Health organizations worldwide, including the American Academy of Pediatrics and the Food Standards Agency in the UK, recommend that babies receive no supplemental fluids during the first 6 months of life. This means no juice OR water -- breastmilk or formula alone are sufficient for your baby's needs. If you give juice to your baby before 6 months of age, you may find that this "fills him up". This will then result in him consuming less breastmilk or formula, depriving him of the nutrients which are crucial to healthy growth and development.
Even in older babies, drinking too much juice may be harmful -- a baby who consumes juice before a solid meal will very likely eat less of his meal. This means he will be consuming less essential fats, vitamins, proteins and minerals than his body needs.
Another very important point to consider is that an excessive intake of juice can cause the body to absorb less carbohydrates -- this can sometimes lead to malnutrition.
Fruit juice can also be responsible for infant tooth decay, tummy pains and diarrhea.
The best option is to offer whole fruit to your baby -- nutritionally, it is far superior to juice and contains the fiber that juice lacks. When your baby reaches an age when supplemental fluids are required, then offer water instead! Many parents say that their babies will not accept water -- however, if your baby has not yet experienced the sweetness of juice, then he won't know what he's missing and is more likely to accept water quite happily!
If you still wish to include juice in your baby's diet, then you need to ensure that the juice you offer is appropriate for an infant.
Avoid juices marked "fruit drink", "fruit beverage" or "fruit cocktail" -- these are composed of less than 100% juice and often contain added sweeteners and flavors.
Any juice given to a baby must be pasteurized -- specially produced "infant juices" are the safest option and do not contain sulfites or added sugars.
Always offer juice in a cup, not a bottle - this prevents the fruit sugars in the juice from pooling around your baby's teeth. Only give juice with a meal - don't allow your baby to endlessly sip juice throughout the day.
Finally, limit your baby's intake of juice to between 4-6oz per day. This is equivalent to one serving of fruit and is sufficient for a baby's dietary needs.