Fruit and vegetables in supermarkets might look shiny and fresh, but often they were picked several days earlier. Some nutrients, particularly vitamin C and folic acid, begin to oxidise as soon as picking happens, but manufacturers use chilling and packaging techniques to minimise the resulting losses. “Lots of these reactions are driven by enzymes, and if you want to slow an enzyme reaction right down you chill it. However, if you are choosing between organic leeks from a distant country or locally grown,  go for the fruit and vegetables grown locally and subject to the  shortest possible supply chain.    As if to illustrate this, a 2003 study evaluated the nutritional content of broccoli kept in conditions that simulated commercial transport and distribution: film-wrapped and stored for seven days at 1 °C, followed by three days at 15 °C to replicate a retail environment. By the end, the broccoli had lost between 71 and 80 per cent of its glucosinolates – sulphur-containing compounds shown to have cancer-fighting properties – and around 60 per cent of its flavonoid antioxidants.

Many kinds of mass-produced fruit and veg – most famously tomatoes – are picked unripe so that they bruise less easily during transit. They are then sprayed with ethylene to ripen them. Some studies suggest that tomatoes harvested early have lower antioxidant activity and less flavour. If a fruit is left on a plant until the end of its life cycle, it’s able to recycle all the energy from the plant.  If you pick it early you truncate that process and get less sugars into the fruit, which are needed to bind the nutrients.

Similarly, processing has become a maligned word in the context of food, but there are some cases where it enhances a food’s health benefits. In fact, you arguably get more benefits from processed tomatoes, such as in purees, sauces or ready chopped in cans, than fresh.

Processed tomatoes tend to be harvested at a riper stage. In addition, lycopene – a compound tomatoes are rich in, and which has been shown to protect against cancer – is much more readily absorbed by humans from tomato paste than fresh tomatoes. “The more processed a tomato is, the more lycopene is available,” says Collins. “Processed tomatoes are often very concentrated, so you’re actually getting a greater quantity than you would use if you made your own sauce.” However, she adds that the heating used in processing destroys vitamin C.

Although salad leaves that have been picked and stored for several days before being eaten are a bit less nutritious than a freshly harvested lettuce, chilling and using packaging to reduce oxygen exposure may slow the nutrient loss. And any loss of nutrients must be weighed against the fact that these products may encourage people to eat better overall.

The bottom line is that although aspects of today’s food production, processing and storage might make what we eat a bit less nutritious, they are also making foods more available – and this is far more important. The majority of us consume far less fruit and vegetables than we ought to. We eat too much fat, sugar and salt and not enough oily fish.

The most important thing you can do is eat more fruits, vegetables and wholegrains, and cut down on highly refined, human-made foods, vegetable oils and added sugars. If you’re worrying about nutrient losses from cooking or whether your food is straight from the farm – those differences are minor compared to the differences you’d get from eating unprocessed foods.