If people don't eat enough food with the right balance of nutrients they can become malnourished - this can lead to slow growth in children.
People can also become underweight and more likely to get infections.
Malnourishment can also lead to tiredness and difficulty focusing. People may find it hard to concentrate at school/work and will have less energy to socialise.
Not enough vitamins and minerals can also cause deficiency diseases.
A lack of vitamin C can cause scurvy, which leads to problems with the skin, joints and gums.
Eating too much, however, can lead to the increased chances of becoming overweight and obese, naturally health problems following suit as well. This may cause issues diabetes and cardiovascular disease.
A bad diet can lead to negative self-image. If people feel unhappy with their appearance they may lack the confidence to socialise.