It's a concept at the cornerstone of most diets: counting the calories of your food intake so you don't go over the limit.
But just how accurate are calorie labels? And are some calories more "equal" than others?
There is a seemingly endless stream of media articles focusing on the latest diet wonder, whether it involves intermittent fasting or feasting on fats.
Although they protest otherwise, most miracle weight loss programmes come down to calorie restriction.
Behind the Headlines takes a look at the science behind calorie counting, examining why it may be only one aspect of healthy, sustainable weight loss.
This article is not asystematic review, where a team carries out a synthesis of the relevant medical research on a given subject
What's in a calorie?
A calorie is a unit of measurement of how much energy is stored in a mass of food.
Confusingly, the "calories" we talk about in daily life are officially described as kilocalories, or kcals, and this is how they appear on food labels. One calorie equals one kilocalorie.
A single calorie is defined as having approximately the amount of energy needed to raise the temperature of one kilogram of water by 1C.
An average man needs around 2,500kcal a day. For an average woman, that figure is around 2,000kcal a day.
These values can vary widely depending on levels of physical activity. For example, some Olympic swimmers have reported eating as many as 12,000 calories a day when they are competing.
How the laws of the universe affect your weight
When we're dealing with energy, there is one law we always have to consider – the first law of thermodynamics.
The first law is one of the immutable laws of the universe, up there with death, taxes and how you can't travel faster than the speed of light.
It states that energy can never be destroyed, only changed from one form to another. Most of the energy in the universe is in the form of mass: solid objects.
It's easy to underestimate how much energy is stored in mass. For example, the energy contained in a single apple is enough to boil a litre of water.
The complex chemical processes whereby the energy in food is transformed into energy in our bodies makes up part of the metabolism. Metabolism is all the chemical processes that go on continuously inside the body to keep you alive and well, such as breathing, repairing cells and digesting food.
Even when you're having a snooze, your body requires energy for automatic processes such as breathing and keeping your heart beating. This minimal energy requirement is known as your basal metabolic rate (BMR).
Your BMR accounts for anything between 40% and 70% of your body's daily energy requirements, depending on your age and lifestyle.
Any extra energy you consume above your BMR will either be used by your body when you perform any physical activity, or will be stored as mass.
If you regularly do anaerobic activities (high-intensity activities that make use of physical strength, such as sprinting and weight lifting), any extra energy should be stored as muscle (or more specifically, in the form of glycogen, a converted form of glucose found in muscle tissue).
But if, like many of us, you are not exercising enough, any excess energy will be stored as fat.
The inventor of calorie counting
Much of what we know about the calorie content of different foods is down to one man: Wilbur Olin Atwater.
Atwater was a 19th century American nutritionist who spent most of his career measuring the calorie content of different foods. He used a variety of methods that became known as the Atwater system.
Key to this system was a device he invented called the respiration calorimeter.
The body needs oxygen from the air we breathe to release energy from the food we eat. In the process, carbon dioxide is released, which we breathe out.
Atwater's respiration calorimeter was a chamber that measured the use of oxygen and the production of carbon dioxide when people were placed inside the chamber after eating certain foods.
By measuring this, the calorimeter was able to estimate the heat and metabolic activity different foods produced.
As well as using the evidence provided by calorimeters, the Atwater system also used mathematical equations to take into account factors such as energy lost through urine, faeces and various gases.
How accurate is the Atwater system?
Atwater was essentially working blind. But with no previous body of research to build on and using Victorian-era technology, his work is surprisingly (but not entirely) accurate.
And while flaws in his original techniques have been widely discussed, no credible alternative methods of measuring calorie contents have been produced.
Calorimeters – albeit more advanced versions – are still used by dietitians and food makers to estimate the calorie content of foods today.
But there is evidence that some of the underlying mathematics of the Atwater system fails to take into account certain vagaries of the human digestive system.
For example, in a 2012 study researchers had the unenviable task of picking through faeces samples from 18 healthy adult volunteers to check for undigested food stuffs.
They found the body has difficulty digesting almonds. The Atwater system failed to take this into account, so, according to the study, wrongly overestimated the energy content of almonds by 32%. The researchers argued a similar overestimation of energy content could apply to other nuts.
According to biochemist Professor Richard Feinman, this flaw in the Atwater system is the result of a failure to appreciate the second law of thermodynamics.
Calories and metabolic efficiency
The Atwater system, and calorie counting in general, is based on the principle of "a calorie is a calorie". What you eat, whether it is honey, hummus or haddock, is unimportant. It's the amount you eat, right?
Feinman argued that this approach, while seemingly logical, is flawed because it does not take the second law of thermodynamics into account.
This law states any complex system will experience increasing disorder over time. All systems have an ingrained inefficiency. The energy that drives organisms, machines and processes will always "leak out", usually in the form of heat.
Our digestive systems, as marvellous as they are, cannot overcome this ingrained inefficiency. Some energy will always leak out.
The level of inefficiency varies depending on the type of food being eaten – a concept known as metabolic efficiency. The higher the metabolic efficiency, the more energy you receive from food.
Foods with poor metabolic efficiency are known as having a "metabolic advantage" – they make your metabolism work harder, so you are less likely to gain weight from eating them.
There are many things that can affect metabolic efficiency, not least cooking. As a 2009 anthropological paper argued, it was the discovery of fire – and, by extension, cooking – that probably kept the human race from dying out during the last ice age.
