"I felt like absolute crap afterwards," he said. "I never intended to give up doughnuts, I never intended to not eat these foods. As I got healthier I realised I no longer wanted them."
Having practised healthy eating for years now, Mr Warren finds himself extra aware of the effect unhealthy foods have on his body.
With 11 years of experience and a masters degree in clinical nutrition, he runs a nutrition practice in Havelock North with a team of 11.
Mr Warren was in Whanganui this week to give a seminar and help Whanganui residents "revitalise their health in 2016".
One of the topics was defining a "clean diet" and helping people figure out what they should be eating.
"Eleven years' experience and I can't actually tell people what diet's right for them, everybody's different," he said. "Everyone's bodies are responding differently to foods."
Mr Warren also spoke on what nutrients people needed and the nutrients many New Zealand foods lacked.
"It's very difficult to get all the nutrition you need from your diet these days because of farming practices. Essentially, the nutrition is no longer in our food," he said. "We can really tell this by the taste of our food."
One reason homegrown fruit and vegetables often tasted better than those bought in a supermarket was because farmers did not put trace minerals back into the soil.
The other reason was the use of nitrogen in growing.
Farmers would give nitrogen to the plants to boost growth, but this meant the plants would not get their nitrogen naturally from the soil and their root growth would be stunted.
That meant they would also have trouble absorbing minerals and vitamins from the soil.
"What we end up with is essentially candy floss," he said.
Mr Warren owns a 6ha farm and grows 80 per cent of his own food.
"It explodes in your mouth, the flavour ... people who have vegetable gardens know this, they get this."
He said humans were "basically pre-programmed to hunt and destroy sugar, salt, and fat".
"In nature, those three things are found with key nutrients."
When food manufacturers added salt, sugar, or fat to relatively nutritionless food, the body craves more because it isn't getting the nutrients it associates with those flavours.
If someone was craving salt, Mr Warren said they should throw a steak on the barbecue with a little salt and eat that, as the body is actually craving protein.
His seminar on Thursday addressed nutrition, and how someone could apply that information to their own life.
He believed there was "no one way" for people to eat.
"Grains are a good source of nutrition; for other people dairy is a good source of nutrition," he said.
However, he said things like sugar and processed grains were leading causes of obesity in New Zealand, adding that processed grains quickly turned to sugar in the human body.
Mr Warren encouraged people to eat leafy greens three times a day, eat liver and other organs as they were nutrient-rich, and have natural, undamaged fat in foods such as butter.
