Body weight: what makes people fat?

In order to survive, people eat. With such a large variety of goods to choose from, most people do not go hungry. Why is it, though, that 10 chocolate bars will make you fatter than eating 10 apples? What determines how hungry I am? In this article, and the next, I hope to answer these questions and more.

Food products are made up of three fundamental entities: sugars (also called carbohydrates), proteins and fats. Interestingly, 1 gram of sugar does not contain as much energy as 1 gram of fat. In fact it contains less than half the amount of energy. The energy contained within:

• 1 gram of sugar is 4 kcal
• 1 gram of protein is 4 kcal
• 1 gram of fat is 9 kcal

The expression kcal is commonly – and incorrectly – referred to as “calories”. In fact, kcal refers to “thousand calories”. But what is a calorie (cal)? A calorie is the amount of energy required to raise the temperature of 1 gram of water by 1 degree Celsius. Therefore 1 kcal is the energy required to raise 1 gram of water by 1000 degrees Celcius or the energy required to raise 1000 g of water by 1 degree Celcius.

Male adults generally require up to 2500 kcal every day and adult women generally require up to 2000 kcal every day. The energy consumed from food allows the body carry out “background” tasks like food digestion (see our blog from last week) and thinking. It also allows the body to move around and carry out everyday tasks like walking and talking.

Why do people get fat from eating too much “bad food”?

Today, obesity is a real problem. About 25% of British people are obese. Why is this? It mostly comes down to what they eat (and how much of it they eat). Different foods contain different amounts of energy. An apple, for example may contain 80 kcal whereas a chocolate bar may contain 250 kcal (obviously this will depend on the size of the apple and chocolate bar).

If you eat 10 apples then you will have consumed 800 kcal (less than your body requires) but if you eat 10 chocolate bars you will have consumed 2500 kcal (exactly the amount of energy a man needs every day and more energy than a women needs every day). People rarely only eat 1 type of food every day though (and people rarely eat 10 chocolate bars either!) The point is, we have to consider the total amount of energy we eat, across all the food items we consume every day.

If the total amount of energy consumed is greater than the amount of energy we need, our body will store it as fat. If we eat less than the total amount of energy than we need, our body will burn the fat within our body to keep itself going.

But why can some people eat more than others?

The answer to this question is that different people need different amounts of energy. The figures given above (2000 kcal for women and 2500 kcal for men) are an average. This means some people will need more and some people will need less. Many different factors will determine how much energy an individual person needs.

• Amount of exercise done: Individuals who do a lot exercise will need to consume more food as they use more energy than someone who does a small amount of exercise.
• Metabolic rate: Individuals with a higher metabolic rate will typically use up more energy in a given period of time than those with a lower metabolic rate. Hence, they need more food.
• Age: A baby obviously needs less food each day than a 20 year old male, for example.

What is meant by obesity?

Obesity doesn’t just mean you have a lot of body fat. Although people with a lot of fat are generally overweight (and possibly obese) there is a scientific way of defining obesity.

If you know your height in m and your weight in kg, then you can get your body mass index as follows:

BMI = weight (kg) / h²

Where h = height in m

If the value obtained is:

• Less than 18.5 then you are underweight
• Between 18.5 and 24.9 then you are normal weight
• Between 25 and 29.9 then you are overweight
• Above 30 then you are obese

The reason obesity is based on your weight and your height is that taller people are generally heavier than shorter people. A tall person who weighs 65 kg would not be considered fatter than a short person who weighs 64 kg, would they? Hence the BMI considers both weight and height.

In reality, BMI is not a perfect measure because some people have a greater muscle-mass than fat mass. A body builder could weigh more than a fat person but because their extra weight is muscle, this would not be considered especially unhealthy. If you are concerned about your BMI, it is always best to discuss your concerns with a trained medical professional.

Our consultants at the London Gastroenterology centre are experts in endoscopy and colonoscopy. They can also offer advice on healthy living and nutrition, and answer any questions you have regarding the information in this article.