British activist Tristram Stuart wants you to clean your plate—or just ladle less food onto it.

The author of the revelatory book Waste: Uncovering the Global Food Scandal was recently named one of National Geographic's newest emerging explorers for his work on reducing food waste. He estimates that less than a quarter of the food wasted in Western countries could feed the planet's one billion hungry people.

Stuart has initiated numerous campaigns to call attention to the food we waste. Among them: Feeding the 5000, a free public feast on what would have been wasted food, and the volunteer Gleaning Network, which harvests surplus produce that would otherwise rot.

When did you first get interested in the issue of food waste?

When I was 15, I was living on a small and disused farm in England, and I decided to buy some pigs and chickens. Buying pig feed was hugely expensive and had God knows what impact on the environment, so I decided to get free waste food from my school cafeteria. The local baker, the greengrocer, the market, and a farmer who was discarding potatoes because they were the wrong shape or size for the supermarkets—this was great as a teenager. I had loads of delicious pork that I sold to my friends' parents.

But I was aware that we were wasting food at every link in the supply chain, from the farm down to the plate. I was an environmentalist from before I can remember. I thought there's surely something better than chucking that food away, and I made that into a public campaign on food waste.

Why was no one paying attention until your book Waste: Uncovering the Global Food Scandal was published?

When I started in 2001—literally it's almost unbelievable, but you couldn't find any information on food waste. You didn't find any overt policies from business about cutting food waste. There was nothing from the government, except for the European landfill directive. But nothing about reducing food waste. It was hidden.

It's hidden by companies that don't want people to know how much they waste, because they know people will be shocked. It's hidden from ourselves, because we put food in the bin and we don't even realize how much it is. A very large part of the work has just been to expose the scale, add it up from farm to plate, and then you've got a really shocking figure, which is: At least a third of the world's food supply is wasted, and in rich countries the percentage is much higher.

How do we waste so much food?

It starts in the field. If you look at fresh produce, shops have cosmetic standards, particularly the big supermarkets. That means all the ugly things, stuff that doesn't comply with this kind of perfection standard gets wasted—and doesn't even come off the farm very often.

Factories waste on a routine scale. So if you're making sandwiches, the crusts are thrown away. If you're making pies, the trimming on your pastries is thrown away. At the retail level they overstock their shelves to create this image of a cornucopia and abundance, because that's what they think people are expecting to see. And it becomes like an arms race. They are treating food almost like Christmas decorations.

We shop in this kind of preemptive way in rich countries. It's like we don't value food enough, so we buy on the off chance that we're going to eat it.

How does waste happen in the developing world?

Waste in the land of hunger. Often smallholder farmers lack the resources to invest in basic, postharvest infrastructure to make sure that the crops they grow get to market. We're talking about simple things: grain stores, which in Africa are so poor that cereals often go moldy. Pasteurization, refrigeration, fruit crates, even shade in markets is absent very often. So crops are exposed to the hot sun, vermin, insects, and it gets wasted.

How can governments help trim food waste?

We can solve the global food-waste scandal without governments, but intervention from governments can be very helpful.

Very often the solution is deregulation, not more regulation. The European Union relaxing the laws on cosmetic standards was progressive. The European farm subsidy shifting away from subsidizing overproduction, in the way that the U.S. farm subsidy program has not—these were progressive, deregulatory measures that reduced food waste.

But there are some regulatory things that have helped. In Belgium they brought in a law that supermarkets should be obliged to offer any surplus foods to charities prior to disposing of it. In the United States you have something called the Good Samaritan Act, which protects companies if they donate food in good faith, and that gives them more confidence to give away their food and not get sued.

Last year the U.S. launched a Food Waste Challenge to reduce, recover, and recycle food waste. Your thoughts?

That kind of initiative has a very strong role to play in educating people in how they can better manage their food waste, so they save money and save impacts on the environment.

Top-down measures can be extremely helpful, but what we need is a popular revolution, a revolt against food waste. We, the public, need to say we're fed up with food waste.

This interview has been edited and condensed.