What happens inside the body when you eat too much or too little food? The body ends up with an unbalanced energy budget, meaning you have taken in more or less food energy than you spent. when more food energy is consumed than is needed, excess fat enters the fat cells in the body's adipose tissue for storage. When the energy supplies run low, stored fat is withdrawn. The daily energy balance can therefore be stated like this:

change in energy stores = energy in - energy out

Too much or too little fat on the body right now does not necessarily reflect today's energy intake of course. Small imbalances in the energy budget accumulate over time to make us underweight, or obese.

The energy in foods and beverages is the only contributor to the "energy in" side of the energy balance equation. Before you can decide how much food energy you need in a day, you must first become familiar with the amounts of energy in foods and beverages. One way to accomplish this is to look up calorie amounts associated with foods and beverages in a good Table of Food Composition. Many people also now have computer programs which can provide this information much more quickly. As an example of calories associated with food portions: an apple gives you about 125 calories from carbohydrate; a regular candy bar gives you about 250 calories, mostly from fat and simple carbohydrate. For each 3,500 calories you consume in excess of expenditures, you store approximately 1 pound of body fat. That is, 1 pound of fat = 3,500 calories above the energy balance. If you consume 500 calories in excess every day for a week (7 days), then you can expect to put on about 1 pound of fat.

Though the energy present in a serving of food or in a day's meals can easily be estimated, no easy method exists for estimating how much energy an individual spends. How much you need. Many authorities have published recommended energy intakes for various age-sex groups in their populations, and these can be found with just a little effort. These figures are good for population studies, but energy needs vary so widely among individuals that it is impossible to estimate that person's energy needs without knowing the facts about their characteristics, such as their basal metabolism, and their lifestyle. The recommendations for energy intake are based on average people. For example, an intake of 2,200 calories per day is recommended for a woman who is assumed to be 20 years old, standing 5'5" tall, and weighing about 128 pounds, of average body fatness, and engaging in light activity. The man, needing 2,900 calories per day, is a healthy 20-year old of average body fatness who stands 5'10" tall, weighs 160 pounds, and is also lightly active. Taller people need proportionately more energy than shorter people to balance their energy budgets because their greater surface area allows more energy to escape as heat. Older people generally need less than younger people due to slowed metabolism and reduced muscle mass that occurs partly because of reduced physical activity. On average, the energy need diminishes by 5 percent per decade beyond the age of 30 years.

In reality though, no one is average, and people's energy needs vary widely. In any group of 10 or 20 similar people with similar activity levels, one may expend twice as much energy per day as another. A 60-year old who bikes, swims, or jogs each day may need as many calories as another person of 30. Clearly with such a wide range of variation, a necessary step in determining any person's energy need is to study that person.

Once way to estimate your energy needs is to monitor your food intake and body weight over a period of time in which your activities are typical of your lifestyle. If you keep an accurate record of all the foods and beverages you consume for a week or two and if your weight has not changed during the past few months, you can conclude that your energy budget is balanced. At least three days of honest record-keeping are necessary because intakes fluctuate from day to day. A week or two is better still. 

Another method of determining energy needs is based on energy output. To estimate output, a person must compute the amount of the two major components of energy expenditure, and then add them together. This method leaves out a third energy component, the body's metabolic response to food. About 5 to 10 percent of a meals's energy value is used up in stepped-up metabolism in the five or so hours after the meal; this category of energy expenditure is called the "thermic effect of food". Although this amount of energy could affect expenditures over the long run, most experts believe its effects are negligible. The two major ways in which the body spends energy are (1) to fuel its basal metabolism, and (2) to fuel its voluntary activities. Basal metabolism generates energy to support the body's work that goes on all the time without our conscious awareness. Reactions such as breathing, heartbeat, thinking, digesting, and cell-division all require energy, and the amount of energy needed to fuel these activities is what determines our basal metabolism. Basal metabolism consumes a surprisingly large amount of fuel, and the basal metabolic rate (BMR) varies from person to person. A person whose total energy needs are 2,000 calories a day may spend as many as 1,200 to 1,400 of them to support basal metabolism (doing no voluntary activity). The hormone thyroxine directly controls basal metabolism. The less hormone secreted, the lower the energy requirements for basal functions; the more hormone produced, the higher the energy requirements are. Thyroxine is produced by the thyroid gland, and proper amounts of Iodine (150 mcg/day) are necessary for it's production. Many other factors also affect the BMR, as you are about to learn.

Everyone wants to know how they can speed up their metabolism to promote fat loss. You cannot speed up your BMR in the space of just a few days. You can however, amplify the second component of energy expenditure, your voluntary activities. If you do, you will spend more calories today, and if you keep doing so day after day, your BMR will also increase. Lean tissue is more metabolically active than fat tissue, so a way to speed up your BMR to the maximum possible rate is to make endurance and strength-building exercise a daily habit so that your body composition becomes as lean as possible. As for fuel for voluntary activities, the amount of energy you spend in exercise depends somewhat on your personal style, and on having the right amounts of all the proper nutrients your body needs to get you through these activities. In general, the heavier the weight of the body parts you move in your exercise and the longer the time you invest, the more calories you spend, The more lean you become, and the more calories you spend through basal metabolic functions. In nutrition science, a calorie is measured as a unit of heat. One calorie is the amount of heat required to heat one milliliter of water by one degree. This heat energy is what allows us to fuel our bodies through basal metabolism and voluntary activities. We are all solar-powered. Get used to it.

