Updated 29 July 2013

Why teenagers get fat

South African teenagers are gaining weight at an alarming rate, a crisis that is also seen on a global scale. What are the reasons for adolescent obesity, and is it preventable?

A recent talk by international nutrition expert Prof Barbara Livingstone, from the University of Ulster in Northern Ireland, highlighted the factors that fuel adolescent obesity and whether prevention is possible. DietDoc takes a closer look.


At the start of her lecture on adolescent obesity, Prof Livingstone pointed out that, "Technological innovation has overwhelmed our capacity of physiological adaptation". What this means is that "improvements" in every aspect of modern life (fast food, public transport, gadgets to save effort and energy expenditure, TV and computers and all the other 21st century must-have electronic equipment that prevents humans from participating in physical activity) have developed so fast that we as humans have not been able to keep up by adapting our physiology to prevent damage to our health (e.g. obesity and all its attendant diseases of lifestyle).

This was probably the most insightful statement that I have heard in a long time.

Perhaps one day, a few thousand years down the line, humans will have adapted to the overwhelming amount of energy that is available to us at every turn and the sedentary lifestyle that prevents us from using up excess energy. I can imagine humans who have adapted by developing the appetite of an ant existing happily on 100 kJ per day, or who no longer absorb fat, but excrete it instead. However, what we need right now are solutions to prevent more of our adolescents from developing obesity.

Factors that fuel adolescent obesity

Prof Livingstone listed the following factors that are being investigated to see if they fuel obesity in teenagers, or not:
  • Dietary energy density
  • Increased portion size
  • Consumption of sugar sweetened beverages
  • Fast food consumption
  • Sedentary lifestyle
While we automatically attribute weight gain in adolescents and also in adults to the above-mentioned factors, Prof Livingstone pointed out that research evidence is not as convincing as we would like to believe. So let’s look at these factors and the research results presently available:

a) Dietary energy density

The theory

The higher the energy density of the diet, the more adolescents will be obese.

The evidence

According to Prof Livingstone there is "moderately strong evidence from a (limited number) of high quality studies" that there is "a positive association between the Energy Density of the diet and increased adiposity [fatness] in children".

The energy density of a food is the quantity of kilojoules or kilocalories that we ingest per 100 gram of that food. If we eat lettuce for example, then 100g of lettuce will only provide us with 67kJ, but if we eat 100g of butter we would be ingesting 3040kJ; butter is, therefore, a much more energy dense food than lettuce (Wolmarans et al, 2010).


Children and teenagers should be taught to eat foods that are predominantly nutrient dense, rather than energy dense. So drinking low-fat milk is a better option than drinking sugar-sweetened cold drinks.

Nutrient dense foods include: fruit, vegetables, low-fat or fat-free milk, yoghurt, cottage cheese, high-fibre or unsifted grains and cereals, lean meat, fish, eggs, and legumes. Nuts and peanut butter in moderate quantities can also be included because nuts are rich in plant protein and monounsaturated fats, but always remember that nuts have a high energy content.

Energy dense foods are usually high in fat such as butter, cream, oils, lard, sugar, syrup, cordials, and all foods that have a high fat and/or sugar content (cakes, pies, pastry, deep-fried foods, chocolate, full-cream desserts), and alcoholic drinks.

b) Increased portion size

The theory

The larger the portions of food that are on offer, the more children and adolescents will eat and the more weight they will gain.

The evidence

Prof Livingstone cited a number of studies that had found the following:
  • As children get older they become susceptible to the lures of larger portions which mirrors adult behaviour
  • People find it difficult to accurately estimate what is an appropriate portion size
  • Most people of all ages have a distorted perception of what is an appropriate portion size, especially because they regard larger portions as "value for money"
  • The tendency to overeat when presented with larger portions affects all people not just overweight adults and children.


The combination of high energy density and increased portion sizes drive overeating and can contribute to obesity in adolescence and adulthood.

Solutions include educating children from an early age to understand what healthy food portions look like and discouraging the sale of larger-sized food portions. Incentives such as tax rebates for manufacturers and fast food outlets that sell smaller portions may be a good idea.

c) Consumption of sugar sweetened drinks

The theory
The intake of sweetened beverages has risen exponentially in tandem with the increase in obesity, so it has been suggested that liquid sugar is more fattening and/or less satiating than solid sugar.

The evidence

It was fascinating to hear that contrary to popular belief in nutrition circles, the sum total of experimental evidence (more than 52 scientific studies since 2008), shows that there is no conclusive proof that liquid sugar is more obesogenic than solid sugar (Livingstone, 2013). However, the increase in energy consumption caused by the intake of energy dense drinks, particularly super-sized drinks, may well be fuelling adolescent obesity.


Adolescents should be educated to select drinks with a lower energy density and to consume smaller beverage portions.

d) Fast food consumption

The theory

Fast food is extremely energy dense and the more young people eat, the fatter they will become.

The evidence

A study conducted by Bowman and coworkers in 2004 showed that 30% of a population of 6 212 American children and teenagers between the ages of 4 and 19 years consumed fast food and that those subjects who ate fast food consumed:
  • more total energy
  • more total fat
  • more sugar
  • more energy dense foods
  • more sugar-sweetened beverages
  • less milk
  • fewer fruits and vegetables


Eating fast food can influence dietary quality in a negative way and possibly cause obesity (Bowman et al, 2004).

e) Sedentary lifestyle

The theory

Because teenagers spend so much time watching TV and playing computer games, they no longer engage in physical activity which, therefore, fuels obesity.

The evidence

Prof Livingstone pointed out that Tucker already warned in 1990 that "The major impact of television is not the behavior it produces, but the behavior it prevents [and prompts]".

In addition research indicates that TV watching is linked to increased snacking and meal frequency, increased fast food consumption and increased fat intake which all may fuel obesity.

If we consider all the factors that can cause an increase in obesity in our young people, then it would seem that the odds have heavily stacked against any intervention(s) succeeding in turning this trend around. Next week, we will look at one solution that may well be on the right track.

(References: (Bowman SA et al (2004). Effects of fast-food consumption of energy intake and diet quality among children in a national household survey. Pediatrics, Vol 113(1 Pt 1):112-118; Livingstone B (2013). Dietary & Lifestyle Risk Factors for Adolescent Obesity: Is Prevention Possible? Paper presented at: Nutritional Solutions CNE, Johannesburg 11 April 2013; Wolmarans P et al (eds) (2010). Condensed Food Composition Tables for SA. MRC, Parow Valley, Cape Town.)

(Photo of overweight woman eating fast food from Shutterstock)

Dr Ingrid van Heerden is a registered dietician and holds a doctoral degree in Nutrition and Biochemistry. She believes that "we are what we eat" and offers free nutrition and weight loss advice via her DietDoc service on Read more of her articles.


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