What is a solid food

The introduction of solid foods is a time of excitement and experimentation for babies and their parents. Learning the mechanics of taking food from a spoon, chewing and transferring it from the front of the tongue to the back of the mouth and then swallowing takes time and lots of practice.

Don't worry if, at first, all your baby seems to do is spit it out. Not only do babies have more sensitive taste buds than adults, they also have many more. Even the most subtle flavours can seem quite strong to a young baby.

The current recommendation from healthcare professionals is that babies should not be offered solid foods until they are six months of age. Even after solids are introduced, milk still needs to be their predominant source of nutrition in the first twelve months of life. Breastfeeding mothers and their babies still gain significant health benefits by continuing to breastfeed for as long as they are both happy to do so.

Why offering solid food is important

Breast milk or formula contains all the nutrients babies need for their body and brain to grow and develop in their first six months of life. After this age, iron and zinc stores start to deplete and need boosting through additional dietary intake. Extra kilojoules are also necessary to fuel growth in the second half of the first year.

What can happen if I offer my baby solids earlier or late than six months?

There are no benefits to introducing solid foods prior to six months. Some parents believe that doing this will support their baby towards improved sleeping overnight. However, there is no guarantee that this will happen. Babies are still entitled to at least one overnight breast or bottle feed until at least six months of age.

  • Constipation is a common outcome if solids are offered too early. Due to gut immaturity it can be difficult for babies to digest anything other than milk before six months of age
  • Infection due to food borne illness. Babies are susceptible to infection and don’t have fully functioning immune systems. Increasing their exposure to germs via food is an avoidable risk
  • Impact on a mother’s breast milk supply. Babies who fill up on solid foods can begin to refuse breastfeeds or not feed as efficiently at the breast
  • Kidney overload because of needing to filter nutrients other than those contained in milk

What about allergies?

Currently, there is no convincing evidence that delaying the introduction of solids past four to six months of age is protective against developing allergies. Babies who are not offered solid foods when they show signs they are ready can become a little delayed in developing mastery of eating. There is a window of time between six to eight months where most babies are receptive, willing and more than ready to start eating solids. Ignoring readiness signs and deferring their introduction can actually prolong a baby’s sucking action, rather than encourage them to start chewing.

When will I know if my baby is ready?

There is a range of classic readiness signs which babies show they are ready to ingest more than just milk. Look for a combination of these:

  • When your baby can hold their head reasonably steady, has some upper body control and is able to sit upright
  • When they are showing interest in what is going on around them, particularly when you and others are eating and they’re keen to do the same
  • When their tongue thrust reflex is not so obvious. This is a sign that they’re ready to stop automatically pushing food back out of their mouth but instead, make some attempts to chew and swallow
  • When your baby does not seem satisfied with just milk feeds. At around six months they may want to feed for longer, more frequently and not be as satisfied with what they’ve been having

What food do I need to be offering?

Soft, pureed and easily swallowed foods are ideal to start with. Don’t be disheartened if, at first, your baby isn’t too impressed. A small amount on a soft, shallow plastic spoon is preferable. Offer one solid meal of a few teaspoons each day until your baby is looking for more. If they are keen, then slowly graduate to a couple of tablespoons two to three times each day. Let your baby guide you in how much they want to eat.

Baby led weaning is becoming very popular and some parents are keen to hand over the onus of eating control to their child from the very beginning. But it is still important to monitor which foods, how much and the consistency of the food you are offering. Babies can quite literally bite off more than they can chew and be at risk of choking.

How much and how often?

There are no hard and fast rules about introducing solids. Other than taking it slowly and being sensitive to your baby’s responses. Offering one new food at a time is ideal; if they have any problems or reactions to a particular food then it is easier to identify which one it is likely to be.

Start off by offering one solid meal each day for a few days until your baby is used to it. If there are no problems, then you can grade them up to two meals each day and after a few more days, offer three.

Why rice cereal?

One of the first foods offered to babies is iron fortified rice cereal. This is because it is easy to digest and poses a very low risk of initiating an allergic response. Rice cereal is also easily mixed with other foods and its consistency can be varied according to each individual baby’s preference. Rice cereal is readily available and cheap to buy.

From 6-7 months start with:

  • Rice cereal mixed with breast milk or cooled boiled water. You can mix a little pureed apple or pear with this to see if it’s more palatable to your baby
  • Pureed or very well mashed vegetables and a range of fruits. Potato, pumpkin, carrots, sweet potato, zucchini are ideal. Mashed (ripe) banana and avocado are perfect as first foods. When you are making homemade solids, especially vegetables, a steamer and blender or mini blender can be invaluable in making this process quick and easy
  • Milky solids such as custard and yoghurts. Be careful of sugar content in prepared dairy desserts; it is preferable to buy unsweetened ones and flavour them yourself with a small amount of fruit instead.

