What we know, says Katz, comes down to what we do with our feet, our forks and our fingers. In other words, it's about exercise, a healthy diet and not smoking.
"We already know enough to add a bounty of years to lives, and a bounty of life to years," he says. And in the face of worldwide declining health, "arguing about whose diet is best is like standing in front of a burning building and fighting over who has the best hose".
So is there an "ideal" diet? It may come as a surprise that despite what seems like much debate on diet, nutrition experts actually agree more than they disagree about what and how we should be eating. It's just that, as noted in research undertaken by Katz and his university colleagues last year, "the clutter of competing claims likely obscures the established body of knowledge and forestalls progress, much like the proverbial trees and forest".
In their research the Yale team aimed to determine which, of an array of popular diets, was best for health. They looked at low-carb diets, low-fat diets, vegetarian and vegan diets, low-glycaemic diets, the Mediterranean diet, the "mixed/balanced" (or Dash diet) and the paleo diet.
And what did they find? No one diet "wins".
Rather, common themes among all the diets tested promote good health. Broadly speaking they are: an emphasis on whole-plant foods; limited processed foods; limited refined starches and limited added sugars.
We could boil this down to the often-quoted mantra of American food writer Michael Pollan: "Eat food; not too much; mostly plants." Or, to put it even more simply: eat real food.
Happily, this is something with which everyone from the most evangelical of paleo followers to the most vigilant vegan would agree.
Nutritionist Sarah Hanrahan from the New Zealand Nutrition Foundation is one who does.
"Feeding your family shouldn't require you to even consider words like carbohydrate, magnesium and omega 3, 6 or 9," she says. The best advice, she says, is: "Food, not nutrients, plenty of whole foods with an emphasis on vegetables."
Dietitian and sports nutritionist Dave Shaw agrees, citing Pollan's words as a philosophy to live by. "It's a simple guideline that empowers everyone and is considerate of every culture and tradition. Personally, I also make sure I have a high quality source of protein at each meal and snack, because it helps to keep you full and satisfied and prevents you from reaching for the treacherous processed treats."
Good examples of healthy eating habits can be seen in the so-called Blue Zones. These are the eight or so places around the globe that are home to the longest-living, healthiest populations. They include Okinawa in Japan, Sardinia in Italy and Nicoya in Costa Rica. What's interesting - and this reinforces the Yale findings - is the diets of those in the Blue Zones are quite different from each other.
The Sardinians eat very little meat but lots of vegetables, wholegrain bread, beans and goats' milk. The Okinawans eat lots of tofu and miso with their stir-fried vegetables. And the Nicoyans' diet is based on beans, rice, corn tortillas and vegetables. Some Blue Zoners eat a little meat or fish; some don't. Some eat more fat, in the form of olive oil and nuts, while others are at the lower-fat end of the spectrum. But what all the Blue Zones have in common is a plant-based diet with little processed food. Without exception, they eat a lot of vegetables.
What's also clear is that the way people in the Blue Zones eat is quite different from the way most of us in the Western world eat. We're exposed to an over-abundance of processed convenience foods, refined carbohydrates, poor quality fats and an overload of sugar, that combined is hard to resist. Couple that with our increasingly sedentary lifestyles, and it's no wonder we're getting fatter and less healthy.
It's a problem public health bodies struggle with and, it seems, struggle to give advice on. The much-maligned "food pyramid" model - with its generic guidelines and no distinction between refined and healthier carbohydrates - has long been superseded.
Nutritionists now mostly favour an "ideal plate" model - vegetables on half the plate, meat or fish on one quarter and carbohydrates on the other. The Heart Foundation's "healthy heart" is also a useful, food-based model.
So when it comes down to it, can we say what a normal, healthy diet looks like?
Assuming you're an omnivore and you're not avoiding foods because of allergy or intolerance, it's not really too tricky. Beyond the basics: lots of vegetables; some good-quality protein; good-quality carbohydrate foods and some healthy fats, stick to thinking about it like a Blue Zoner: choose as much real, whole food as you can. "If you can picture what the food you're eating looks like in nature, then it's probably going to be good for you. If you can't, then chances are you may be better off eating less of it, or not at all," advises Dave Shaw.
He also points out something that often goes overlooked in conversations about nutrition: food is about more than fuel. "I think we need to stop viewing healthful eating as being punitive, because it's actually [about] conviviality and pleasure ... everything from growing your vegetables and herbs, going to the supermarket, butchers and produce store, preparing, cooking and dining at the table with your family and friends - occasionally with a beer or glass of wine. When we do that, we actually don't have to think or worry about what to eat."
Brazil seems to be heading in the right direction with its recently introduced Ministry of Health dietary guidelines. They're a great example of eating rules anyone can understand, and they take into account the social and cultural aspects of eating.
As well as "what to eat" advice (real food again), they also include advice like "Eat in company whenever possible"; "Develop, practise, share and enjoy your skills in food preparation and cooking"; and "Plan your time to give meals and eating proper time and space".
Following this advice means we can give more time and attention to what we put in our mouths, which can only have a positive impact on health.
The guidelines are also not overly prescriptive, giving us lots of room to choose a way of eating that suits us. Because, as all the experts agree, ultimately the diet that works best is the one you can stick to for life.
• Niki Bezzant is the editor-in-chief of Healthy Food Guide magazine. She is a regular columnist, blogger and contributor to TV and radio and the author of two cookbooks.
A day of ideal eating
This day's food for one person (based on a woman, aged 31-50, 60kg/160cm, light activity, who needs about 8900kJ a day) ticks all the nutrition boxes. It has eight servings of vegetables, plenty of fibre and all the vitamins and minerals you need. It's roughly 22 per cent protein, 33 per cent fat and 45 per cent carbohydrates, if you're tracking such things, but the main thing to note is it's a day of simple, real food.
Breakfast
2 slices grainy bread
2 eggs
Large tomato
Trim latte
Snack
1 kiwifruit
Pottle low-fat yoghurt
2 brazil nuts
Lunch
Soba noodle salad
90g soba noodles
2 cups mixed slaw
1 tbsp mixed nuts
½ tsp ginger
95g can salmon
Dressing:
2 tsp sesame oil; 1 tsp lime juice; 1 tsp mirin; 1 tbsp rice wine vinegar
Snack
Banana
Dinner
150g lamb loin/steak (cooked with 1tbsp spice paste)
150g kumara
Salad:
½ can no-added-salt lentils
¼ cup parsley
1 tomato
¾ cup green beans
¼ onion
½ clove garlic
¾ cup carrot
2 tbsp sunflower seeds
2 tbsp vinaigrette
(2:1 vinegar to oil)
