Locally grown food is a good choice for sustainability.
What is sustainability?
The Earth's increasing population means more pressure on finite resources. It is estimated that if all six billion people on Earth lived a European lifestyle, we would need more than two-and-a-half planets' worth of resources to support us.
Rather than steaming ahead at the current rate, with little thought for future generations, sustainability is about seeking ways of providing food, water and energy that are long-lasting and have less of an impact on the environment.
Sustainability therefore becomes an issue for anything from food production and transportation to its disposal.
Eating according to the seasons has disappeared for most people in the developed world. You can buy asparagus and strawberries all year round thanks to refrigeration, heated greenhouses and, of course, global food transportation.
All these technologies and so-called "food miles" - the distance the food travels from producer to consumer - bring with them implications for greenhouse-gas emissions and consequent effects on the climate.
Although eating local food according to the seasons is the most sustainable option, if this cannot be done it is important to consider how producers make the food available out of season. For example, some studies show importing items such as lettuce from abroad results in fewer emissions than freezing locally grown produce. [Let's hope these studies didn't freeze lettuce!]
What does sustainable food mean?
Sustainability takes into account the process from field to plate. A key word in the sustainable food dictionary is "local" - minimising energy used in food production, transport and storage.
If you are not about to go to the extremes of rearing your own livestock and growing your own vegetables, buying with sustainability in mind is a matter of finding the right balance.
Sustainability campaigners recommend buying local food from farming systems that minimise harm to the environment, such as certified organic produce.
Others look to science to provide a solution - supporters of genetically engineered or modified food say they will be essential to deal with an ever-growing population and diminishing water supplies. The modified crops can deliver higher yields, require fewer nutrients and pesticides, and can also cope with poorer soil conditions and less irrigation.
But critics of GM foods say they are not all proven to be safe, they allow multinational companies to increase control of the food system and have disempowered small farmers all over the world.
Buying animal products of any sort raises its own questions of animal welfare and greenhouse-gas emissions, as livestock farming is said to be one of the most significant contributors to climate change.
Should we eat fish?
Fish stocks are threatened by overfishing and practices that destroy habitats, and by quotas that result in overcaught or undersized fish being thrown back dead. Some quota systems have helped dwindling populations of some species restock.
The Marine Conservation Society has identified species that are at risk and others who are in healthy abundance.
For example, Atlantic halibut has been overfished, which means it is caught in such high numbers that a sustainable stock cannot be maintained and the species is classed as endangered by the World Conservation Union. The MSC recommends choosing Pacific halibut or farmed Atlantic halibut instead.
In the UK, the flatfish dab has healthy stocks and is recommended as a fish to eat, especially if caught by seine netting, which causes less damage to the seabed than other methods of trawling.
What about food from abroad?
Fairtrade food often comes within the wider definition of sustainable food. And it is the developed world markets that sustain many producers in poorer countries.
If you are buying foreign foods out of season, campaigners say the sustainable option is to choose Fairtrade-certified products imported from poorer countries, to ensure a fair deal for disadvantaged producers.
The Soil Association, which certifies 70% of the UK's £1.9bn organic food sector, requires foreign farmers to prove they provide employment at fair pay and that they are trying to lessen dependence on air travel. It wants all air-freighted food to meet tough "ethical trade" standards.