Most healthy diets do tend to have things in common. Lots of plants with a moderate amount of other things and not much sugar, salt or saturated fat.

So why do we find it so hard?

It’s difficult to do this question justice, as it involves complex psychology, thermodynamics and tableware. It also takes in everything from philosophy, through digestive physiology to economics. Entire books can and have been written about this topic, but I’ll try to answer concisely.

One big gap in many people’s knowledge is: What is a food calorie? According to Merriam-Webster it is “…the amount of heat required to raise the temperature of one kilogram [about 1 litre] of water one degree Celsius…”

So talking about calories in food metabolism is a bit of an approximation. Since it simplifies the whole of human metabolism to burning fuel to heat water in a steam engine. Even different steam engines differ in what fuel they best use.

A better question might be: What is the healthiest diet for you?

Most of us are mammals (a big hello to all our reptilian readers [flicks forked tongue]). This means we keep our bodies at optimum temperature (about 37 Celsius) by producing heat rather than relying on ambient temperature.

We do this by deliberately burning more calories than we need to in order to make heat. Therefore temperature will have an effect on how many calories we burn. Polar explorers need to eat far more than most people just to keep the heating on.

Exercise also has an impact. Professional athletes such as Tour de France cyclists are estimated to need between 6000-8000 calories per day, compared to the approximately 2000 calories required by the average person (average people please raise your hands now).

An amusing analysis of this can be found in the wonderfully titled Simply Walking into Mordor: How Much Lembas [elf loaves] would the Fellowship Have Needed?

Exercise also tends to require different sources of energy according to how intense it is. Broadly speaking, carbohydrates (found in sugary and starchy foods) burn quickly, protein (found in seeds and meat) and fat (found in oil and meat) more slowly. It’s a bit like the difference between sticks and logs on a fire, carbs (sticks) burn responsively but don’t last, fats (logs) burn for a long time but are hard to get going. Therefore different intensity of exercise requires a different fuels.

With regards to fats and carbs, scientists can’t agree which is the body’s primary fuel and which is the backup. Do we mostly burn carbs and use fat when carbs are not available or burn fat and use carbs as a boost when they’re available? As is often the case, there is evidence for both.

Also different types of food move through people and are digested at very different speeds. Simple carbs, found in sweetened drinks and cakes, are digested very quickly. More complex carbs, found in “wholegrain” foods take longer.

Proteins and fats are digested more slowly and tend to reduce the speed at which food leaves the stomach, so that it has longer to break them down. This provides some credibility for “fuller for longer” high protein foods and low carb diets.

Fibre is digested very slowly, if at all. Surprisingly, this is a good thing, hence the importance of vegetables. This is because fibre also traps other things slowing down their digestion.

This is why apples are healthier than cakes or apple juice, despite having the same sugar content .

This relates to the idea of “Free sugar” defined by the WHO as “… all monosaccharides and disaccharides [sugars] added to foods by the manufacturer, cook or consumer, plus sugars naturally present in honey, syrups and fruit juices [i.e not bound up in solids]…”

Different peoples nutritional needs will also be different, food gives the body material as well as fuel. If you are trying to get bigger muscles or are having a “growth spurt” that means you might need a little more protein and energy. The key question might not be “are you gaining weight?” but “are you growing taller, fatter or more muscular?”

So far, this has all been relatively simple: how much food do you eat and how much do you burn. However when you are trying to work out how to change either food intake or calorie consumption things get complicated because of people’s preferences and willpower.

We eat when we are hungry until we are full, it’s that simple, till about age 3. However, our readers above the age of 3 may find that they sometimes eat when they’re not hungry, and sometimes don’t eat when they are hungry. Being an adult is hard.

In fact, it turns out that even something as simple as the size of the plate or the way we pay for the food can influence our choices, by tricking our brains into thinking we are eating more or less than we actually are.

What foods you like will make certain diets harder to follow. How much you follow the diet plan is the key determinant. In order to have a healthy diet, we need to ignore the short term temptation of simple pleasures to gain long term, more complex goals . To do this we need to be completely aware of all the weird ways our environment and wishes distort our perception of reality and keep in mind who we want to be months down the line - quite a feat for a brain designed for picking fruit and getting laid. This is what is often called willpower, and is important for loads of other things, like drinking responsibly, quitting smoking and not strangling your editor with your bare hands.

There is a theory that these other uses of willpower leads to it becoming depleted over time, making dieting harder as it goes on and willpower becomes more depleted. However there is some evidence that willpower does not deplete as much as we thought, and scientists remain divided.

If I might offer a piece of advice: just because you slipped up once does not mean the diet has failed and should be abandoned: you’ve already made great progress.

References and further reading

Merriam-Webster definition of the calorie:

http://www.merriam-webster.com/dictionary/calorie

Calorific intake of Tour de France athletes:

http://www.telegraph.co.uk/men/active/recreational-cycling/11729780/6000-calories-What-a-Tour-de-France-rider-eats-in-just-one-day.html

http://www.mensjournal.com/health-fitness/nutrition/the-tour-de-france-diet-how-to-consume-8-000-calories-a-day-20140710

WHO technical report series - “Diet, nutrition and the prevention of chronic disease”:

http://apps.who.int/iris/bitstream/10665/42665/1/WHO_TRS_916.pdf?ua=1

Serving portion size influences 5-year-old but not 3-year-old children’s food intakes:

http://www.andjrnl.org/article/S0002-8223%2800%2900070-5/abstract

Our gigantic problem with portions: why are we all eating too much?

https://www.theguardian.com/lifeandstyle/2016/apr/25/problem-portions-eating-too-much-food-control-cutting-down

Will paying with contactless cards make us less healthy?

http://www.bbc.com/news/business-36358030

The faulty walnut - exploring the weaknesses of the human brain:

https://www.youtube.com/watch?v=ilEwSPljeFc