Obesity is a condition in which a person's weight is more than 20% higher than what is recommended for their height. It occurs when there is an imbalance between energy intake and energy expenditure. In other words, you consume more calories than you can use in your daily activities. Lipids are a major cause of obesity and can be found in foods such as margarine, cream, whole milk, eggs and avocadoes. Foods containing fats and lipids are a dense source of fuel, as 1 gram provides 37 kilojoules of energy. Because of this, it is important to eat fatty foods, such as McDonalds, in moderation. Gender is also and important factor, as males tend to have a higher metabolic rate then females, therefore males require more calories to maintain their body weight. Additionally, when females become post-menopausal, their metabolic rate slows down significantly. Your metabolic rate also slows down as you grow older and you do not require as many calories to maintain you weight.
For example you have more than like heard people say that they can eat the same and do the same activities as they did when they were 20 years old, but at 40, are gaining weight. Some people also have an increased chance of becoming obese because of hereditary factors, however genetics account for only a small fraction of human obesity.
Because each fat cell weighs a very small amount, about 0.4 micrograms, it is not practical to count the number of fat cells in a person's body, so science has come up with an easier way to determine whether or not a person is obese. This is called the Body Mass Index. A person's BMI is calculated by dividing the weight of a person (in kilograms) by their height squared (in metres).