What is a sustainable diet? This innocuous question raises fiendishly complex issues. Do we eat for health? Cost? Environment? Social values? What we like or are given? Only what we know or afford? Global or local? Follow the evidence or seasons?

The sustainable diets question is rapidly rising up the policy agenda. Two weeks ago, the US Institute of Medicine discussed it at the National Academies of Science. The US government will shortly announce the committee to revise its national dietary guidelines. The members will surely address diet’s environmental impact.

It was discussed over here in Europe at a huge international nutrition and behaviour conference in Ghent this week. Next week, UK, French and Belgian nutritionists meet for a joint deliberation. Clearly, the issue crosses borders. It’s also contentious, as was seen when Australia revised its dietary guidelines this year.

The issue will also be on the agenda at the preparatory meeting in Rome this November ahead of the UN’s Second International International Conference on Nutrition scheduled for late 2014.

“Facts and ideology inevitably mingle. But climate change is about facts”

One reason it’s so hot is greenhouse gas emissions, from meat and dairy particularly. This isn’t lost on food industry people. One mused to me recently: we can de-carbonise food so much but eventually consumers will have to change diets. How animals are fed is key: grain or grass?

The issue won’t go away, not least because the evidence for a rethink about food systems and diet is so strong. But, hey, what’s evidence got to do with it? Policy wonks talk evidence-based policy, but that’s easier said than done. Even so, I was shocked to learn this week that Defra has cut its climate change team from 38 to six people. This is Orwellian.

What should ministers do about evidence? Easy. Restrict its flow! Civil servants are often derided but this Defra team had a clear brief: to help meet the legal target to reduce climate change emissions by 80% by 2050.

You could just say facts and ideology inevitably mingle. But climate change is about facts. So it’s shocking to see Defra cut this team. The fury of former senior civil servant John Ashton at the Royal Society of Arts last week was understandable.

Tim Lang is professor of food policy at City University London

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