How do I calculate my BMR?

Use the BMR factor     1.0 calorie per kilogram (2.2 lbs) of body weight per hour for men

                                    .9 calories per kilogram (2.2 lbs) of body weight per hour for women

The factors are different for men and women because men generally have more muscle tissue. Example, for a 150 pound woman.

1. Change pounds to kilograms:

150 pounds/2.2 pounds per kilogram = 68 kilograms

2. Multiply weight in kilograms by the BMR factor:

68 kilograms X 0.9 calories per kilogram per hour = 61 calories per hour

3. Multiply the calories used in one hour by the hours in a day

61.2 calories per hour X 24 hours per day = 1,469 calories per day.

Use the same formula for men, only use 1 calorie per hour, instead of 0.9 calories.

Determine your lifestyle

Sedentary: You sit down most of the day and drive or ride whenever possible. A person who does small amounts of cooking or housework, or types at a computer would be considered sedentary.

Light activity: You move around some of the time, as a teacher might during working hours. A foreman at a factory. A salesperson in a store. Housework taking more than an hour or so continuously.

Moderate activity: You engage in some intentional exercise, such as an hour of jogging four or five times a week, or your occupation calls for some physical work.

Heavy activity: Your job requires much physical labor, such as a roofer or a carpenter. Construction worker. More intentional exercise than that described in moderate activity, including strength training.

Exceptional activity: The exceptional category is reserved for those few who spend many hours a day in intense physical training. Professional or college athletes mainly.

The second component of energy expenditure, physical activity, is calculated by multiplying the BMR calories by a percentage that varies by activity level (refer to levels above). These percentages are estimates or approximations of energy expenditure based on the amount of muscular work a person typically performs during the day. You will multiply your BMR by the numbers below. If you are:

Sedentary: Men, 25 to 40 percent; Women 25 to 35 percent.

Light: Men, 50 to 70 percent; Women 40 to 60 percent.

Moderate: Men, 65 to 80 percent; Women 50 to 70 percent.

Heavy: Men, 90 to 120 percent; Women 80 to 100 percent.

Exceptional: Men, 130 to 145 percent; Women 110 to 130 percent.

Calculate your total energy requirement

Calculate your energy expenditure using both the upper and lower ends of the range of percentages given for your gender and activity level. Suppose the 150-pound woman used in the earlier example bikes about ten minutes a day, and walks to complete errands, but otherwise sits or does light housework most of the day. She would fall into the light activity category, so we can estimate the range of energy she needs by multiplying her BMR calories per day by both 40 and 60 percent:

1,469 calories per day X 0.40 = 588 calories per day.

1,469 calories per day X 0.60 = 881 calories per day

The woman needs between 588 and 881 calories per day to fuel voluntary activities. So total the metabolic and activity components, first using the lower number for activity energy, then using the higher number. In one day, the woman in our example spends either:

1,469 + 588 = 2,057 calories

1,469 + 881 = 2,350 calories

Estimate the woman's needs as a range of rounded values: 2,000 to 2,400 calories per day.

By now, you have a very good idea of how many calories you need in a day. You also know that one pound of fat = about 3,500 calories. Furthermore, you know that lean tissue is more metabolically active. So, decide what you want to do. If you want to lose weight you must create an energy deficit that will entail using about 3,500 calories less than you need during the average week, or 500 fewer calories per day. (You can do it more slowly) Or, you must increase your activity level while remaining at a constant energy intake. This will also produce an energy deficit. Keep in mind that as your body becomes leaner, your BMR may also speed up.  The value for a woman of 0.9 calories per kg of body weight per hour is an approximation based on the average healthy woman. While thinking about all of this, keep in mind that fat from foods contains more than twice the calories of either carbohydrates or protein. Fat contains 9 calories per gram, as opposed to 4 calories per gram of either carbs or protein. Remember also, that while our bodies can store only a limited amount of carbohydrate and protein, we can store an unlimited amount of fat. Furthermore, fuel from carbs and proteins that isn't used for their intended activities can be converted and stored as fat. This is important to note because many actions inside your body can only use carbohydrate or protein as fuel. Examples are nervous system function (carb), cerebral function (carb), tissue building (protein), and immune response (carb, protein). Many of these critical activities can't use the calories in fat as fuel. Fat is simply fat, and unlike other nutrients, a fat cell can grow theoretically as large as it needs to. In cases of extreme starvation fat can be used to fuel nervous and brain function, but by that point a person's health is almost ruined beyond recovery. Fat can still be used to fuel aerobic (oxygen-requiring) exercise activities though, this is why exercising for extended periods allows us to lose weight, we're using the fat. More on that in another section.