From 7-8 months:

  • Offer more mashed, rather than finely pureed or sieved foods. Your baby can be having three solid meals each day now. Aim for breakfast, lunch and dinner after their milk feeds
  • Offer a range of cereals, fruits, vegetables, meat and chicken and milky desserts. Two courses at lunch and dinner is common
  • A couple of tablespoons to ½ cup is an ideal volume to be offered at each meal time now.

From 8-12 months

  • Foods chopped into small pieces, minced or in finger size are popular with this age group. Self feeding is beginning, so encourage your baby to explore their food and have some control over their meal times. Giving them a spoon to practice with is a good idea
  • Offer a range of fruits, vegetables, meats, cereals, pasta and grains. Your baby’s brain growth at this age is significant and they need lots of iron in their diet to fuel this.

How will I know when my baby has had enough to eat?

It is important to be sensitive to the cues or prompts your baby will give which means they have had enough to eat. Overriding these or ignoring them can lead to overfeeding. Common signs of fullness include the following:

  • Turning away from the spoon as it is coming towards their mouth
  • Closing their mouth and not opening it in anticipation of the next mouthful
  • Becoming upset, crying and restless when previously they have been interested and engaged
  • Vomiting, gagging and appearing sleepy. All of these cues are a baby’s best attempts to give clear signals that they have had sufficient

It is important to establish meal time routines with your baby so they understand it is time to eat. There are many feeding utensils available which incorporate educational designs to take the focus off the food and make feeding more engaging and fun for little ones.

When can my baby eat the same food as everybody else in the family?

From around 12 months of age, a family diet is ideal. Avoid making your baby special meals which are different to the rest of the family’s.

Check with your child health nurse to ensure your baby’s dietary intake is sufficient for their needs. Weight gain slows after six months of age, to between 70-90 grams/week. It is important to ensure your baby is either tracking on the same line or climbing on their percentile (growth) curves for their weight, head circumference and length. Dropping to a lower curve can indicate a need for improved nutrition.

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Humans are the only mammals that feed our young complementary foods before weaning and the only primates that wean our offspring before they can forage independently (1). The human anomaly of early weaning is likely due to our slow post-natal growth and maturation which in turn are likely due to our brain size. The human brain is more than three times larger than our closest relative the chimpanzee. In human adults, the brain accounts for 2% of body mass but consumes 25% of resting energy expenditure. An average adult needs 600 kilocalories per day to feed his or her brain (2). In other primates, the brain consumes <10% of resting energy expenditure (3).

In non-human primates, time spent feeding increases with body mass; the bigger the animal, the more time spent feeding. On a raw diet similar to what non-human primates eat, humans would need to feed 12 h every day to meet our energy needs however, the actual value is only an hour (4). We solved this paradox with cooking; cooking enables us to eat a diet much lower in fiber than any other ape. The move from raw food to cooked food was associated with a significant reduction in gut size as compared to our ancestors and allowed us to divert more energy to brain growth (5) and increased the time available for social and other cognitively demanding activities which in turn imposed positive pressure for increasing brain size (6).

Humans have the longest period of juvenile dependency of all primates, and the shortest period of lactation. If humans followed the great ape pattern of supplementing our offspring via lactation and not conceiving again until juveniles were nutritionally independent, our interbirth interval would be between 10 and 15 years. Early weaning allows human mothers to resume reproduction much sooner than expected. Moreover, human mothers are unique in that they simultaneously care for multiple dependent offspring at different stages of maturity (7). Cooking and making food more digestible allows children to be weaned onto adult foods much more quickly and allows mothers to stop nursing sooner; all human societies prepare some form of weaning food (7, 8).

When Do We Start Feeding Infants Solid Foods?

As a result of feeding our young complementary foods, there does not appear to be a specific age for weaning in humans, and weaning is variable across cultures. In pre-industrial populations, complementary foods are usually started between 4 and 6 months and breast-feeding has completely ceased by 30 months however, in some cultures solids are started substantially earlier (1). In general, the more active women are in subsistence activities, the earlier supplementary foods are started however, there is variation that seems the result of the work women are expected to do (9).

Since 2002, the WHO has recommended infants be exclusively breast fed for the first 6 months of life to optimize their growth, development and health (10). The American Academy of Pediatrics, the American College of Obstetrics and Gynecology and the American Academy of Family Physicians all recommend solid foods be introduced at approximately six months of age (11–13) whereas, the European Society for Pediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) recommends “complementary foods should not be introduced before 4 months but should not be delayed beyond 6 months” (14). Despite these recommendations, in the U.S., 40% of mothers introduce solids before 4 months of age, and in those who start solids before 4 months, the average age is 12 weeks, and 9% of mothers start solids before 4 weeks of age (15). In Australia, 50% of mothers have fed their babies solids by 4 months of age, and 90% of mothers feed solids before 6 months (16). Comparable patterns are reported in the U.K. and Italy (17). In all these studies, the most commonly cited reasons for starting solids early are the baby was hungry all the time, the baby seemed interested in eating solids and/or starting solids would help the baby sleep (15–17). There are studies that confirm that the early introduction of solid foods is associated with longer infant sleep duration and less frequent waking at night, (18, 19). It is worth noting that most of the studies examining when infants commence eating solid foods describe feeding practices before 2010, and the increased emphasis on the benefits of exclusive breast feeding over the past decade may have changed mothers' behaviors.

