, which evidence suggests may contribute to cardiovascular and metabolic health. Polyphenols are a large group of different compounds that are produced naturally in many different fruits and vegetables, often responsible for giving them their bright colours or distinctive tastes.
Many polyphenols are also bioactive compounds, which means they can interact with and affect cells and tissues in the body, often in ways that would be expected to benefit health. Combining studies of how these compounds affect our bodies and cells with larger dietary studies that link diets with health outcomes provides the knowledge needed for developing evidence-based dietary advice, as well as improved foods for health.
This is how experts like Dr Paul Kroon from the Quadram Institute help answer questions such as “Why are apples healthy?” Paul is an expert in dietary polyphenols and leads a research group that aims to understand how consuming foods or diets that are rich in polyphenols can deliver health benefits.
Recently, he has been working with researchers from King’s College London to analyse whether apple polyphenols might help reduce glucose “spikes” after eating high carbohydrate meals.
If you eat a meal high in carbohydrates that are quickly and easily digested, i.e. with a high glycaemic index, your blood glucose levels rise sharply for around 30 minutes to an hour. Avoiding this postprandial glycemia is crucial for people who need to manage their blood glucose levels, and also reduces the risks of developing Type II diabetes. There has been a lot of interest in ways of helping moderate postprandial glycaemia, especially given the predominance of refined, carbohydrate-rich diets in industrialised countries.
One line of research has looked at using apple juices and extracts to slow down the absorption of glucose, by inhibiting the enzymes that break down starch and sugars, and also blocking the systems used to transport glucose from the gut into the blood stream. This has focused in on one particular polyphenol called phlorizin, that is only found in high concentrations in apples.
The King’s College team recently showed in a clinical trial that people who had an apple extract drink at the same time as having a high carbohydrate meal had markedly reduced blood glucose concentrations up to 30 minutes after the meal, compared to people who were given a placebo. However, the dose of polyphenols in the extract was very high, and to get the same level from apples you would need to eat about five of them. In a new study, published in the Journal of Nutritional Biochemistry, the team have now looked to see whether lower doses of the polyphenols still produce an effect. If so, this makes it more feasible for people to get the benefits either from apples, or from including apple extracts in functional foods and drinks.
In this study, volunteers were given 4 different drinks, containing the high level of apple extract, or drinks with intermediate levels, or no extract at all. They had one drink at each visit, with a week between each visit.
QI’s Mark Philo, who carried out polyphenol metabolite analysis
The extract was served mixed with a fruit squash preparation, so each drink tasted the same, and neither the volunteers nor the researchers knew which level of apple extract they had. Alongside the drink, they ate a thick slice of white bread with a measured amount of apricot jam, which allowed the researchers to control precisely the carbohydrates eaten.
Blood samples were then taken at time points up to 4 hours for glucose analysis, and also for the Quadram Institute’s experts to analyse the polyphenols that had been absorbed and metabolised.
They found that each level of apple extract reduced the rate of glucose absorption in the blood in the first 30 minutes after eating. The same amount of glucose is absorbed in total, but it is spread out over a longer time, reducing the “spiking” and better moderating the glucose levels.
Although the effect is strongest with the extract containing the highest levels of polyphenols, it was still seen in the lowest as well. The total polyphenol content in this low extract was equivalent to that in one or two apples, showing that a positive effect is achievable on regular basis for most people.
So could an apple (or two) a day really keep the doctor away? Well it might help, especially if you are having to keep an eye on your blood glucose levels. Preventing those sharp exaggerated glucose peaks could help reduce progression to Type II diabetes and cardiovascular injury.
But it won’t help you get away with having an unhealthy diet. Those carbs will still be absorbed, just at a steadier rate. Whilst apples boost your overall polyphenol intake, one key polyphenol, phlorizin, is poorly absorbed by the body. And, as the extracts are enriched, you’d need to eat 14 to get the lowest dose of this one. These studies didn’t look at consuming whole apples, just polyphenol extracts, so we also need to ensure that factors like the polyphenols being locked inside plant cells don’t affect their availability to be absorbed.
Individuals may differ in the benefits they get, but what this study does also do is provide more evidence supporting dietary guidelines encouraging fruit and vegetable consumption. At the population level, encouraging apple consumption could have significant public health benefits, especially if it helps keep at-risk populations from progressing to chronic disease.
The study also suggests that developing foods or beverages contains apple polyphenol extracts might be a way to deliver the benefits seen in these experiments. These “functional foods” are increasingly being explored as a way of targeting specific conditions, but to protect the consumer it’s important they are backed up by sound science. The research team suggest studies using these apple extracts with volunteers with, or at risk of progressing towards diabetes could demonstrate specific benefits for this group of people – and maybe that will help keep the doctor away.
Apple polyphenol-rich drinks dose-dependently decrease early-phase postprandial glucose concentrations following a high-carbohydrate meal: a randomized controlled trial in healthy adults and in vitro studies, Prpa EJ,Corpe CP,Atkinson B,Blackstone B,Leftley ES,Parekh P,Philo M,Kroon PA,Hall WL. Journal of Nutritional Biochemistry doi: 10.1016/j.jnutbio.2020.108466