Macronutrient structure and Metabolism

In order to survive, the human organism needs to take in oxygen, water, and food. We can survive only about three minutes without air, three days without water, and three weeks without food. Because food is so important to our survival, it's been studied extensively.
But, to some extent, the focus on nutrients rather than food has confused many people. A basic understanding of the nutrients can give us a helpful background of our discussion about food.Ultimately people eat food,  not nutrients.

Dietary carbohydrates are combination of sugar units that come in both complex and simple form.
Simple carbohydrate include monosaccharides, or single sugar units like glucose or fructose. As well as disaccharides, or two-sugar units like sucrose, or table sugar. complex carbohydrates, or polysaccharides, include dietary starches that our body can digest and breakdown, and indigestible polysaccharides that make up dietary fibres. During the process of digestion, carbohydrates are broken down and converted to glucose, which can be metabolized by the body to produce usable energy in the form of ATP. If energy demands are low, glucose can be stored, and most of the time it is stored as adipose tissue.

Dietary proteins are also broken down into their component part, amino acids, during the process of digestion. These amino acids and build and repair the lean tissues in the body and perform many other important functions. But amino acids can also be broken down and used for energy. And if they're consumed in excess, they can contribute to fat stores in the body as well.

Dietary fats can also be broken into smaller components and used for energy. Or, they can be stored as adipose tissue, depending on our energy needs.

Fats are most energy dense storage form, providing nine calories of energy for every gram, and alcohol provides seven calories per gram. In contrast, carbohydrates and proteins provide only four calories per gram. This is one of the reasons we've evolved to store excess nutrients as adipose tissue, and this brings us to the underlying physiological cause of  overweight and obesity. Any calories that aren't converted into usable energy in the form of ATP are stored in the body for later use. Most of the time they are stored as adipose tissue. So disturbing the energy balance to favour energy expenditure over energy storage need to be one of the priorities for people who are trying to loose excess weight. This can be achieved by consuming fever calories, burning more calories through exercise, or a combination of both. But in addition, when long term health is the end goal, the quality of food matters just as much as the number of calories we consume. Within each category of macronutrients, we need to choose foods that support our health, rather than working against it.

A socioculture history of obesity

The human body runs on food. Once food shortage was the major concern. Technological advances have led to a new era that is characterized by an overabundance of inexpensive food, and relatively little physical activity. Other socio-cultural shifts continued to contribute to the changing way we ate.
The processed food industry have started to capitalize on our need for fast convenient food. Fewer meals are being cooked at home, and since convenient foods are generally higher in calories than home-cooked meals, the average persons caloric intake has also increased.

The changes in the way, we, as a society eat, has led to the emergence of obesity as a recognized chronic disease with well defined health consequences, and medical recommendations are made to try and address this growing health crisis. In recent years a lot of attention was focused on reducing saturated fat, and total fat in our diets. And the processed food industry responded by giving us what we wanted. But they still had vested interest in selling their product, so they found other ways to make the reduced fat products taste good. One way they did this was by adding significantly more sugar, like corn syrup to almost everything we ate. This not only made the reduced fat foods more appealing, but it also increased their shelf life. So the food industry had a huge incentive to add corn syrup, and other sweeteners to packaged foods. The resulting increase in our intake of simple sugar fueled our modern epidemics of obesity and diabetes. These shifts in our food consumption pattern has led us to a point in history where our physiological adaptation, our ability to store energy as fat, has become maladaptive. The balance between food consumption and energy expenditure has been disrupted, and has left us with an exponential increase in the incidence of obesity over the past 60 years. An epidemic that World Health Organization has labeled a worldwide public health crisis.