Cooked foods, especially cooked carbohydrates, have a far superior metabolic efficiency compared with raw fruit and vegetables.
Cooking may also have been responsible for the rapid development in brain power that occurred over the past 100,000 years (our brains use 20% of the body's total energy intake).
This is great if you are trying to survive in your cave through the winter. But it's not so great if you are tying to lose weight.
Calories in processed foods
In industrialised countries, much of our diet is made up of processed foods such as crisps, biscuits, burgers and ready meals. Processed foods have been found to have a very effective metabolic efficiency.
A 2010 US study bears this out. In the study, a small group of volunteers were assigned to eat one of two cheese sandwiches:
a processed-food sandwich – made up of white processed bread and processed cheese "products"
a "whole-food" sandwich – made up of multigrain bread and cheddar cheese
The interesting part of the experiment is that both sandwiches had roughly the same nutrition content:
A spirometer (a device used to measure airflow in and out of the lungs) was then used to estimate how much energy the body used (in terms of calorie consumption) when digesting the sandwiches.
The whole-food sandwich took around 137 calories to digest, which accounted for 19.9% of the total energy provided by the meal. The processed-food sandwich only took 73 calories to digest, which accounted for 10.7% of the total energy provided by the meal.
Imagine we took two identical twin brothers – Alan and Bob – and made them stick to different diets over the course of a year, but did not allow them to exercise.
Bob – eating the processed food sandwich – would theoretically put on around twice the amount of weight as Alan, even though the nutritional content of their diet was identical in terms of protein, carbohydrates and fats.
Another concern about processed foods is that they tend to have a high sugar content. Even foods you would never suspect, such as pizza, yoghurt and cheese, are often fortified with sugar.
Focusing solely on the calorie content of your food at the expense of its nutritional value could lead to health problems further down the line.
Interestingly, a 2014 study looked at a series of the more fashionable diets and found they were all pretty similar in terms of achieving weight loss.
Many of these diets are based on the idea that excluding certain types of food can essentially "hack" the metabolism, causing it to burn off weight at an increased rate and increase your metabolic advantage.
For example, the Atkins diet is based on the principle that by cutting carbohydrates out of your diet, the body is forced to look elsewhere to find glucose so it starts burning fat – a process known as ketosis.
Attempting to "cheat" the metabolism comes at a cost. In the short term, ketosis resulting from a low-carb diet can lead to symptoms such as nausea and bad breath. But in the long-term, it may cause kidney problems such as kidney disease and kidney stones.
Ultimately, there is no such thing as an inherently "bad" type of food. It is common for certain types of food to be demonised in the press and by the diet industry: one month it's carbohydrates, the next sugars, and the month after that saturated fats. The truth is we need all three to function properly. The important thing is to get the balance right.
Current recommendations say the main staples of your diet should be fruit and vegetables, as well as starchy foods such as rice and pasta. We should also include a moderate amount of protein, such as meat and eggs, and a moderate amount of dairy products such as milk and cheese. And then just a small amount of saturated fats and sugars completes a balanced diet.
As well as focusing on the physical aspects of your diet, it may also be beneficial to look at your emotional and psychological attitudes towards diet, eating and the role of food as a reward or addiction.
Focusing on only the physical issue of calories and ignoring the psychological aspect of your eating habits will probably not lead to sustainable, long-term weight loss.
There is a growing body of research to suggest that combining a calorie-controlled diet with a talking therapy known as cognitive behavioural therapy (CBT) can be effective in helping people lose weight.
CBT is based on the principles of identifying unhelpful and unrealistic patterns of thinking and behaviour, and then trying to replace them with more helpful and realistic patterns to improve health outcomes.
Many researchers are now combining elements of CBT with traditional calorie-controlled diets in an approach known as "behavioural weight loss".
A 2011 study looked at how well behavioural weight loss fared when compared with the standard CBT programme. It found people treated with CBT were more likely to have remissions from bingeing behaviour, but also lost a little weight.
Those treated with behavioural weight loss had a lower rate of remission (36% compared with CBT's 51%), but experienced a statistically significant drop in BMI.
And over the course of 2014 there have been three interesting studies covered by Behind the Headlines about the impact of psychology on eating habits:
If you think you could benefit from CBT or behavioural weight loss, your GP or the doctor in charge of your care should be able to provide more information.
Even if you are not a binge eater, you may find there are certain triggers that cause you to throw your good intentions out the window and have a sudden splurge.
These triggers may be emotional, such as feeling stressed, anxious or bored. They can also be environmental, such as going to the cinema, the local pub or dining out with friends.
Environmental "cues" that can trigger overeating shouldn't be overlooked. A study from August 2014 found people who ate in "posh" restaurants consumed just as many calories as those who ate fast food. Learning to spot these "diet danger zones" is a useful exercise – being forewarned is to be forearmed.
Many psychologists with an interest in weight loss have warned against adopting an extremely rigid attitude towards calorie consumption. The more rigid the rules of your diet, the more likely it is you will just give up on the whole thing if you find yourself breaking the rules.
Rather than setting a strict daily limit on calories, it may be a better idea to set weekly limits. So if you do find yourself slipping up one day, you can always make up for it over the rest of the week.
Do calories matter?
Calories do matter. There is no getting away from that fact. If you repeatedly eat more calories than you burn off, you will put on weight. It's the first law of thermodynamics.
But is obsessively focusing on the calorie content of everything you put in your mouth a healthy and sustainable way to achieve weight loss and improve your health? Possibly not.