Kcal is the abbreviation for kilocalorie. That is a unit of energy needed to raise 1 kg water from 15 to 16 °C. A kilocalorie is equivalent to 4.184 kJ. In common speech a kilocalorie is actually refers only as a calorie.
1 cal = 4,184 J (1kcal = 4,184 kJ) / 1 J = 0,239 cal (1kJ = 0239 kcal)
Energy (kilocalories) comes from three main macronutrients: proteins, carbohydrates and fats. 1 g of fat provides 9 kcal (38 kJ) and 1 g carbohydrate provides 4 kcal (17 kJ) of energy as well as 1 g protein. Everything what we eat is a combination of these three macronutrients.
The first law of thermodynamics states that energy can be neither created nor destroyed, but only transformed. The human body is constantly transforming energy (kilocalories) by combusting foodstuffs to produce heat.
From a thermodynamic point of view, a calorie is of course a calorie. Each macronutrient can be used as an energy source, and technically each produces the same unit of energy (kilocalorie). However, nothing is as simple as it seems. The macronutrients all have very different jobs in our body. Let’s have a look at each one. Carbohydrates are primarily the fastest energy source, proteins are important for the construction and protection of muscle mass, organs and binders, and are also used as enzymes, hormones and antibodies. Proteins offer not only a high thermic effect, the amount of energy required to digest and process the food we eat, but also provide a the highest degree of satiety of all macronutrients, helping to significantly reduce your temptation to snack between meals. Fat is an essential structural component of body cells, hormones and other substances, provides a vitamin transfer, protective and thermal isolation of body organs.
The different effects of macronutrients in the human body were also confirmed by A. C. Buchholz and D. A. Schoeller (2014). They evaluated weight loss and energy expenditure in adults consuming diets high in protein and/or low in carbohydrate with those in adults consuming diets low in fat. After reviewing the individual studies, the high protein diets’ efficiency in weight reduction compared to low-fat diets (at the same total daily energy intake) was confirmed.
Thermodynamics dictate that a calorie is a calorie regardless of the macronutrient composition of the diet. It’s a drastic oversimplification to say “a calorie is a calorie.” The big difference is to eat 2,000 kcal through fat, carbohydrates or protein. We are what we eat. According to European Food Safety Authority (EFSA), total daily energy intake should be optimally consists of 10-15% protein, 20-35% fat and 45-60% carbohydrate.
BUCHHOLZ, Andrea C a Dale A SCHOELLER. Is a calorie a calorie?. The American Journal of Clinical Nutrition [online]. 2004, 79(5), 899S-906S [cit. 2018-03-22]. DOI: 10.1093/ajcn/79.5.899S. ISSN 0002-9165. Dostupné z: https://academic.oup.com/ajcn/article/79/5/899S/4690223
Dietary Reference Values for nutrients Summary report. EFSA Supporting Publications [online]. 2017, 14(12), – [cit. 2018-03-22]. DOI: 10.2903/sp.efsa.2017.e15121. ISSN 23978325. Dostupné z: http://doi.wiley.com/10.2903/sp.efsa.2017.e15121
SKOV, AR, S TOUBRO, B RØNN, L HOLM a A ASTRUP. Randomized trial on protein vs carbohydrate in ad libitum fat reduced diet for the treatment of obesity. International Journal of Obesity [online]. 1999, 23(5), 528-536 [cit. 2018-03-22]. DOI: 10.1038/sj.ijo.0800867. ISSN 0307-0565. Dostupné z: http://www.nature.com/articles/0800867