Dr Wynnie Chan explains why a good, balanced diet is absolutely vital for teenagers and their future health - whatever they prefer to think

A healthy diet during childhood and adolescence is an important factor in helping to prevent the development of a number of diseases in adulthood such as obesity, iron deficiency anaemia, coronary heart disease, hypertension, dental caries, osteoporosis and cancer.

During adolescence (from 10 to 18) puberty requires an increase in the amount of energy and nutrients from the diet. In boys, the rate at which their height increases is greater than in girls; it is also accompanied by an increase in the growth of muscles. Bone density also increases quickly for both boys and girls during the period of adolescence.

Energy and nutrient requirements
The amount of energy and nutrient requirements vary for boys and girls as well as varying according to age (bearing in mind that a very active teenager with have slightly higher requirements). The table below is based on government guidelines of daily requirements.

	Boys			Girls
Age	11-14		15-18		11-14		15-18
Energy (kcal)	2,200		2,755		1,845		2,110
Fat (g)	86		107		72		82
Saturates (g)	27		34		23		26
Carbohydrates (g)	293		367		246		281
Protein (g)	42		55		41		45
Iron (mg)	11.3		11.3		14.8		14.8
Calcium (mg)	1,000		1,000		800		800
Zinc(mg)	9.0		9.5		9.0		7.0
Magnesium (mg)	280		300		280		300
Phosphorous (mg)	775		775		625		625
Sodium (mg)	1,600		1,600		1,600		1,600
Vitamin A (µg)	600		700		600		600
Thiamin (mg)	0.9		1.1		0.7		0.8
Riboflavin (mg)	1.2		1.3		1.1		1.1
Niacin (mg)	15		18		12		14
Vitamin B6 (mg)	1.2		1.5		1.0		1.2
Vitamin B12 (µg)	1.2		1.5		1.2		1.5
Folate (µg)	200		200		200		200
Vitamin C (mg)	35		40		35		40

* Figures are from Department of Health (1991)

Although there aren't any specific figures for fibre, the Department of Health recommends that children have proportionally lower fibre intakes than adults. Adult daily recommendation is 18g fibre. There are also no figures for vitamin D, as it's assumed that most people can obtain an adequate amount via the action of sunlight on the skin.

Adolescent rebellion
Of course knowing what our teenagers should be eating and putting that into practice are separate issues. As children grow older and become increasingly independent, parental influences tend to decline, while friends, peers and media-led fashions take over. Developing independence often leads to adolescents rebelling over what's eaten within the family setting, which may be also coupled with a refusal to eat anything that their parents or teachers consider as 'healthy'. 'Junk' foods are often associated with independence, friends and enjoyment.

Idealism over environmental issues may also change teenagers' attitudes towards eating. Vegetarianism is common among teenagers: a recent government report, 'The National Diet and Nutritional Survey of Young People Aged 4-18 yrs', published in 2000, found that one in ten girls between the ages of 15 and 18 years old was either vegan or vegetarian. Ethical or moral reasons and not liking the taste of meat are among the reasons put forward by teenagers for abstention of meat.

Pressure to be thin
There are other social influences that affect what teenagers eat; one of the most important influences is pressure to be slim. Attitudes towards body image, weight and appearance are important during adolescence. The government survey showed that 16 per cent of girls aged 15 to 18 were currently trying to lose weight, compared with three per cent of boys in the same age group.

Dr Andrew Hill, a senior lecturer in psychology at the University of Leeds, believes that the advent of television has meant that children can understand adult values regarding fatness and thinness simply because the information is visual and does not rely on verbal reasoning or skill. Dr Hill has shown in a recent study that girls as young as nine were restrained eaters and had reported frequent bouts of dieting. The implication is that if the child is not overweight or obese but imposes diet restrictions on herself, this may lead to anorexia or bulimia nervosa. So the emphasis on healthy nutrition needs to be put in context to ensure that it doesn't enhance the culture of weight loss. This could tip the balance in favour of eating disorders, which could lead to serious illnesses and even premature death.

Slimming diets often encourage the exclusion of 'fattening' foods such as potatoes, bread, cereals, pasta, meat, milk and dairy products. Eliminating these sorts of foods altogether will have serious implications for their intake of carbohydrate, fibre, iron, calcium and other vitamins and minerals, which are all important for maintaining health and preventing diseases such as constipation, anaemia, bone health and osteoporosis.

Messages about healthy lifestyles - not just a varied and balanced diet but increasing physical activity - should be consistently delivered by parents, schools, government and the media. Food manufacturers, caterers and retailers should be encouraged to develop products that help support the messages of healthy nutrition.