A reducing diet is a change in eating habits intended to decrease the amount of adipose tissue (also called body fat) in a person's body by eating fewer calories than the person uses.
The body mass index (BMI) is an estimate of body composition. In the mid-19th century, Belgian sociologist Adolphe Quetelet defined what is now called BMI as an individual's mass in kilograms divided by the square of height in meters. This produces units of area density: kilograms per square meter (kg/m²), often omitted. It can also be calculated using American units as weight in pounds times 703 divided by square of height in inches. If the size of one's feet is assumed proportional to height, one can interpret it physically as pressure on the soles of the feet.
A rule of thumb holds that BMI more than 25.0 kg/m² is overweight, and more than 30.0 kg/m² is "obese," or overfat with an increased risk for falls, circulatory disease, type II diabetes, or degenerative joint disease. It's a decent first approximation for the general population, though because it confuses overweight due to muscle with overweight due to fat, it has limits in more athletic or taller people.
Most weight is lost through carbon breathed out. As calories out exceed calories in, the body dips into the roughly 500 grams of glycogen in the muscles and liver. Each gram of glycogen has 4 calories and is bound to 3 to 4 grams of water, for a total of roughly 5 pounds of glycogen hydrate. Thus the first week will produce an atypically large loss as excess water formerly bound to glycogen is shed as urine. Beyond that, weight is lost from adipose tissue at 3500 calories per pound. It's considered safe to lose 1 to 2 pounds (450 to 900 g) of fat per week without stressing the kidneys, which translates to a deficit of 500 to 1000 calories per day. In many men, a diet of 1500 calories per day produces a deficit close to 1000 calories per day.
Excess calories burned through physical activity can offset eating on the same day. Be careful not to double-count an offset, as many published charts of calories burned during exercise include basal metabolic rate (BMR), which a diet's daily allowance already includes.
In 2014, Coca-Cola and two universities founded the short-lived non-profit organization Global Energy Balance Network (GEBN) promoting reliance on exercise offsets. However, over the course of the average person's day, BMR greatly exceeds exercise excess, limiting the effectiveness of such an approach.
The 1-day food diary
A realization that a 1500-calorie diet is roughly 100 calories per waking hour led to a breakthrough in one editor's weight loss journey: the 1-day food diary. Scheduling in the morning what to eat when over the course of the following day can help someone stick to a reducing diet. Grazing on portions of 50 to 300 calories, such as a 70-calorie can of green beans, helps avoid the sense of deprivation associated with going several hours without food.
This reducing diet incorporates a mechanic analogous to what WW used to call "flex points." If 100 or more calories remain at bedtime, the remaining calories go to the bank and can be used to offset mild overeating on later days. Banked calories expire on the 7th day after they are earned. To build bank, distract self from time to eat with work or exercise. Then after eating, push scheduled times of following portions forward.
Every Saturday morning, before I eat or drink anything, I'll weigh myself. A diet goal file lists a target weight for each weigh-in, decreasing by 1 to 2 pounds per week. After weighing in, if ahead of forecast, adjust the next two weeks' forecast to lose no less than 1 pound each, then forecast 1.5 pounds per week until back on schedule, or interpolate to put BMI milestones on weigh-in days. Then drink two cups of water per lost pound to ensure lost weight is fat, not water. A loss of more than 2 pounds in a week after the first week is an excuse for dipping into the bank for a cheat day.