"A calorie is a calorie" is a tautology used to convey the speaker's conviction that the concept of the "calorie" is in fact a sufficient way to describe energy content of food.
It has been a commonly cited truism since the early 1960s. The tautological phrase means that regardless of the form of food calorie a person consumes (whether a carbohydrate, protein or fat calorie) the energy chemically extracted from the food, or the work necessary to burn such a calorie, is identical to any other. One dietary calorie contains 4.184 kilojoules of energy. With this knowledge, it is easy to assume that all calories have equal value.
In 1878, German nutritionist Max Rubner crafted what he called the "isodynamic law". The law claims that the basis of nutrition is the exchange of energy, and was applied to the study of obesity in the early 1900s by Carl von Noorden. Von Noorden had two theories about what caused people to develop obesity. The first simply avowed Rubner's notion that "a calorie is a calorie". The second theorized that obesity development depends on how the body partitions calories for either use or storage. Since 1925, a calorie has been defined in terms of the joule. The definition of a calorie changed in 1948, which became one calorie is equal to approximately 4.2 joules.
The related concept of "calorie in, calorie out" is contested and despite having become a commonly held and frequently referenced belief in nutritionism, the implications associated with "a calorie is a calorie" are still being debated. The wisdom and effects of skipping meals in an attempt to limit caloric intake is also still largely debated.
Calorie amounts found on food labels are based on the Atwater system. The accuracy of the system is disputed, despite no real proposed alternatives. For example, a 2012 study by a USDA scientist concluded that the measured energy content of a sample of almonds was 32% lower than the estimated Atwater value. Furthermore, it is known that some calories are lost in waste, without ever having been chemically converted or stored. The driving mechanism behind caloric intake is absorption, which occurs largely in the small intestine and distributes nutrients to the circulatory and lymphatic capillaries by means of osmosis, diffusion and active transport. Fat, in particular is emulsified by bile produced by the liver and stored in the gallbladder where it is released to the small intestine via the bile duct. A relatively lesser amount of absorption, composed primarily of water, occurs in the large intestine.
In 2007, a group of Wake Forest University researchers published a report from a six-year longitudinal study in which they fed two groups of monkeys the same number of calories and dietary levels of fat, with the only difference being that one group was fed foods higher in trans fat. The high trans fat group gained 30% more belly fat compared to their lower trans fat counterparts. The researchers concluded that the type of calories consumed do affect body weight.
It should be noted that the physical activities of the monkeys were not taken into account during this study and the sample size was 38.
In 2011, a group of Harvard University researchers published the results of a study in the New England Journal of Medicine that followed 120,877 highly educated men and women over a period of 12 to 20 years. The study focused on factors that influence weight gain including diet, exercise, sleep, smoking, alcohol intake and television watching. Participants began the study as healthy adults. Every two years, they would complete detailed questionnaires about their eating and other habits. The results found that an array of factors influenced the fluctuation of a person's weight. The average participant gained about one pound per year. Among the results of the study, it was found that the types of foods people ate had a larger effect on weight gain than physical activity. According to the researchers "Consumption of processed foods that are higher in starches, refined grains, fats, and sugars can increase weight gain." and "These results suggest that future policies and research efforts to prevent obesity should consider food structure and processing as potentially relevant dietary metrics." The lead author of the study concluded in an interview that trying to count calories, in an effort to lose weight, would be futile unless one is examining the kinds of calories being consumed.
One dietary Calorie contains 4184 joules of energy, this may cause individuals to believe that all calories are the same. The human body is a highly complex biochemical system that undergoes processes which regulate energy balance. The metabolic pathways for protein are less efficient than the metabolic pathways for carbohydrates and fat. Protein contains four calories per gram, although a large part of the calories are lost as heat when metabolised by the body.
It may be easy to consume 500 calories worth of ice cream or chocolate in one sitting, although it may be difficult to eat 500 calories of eggs or carrot in one sitting.
In 2012, the Journal of the American Medical Association published the results of a clinical trial performed by a group of researchers that investigated whether dietary composition affected weight loss. The study tracked 21 individuals. The individuals first lost at least 12.5% of their body weight, and were then placed on one of three different dietary regimens:
- A diet high in protein and fat, but with fewer carbohydrates
- A diet low in fat, emphasizing whole grains, fruit and vegetables
- A diet with a low glycemic index, focusing on the type of carbohydrates consumed
The results showed that the first group burned the most calories, but also displayed increased markers of stress and inflammation in the body, which can lead to cardiovascular disease, among other health problems. The second group burned fewer calories than the other two groups, and also displayed certain metabolic indicators that typically precede weight gain. The third group burned a reasonable number of calories, but notably did not display increased markers of disease-causing stress. The researchers concluded that the type of calories consumed does affect the number of calories burned by an individual. This conclusion is in direct contrast to what the commonly held belief implies.