What are calories?
A calorie is a unit measure of energy. Calories on food packages are actually Kilocalories (Kcal), or 1,000 calories. A Kcal is a measure of energy needed to raise the temperature of 1 kilogram of water 1 degree Celsius. The energy content of food can also be expressed in kilojoules (KJ); one Kcal equals 4.184 KJ. Originally, Kcal in foods were measured by determining the calories in a bomb calorimeter by burning the food and recording water temperature increases. Today, food label calories are determined by indirect methods (the Atwater System), by looking up caloric values in tables provided by the USDA and other reference tables. 4 Kcal/g is assigned to proteins and fats, and 9 Kcal/g is assigned to fats. Values of 4 and 9 assume a food is fully absorbed, and represent total energy (TE). If a food is not completely absorbed, with all or part of it ending up in stool, then only a portion of the food can be used by our bodies to provide useful metabolic energy. In these instances, the manufacturer provides energy data to the US Food and Drug Administration (FDA), and petitions for a reduced energy value, representing digestible energy (DE; TE-fecal energy=DE). Full fibers have a caloric value of 0, sugar alcohols each have a unique DE and hence their own label calories, but are generally considered to have 2 Kcal/g, with exception of erythritol (0 Kcal/g). On April 18, 2019, the FDA concluded, following petitions, that the rare natural sugar allulose can have a value of 0.2-0.4 Kcal/g on food labels. High intensity sweeteners (HIS) also have reduced caloric values, and there are many other examples of foods and ingredients with reduced caloric values.
How many calories should you measure in a day?
To maintain constant body weight, one should be in energy balance, meaning energy in (usable food energy, most correctly DE or metabolizable energy, ME) equals energy out (energy expenditures). Energy expenditures equal resting metabolic rate (RMR), the thermic effect of feeding (TEF), and the thermic effect of activity (TEA). RMR represents the minimum amount of energy necessary for a person's body to keep functioning at a healthy homeostasis: lungs breathing, heart beating, brain working, and a normal body temperature. Basal metabolic rate (BMR) is not identical to RMR but is a related concept. RMR represents 60-75% of one’s daily energy expenditure. RMR is increased in response to exercise. Heat given off from eating food (TEF) represents 10% of energy expenditure. TEA represents energy devoted to muscular activity (includes shivering and fidgeting) and physical work (exercise), and represents 15-30% of energy expenditure. Formally, each person should calculate their calories to maintain current body weight or to lose a given amount of weight, but this is not very practical on a food label, but may be measured by competitive athletes.
Thus, on food labels, a value of 2000 Kcal/day is typically represented on food labels for adults, and percent daily values (DVs) are based on this amount of calories. There is a long history on how and why this value of 2000 was assigned. For most of us, 2000 calories is likely insufficient to maintain energy balance, but the thinking is that if a higher caloric value were assigned, people would eat more. Considering our current obesity epidemic, for practical reasons, it is reasonable to use the 2000 value. Clearly if you are a Tour de France cyclist, you can consume up to 7000 calories per day, without gaining weight. So one must use common sense when determining the amount of calories to consume each day; and determine what your body weight and body composition goals are.
How can you reduce your intake or burn more calories per day
Set point theory, based on evolutionary principles, dictates that whether one is a bit underweight or overweight, there are metabolic and hormonal forces driving us to maintain current body weight. So, reducing caloric intake is never easy. Here are some suggestions to decrease caloric intake. Increase your exercise level (TEA Physical work), and avoid post-exercise, typical compensatory increases in caloric intake by consuming lots of liquid, and fibrous, filling foods. Consume foods with a high satiety index (SI), that is, foods eaten at one meal, that lead to less foods consumed at a subsequent meal. Potatoes (baked potatoes) are an example of a food with a very high SI. Some fatty foods are known to diminish appetite, which relates to how much food is consumed at a current meal. Foods such as avocados, flax, Greek yogurt, legumes, soups, cottage cheese, oatmeal, meats, and almonds (whole) are reported to diminish appetite. Minimize consumption of addicting foods like sugary deserts, which bypass our normal appetite regulatory mechanisms. Avoid foods rich in both fat and sugar such as donuts, because humans have not evolved the control mechanisms to regulate intake easily. Foods rich in both fat and sugar do not occur in nature.