Even siblings consuming similar diets may respond to calories differently, let alone people of different age, shape, gender, and lifestyle.

One calorie for one person is often more, than one calorie for someone else, and this is one reason why weight-loss diets, and weight maintenance diets, can fail.

AGE DIFFERENCE. Younger women of the exact same shape, weight, height, and even genetic background will always lose weight faster on the exact same diet as older women.

You are not your 20+ year old daughter. She is at the peak of her procreation mission with a metabolism to match. She is still learning about the world around her and has an incredibly busy brain, which is a large consumer of glucose. Her body may still be growing. She is healthier. She may be taller. Her body has more muscles, even if she is the exact same size and shape as you are. She has only half of your genes; the other half is from her father. She is more active simply because she can be. She sleeps better than you, even if she sleeps less. She needs more calories simply because she is younger. As the body ages, it needs less and less calories to maintain the same weight.

SMALLER MUSCLE MASS. The female body contains significantly less muscle than males of similar shape and weight. Women experience faster loss of muscle throughout pregnancy, breastfeeding, and natural aging. Since muscles are one of the most demanding users of energy, their age-related loss reduces the rate of energy uptake, and, correspondingly, increases the gain of body fat.

GLUCOSE UPTAKE BY MUSCLES. Along with the brain, central nervous system, and blood, muscles are the most prolific consumers of glucose. That is why most men on a low-carb diet lose weight faster than women and don’t gain it as quickly with a larger intake of carbohydrates.

Women require fewer carbohydrates than men, yet at the same time improperly structured ultra-low-carbohydrate diets can be inappropriate for older women because these may tend to accelerate muscle wasting.

GREATER FAT MASS. Women have a higher ratio of body fat to total body weight than men. Body fat is essential for reproductive functions, healthy pregnancy, and nursing. As body fat falls below a certain level — around 10% to 15% — infertility and amenorrhea (the absence of a period) set in.

Because body fat plays such an essential role in female reproductive and overall health, women gain fat faster than men on similar or smaller amounts of foods.

THERMOS  EFFECT. As one gains fat, the body lowers the internal rate of energy metabolism (i.e., produces less heat) because the internal organs are cuddled in the warm blanket of one’s own fat, or, as doctors would say, adipose tissue. That is why overweight people are far less sensitive to cold than skinny ones. Unfortunately, this effect has a profoundly negative impact on the ability to lose weight because the rate of metabolism is so much lower, and this has little or nothing to do with thyroid or adrenal glands that one might think are “underactive.”

HEIGHT. A person’s height is an important factor in energy metabolism and, correspondingly, in obesity and weight ...