Furthermore, they are promoted for infants from the age of four months -- an age when they should still be on an exclusive breast milk diet, say the researchers.

They wanted to find out what sort of products are available in the UK for weaning infants from a predominantly milk based diet to a family food based diet, and to assess their nutritional value.

The weaning process aims to introduce infants to a wider range of tastes, textures, and flavours, to encourage them to accept different foods, and to boost their energy and nutrient intake.

UK government recommendations on weaning foods stipulate that these should be introduced gradually, starting with cereals, vegetables and fruits, followed by protein-rich foods and should not be started before six months, in line with recommendations for exclusive breastfeeding until that time.

The authors therefore analysed the nutritional content of all infant foods intended for weaning and produced by four major UK manufacturers and two specialist suppliers between October 2010 and February 2011.

The products included ready-made soft, wet foods, powdered meals to be reconstituted with milk or water, breakfast cereals, and finger foods, such as rusks.

The authors collected their information on the calorie density, added salt and sugar, and the protein, iron, calcium, and carbohydrate content, from the manufacturers' websites, labels on products in store, and via direct email inquiry.

Most (79%) of the 462 stand-alone products assessed were ready made spoonable foods, almost half of which (44%; 201) were aimed at infants from the age of four months onwards.

Analysis of the 410 spoonable foods revealed that their energy content (282 kiloJoules per 100 grams) was almost identical to that of breast milk (283kJ/100g). And their protein content was only 40% higher than formula milk.

Products containing meat had the highest iron content, but this was again no higher than formula milk, and not much higher than products that did not contain meat.

Dry finger foods had a much higher energy and nutrient density overall, but they were also particularly high in sugar.

Around two thirds (65%) of the stand-alone products were sweet foods. Babies have an innate preference for sweet foods, which might explain why sweet ingredients feature so prominently in commercial products, say the authors.

"However, repeated exposure to foods during infancy promotes acceptance and preferences," they write, and the inclusion of fruit sugars rather than refined sugars won't make any difference in terms of the risk of tooth decay, they say.

The nutritional content of the shop-bought products was compared with that of typical family home-made foods commonly given to infants and toddlers.

The savoury ready-made spoonable foods generally had much lower nutrient density than typical home-made foods, with the exception of iron content.

But it still means that 50g of a spoonable family food would probably supply the same amount of energy and protein as 100g of a similar commercial product, say the authors.

They emphasise that the main point of weaning foods is to increase the energy content of the diet and provide richer sources of nutrients, such as iron.

"Yet the most commonly used commercial foods considered in this study supply no more energy than breast or formula milk" and yet they are promoted at an age when they will replace the breast (or formula milk), which is all that babies under six months really need, they explain.

"While it is understandable that parents may choose to use [these products] early in the weaning process, health professionals should be aware that such food will not add to the nutrient density of a milk diet," they conclude.