Calorie density is the amount of energy a food carries for its weight.
It is not a fashionable idea and we have never seen anybody build a diet brand around it.
It is also the most useful thing we ever learned about eating, and it explains almost every plant-based meal that has left somebody hungry.
Your stomach measures volume rather than calories.
There are stretch receptors in the wall of it, and when enough physical bulk arrives, they report back that you have eaten.
So whether a meal fills you depends on how much room it takes up. Whether it keeps you full depends on how much energy arrived with it.
Most foods are good at one of those two things.
The useful ones are good at both.
Vegetables, a hundred a pound
Vegetables are the most nutrient-rich food available to you, and they are also almost entirely water and fibre.
You could eat two pounds of them, feel pleasantly full, and have taken in two hundred calories.
That is a snack arriving in the shape of a meal.
This is why we treat a plate of vegetables as a wonderful thing to eat alongside dinner and a poor thing to eat as dinner.
Fruit, two to three hundred a pound
Fruit carries two or three times what vegetables carry.
It is still not enough to run a whole day on, unless you eat a quantity that becomes impractical rather quickly.
We keep fruit for between meals for exactly that reason. It is sweet, it needs no preparation, and it fills a gap without ruining dinner.
Starches and legumes, five to six hundred a pound
Potatoes and sweet potatoes. Beans, lentils and chickpeas. Rice, oats, quinoa, buckwheat and corn.
These carry five or six times what vegetables carry, in a form that still takes up room in the stomach.
That combination is what makes a meal hold.
It is no accident that these are the foods people actually lived on for thousands of years.
Rice across Asia, potatoes across Europe, corn and beans across the Americas.
Not because anybody had worked out the numbers, but because these were the foods that let a person finish a day of work without thinking about food during it.
Nuts and seeds, around 2500 a pound
Nuts are the opposite problem.
They carry enormous energy in almost no volume, which is why a handful of them disappears without ever registering as food.
They are genuinely good for you and we eat them every day. They belong in a dressing, in a sauce, or scattered over something else.
A bowl of nuts is not dinner and it will never feel like dinner, whatever the calories say!
What to do with four numbers
We build the meal on the five-to-six-hundred foods and put the vegetables around them.
Use the nuts to make it taste good, and keep the fruit for between meals.
That is the whole of it, and it is why our motto has always been that the fewer the ingredients, the better.
A bowl of lentil soup with bread is four ingredients and it will hold you for five hours.
An elaborate salad can run to fourteen ingredients and still leave you opening the fridge before bed.
Once we could see which foods carry weight, meals stopped being a question of discipline and became a question of arithmetic.
The next problem is not knowing any of this. It is being nowhere near your own kitchen at one in the afternoon.
And, always, seek health consciously.
