Carbohydrates are a complex range of compounds consisting of starch, sugar and fibre. Plant foods all have fibre and many, especially fruit, have natural sugars. There's no fibre in animal products, although manufacturers sometimes add it, he says.
Starchy foods are plant foods such as wheat, barley, corn, rice, legumes such as beans and chickpeas, vegetables such as potatoes, kumara and taro, and products made from them such as bread and pasta.
Sugar and white flour, two highly refined carbohydrates that have very little fibre, are often seen as demons in Western diets.
''Sugar eaten as part of fruit - it seems you can't overdo it. There have been studies ... where people were put on essentially all-fruit diets and they were consuming huge amounts of sugar, way more than we consume in our society and they did well, so the sugar itself was not the problem. It's the way sugar is being used in modern society that's the problem.''
Sugar has a two-pronged damaging effect, he warns.
''One is in drinks.The evidence is very good that people who drink sugary drinks tend to put on weight. 'Don't add calories to your drinks' would be a good message.''
The other is the combination of sugar with fat in things such as chocolate, ice cream, cakes and biscuits which are energy-dense foods, he said.''People don't really understand energy density. One way of looking at it is that if you are adding fat to food you are definitely going to increase the energy density of the food.''
Animal foods tend to be energy dense as well, he adds.
Starchy foods also give us energy but they are low-energy foods, so you can eat a lot of them, he says.
''When people go on to starch-based diets, they spontaneously lose weight because you can't actually eat volumes big enough to maintain your overweight status, so it's really your ideal weight solution - as long as they are plant-based whole foods, not processed ones such as sugar and white flour.''
He warns not to add fat to your starchy foods - don't put lots of butter on your potatoes, creamy sauce on your pasta, spread on your bread, or cheese on your pizza. Instead, add vegetables for flavour.
''People need to start thinking about more whole foods and eating more fruits and vegetables, including starchy vegetables.''
Dr Venn looks to the ''blue zones'', areas in the world where people live long and healthy lives into their 80s and 90s, and even 100s.
These zones are the Greek island of Icaria, Okinawa in Japan, the Nicoya Peninsula in Costa Rica, the Italian island of Sardinia and the Seventh Day Adventist community in Loma Linda, California, he said
''A lot of work has been done looking at these people and how and why they live long and healthy lives. There are a number of factors, of course, including low stress. They have community around them, so there are non-dietary factors, but very much in the forefront are the diets of these people.
Typically, they are predominantly plant-based diets. There's a little bit of animal food in there, and there's a little bit of alcohol in most of them, but predominantly they are plant-based. Activity is certainly part of it, but you are active because you are well, so it's really the diet that is at the centre of all of this.''