What Solid Foods Should We Feed Infants?

Fetuses and newborn infants appear to have an innate preference for sweet and an innate distaste for bitter and other taste preferences appear to be learned through exposure and experience (20, 21). In a study at the Pennsylvania Hospital, newborns were offered water or sucrose solution and allowed to drink for 5 minutes. The infants took nearly three times as much of the sucrose solutions as water. At 6 months, the investigators managed to recall 131 of 151 infants. At the time, many parents fed infants water sweetened with sucrose, honey or Karo syrup as a supplement. The infants who had been fed sweetened water as newborns drank significantly more of the sucrose solution than infants who had not been getting sweetened water as newborns (22).

In utero events and exposures appear to influence taste and flavor preferences (23). What mothers eat during pregnancy influences the flavors the fetus is exposed to in the amniotic sac (24). In one study, prior to undergoing amniocentesis, women were randomized to eat garlic capsules or placebo. The smell of garlic was easily detected in the fluid of the women who had eaten garlic but not in those who had not (25). Fetal swallowing frequency increases when they are exposed to sweet solutions in the amniotic fluid and swallowing frequency decreases when they are exposed to bitter solutions, and similar behaviors are observed within hours of birth (26). Newborn infants orient and begin mouthing when exposed to anise or garlic if their mothers had consumed anise or garlic during pregnancy (23, 27).

These prenatal exposures may have long lasting effects (28). At 35 weeks gestation, Irish mothers were randomized to eat four meals a week containing fresh garlic or to not eat any fresh garlic. Eight years later, their children were given a test meal containing two portions of potato gratin, one which contained garlic and one which did not. Remarkably, the children whose mothers ate garlic during the last month of pregnancy ate twice as much of the garlic containing potatoes as did the control group (29). Analogously, what mother eats and drinks while she is nursing influences the flavor of her breast milk (30) and these taste experiences appear to influence her infant's acceptance of new flavors and what flavors they will accept and like later in life (31).

There appears to be a sensitive period during the first several months of life when infants are most receptive to a wide diversity of flavors, and what they taste during this sensitive period can influence their taste preferences later in life (32). Most infants <4 months of age readily drink formulas that contain hydrolyzed casein which are extremely bitter, however beyond 6 months, infants who have never been exposed to these formulas refuse to drink them (33). Moreover, infants fed hydrolysate formulas in the first several months of life are more willing to eat savory, sour or bitter foods than are infants fed standard milk-based formulas and as compared to children who were never fed a hydrolysate formula, 5 year old children who were fed a hydrolysate formula during infancy more readily eat foods with sour or bitter tastes or aromas (34).

Repetition of taste exposures seems to matter as well. In one study, mothers of seven month old infants were asked to identify a vegetable their infant disliked and were instructed to offer that vegetable on alternate days for 16 days, and to offer a well-liked vegetable, typically something sweet like carrots, sweet potatoes or squash, on the other days. On the first day, infants ate substantially less of the disliked vegetable than the well-liked vegetable however by the eighth repetition, the intake of the liked and unliked vegetables were identical (35). A number of studies have demonstrated similar findings with anywhere between five and ten repetitions (36, 37).

In another study 5 month old infants were randomized to three patterns of introduction of complementary foods. A third were randomized to a “no variety group” and got carrot puree every day for 12 days, a third were randomized to a “low variety group” and got puree of carrot, artichoke, green bean, and pumpkin each for 3 days, and the final third were randomized to a “high variety group” and got carrot puree the first day, followed by artichoke, green beans and pumpkin and back to carrot, etc. When the children were six, they were offered a test meal and the children who had experienced a wider variety of vegetables at weaning ate far more new vegetables than those who had experienced little or no variety at weaning (38).

There also appears to be a sensitive period when infants are particularly receptive to different food textures. Infants and young children typically prefer smooth foods over foods with lumps or chunks however, most fruits and vegetables have complex textures which require the infant to use their tongues to move the food around their mouths in preparation for swallow. Some authors suggest infants are not ready to eat solids before 6 months of age when they have developed a phasic bite and release, have good head control, have good trunk stability and are sitting with minimal support, and are bringing their hands and other objects to their mouth however, the ability of infants to use their tongues and move food around their mouth appears to be more dependent upon their experiences with textured food than on any particular age or developmental stage (39). When infants are exposed to a variety of textures, they are more willing to eat and enjoy chopped or chunky foods at 12 months (40), are more likely to eat a variety of fruits and vegetables at 7 years, and are less likely to suffer from feeding problems during childhood (41). In contrast, children who are not introduced to solids until after a year are more likely to develop oral defensiveness and refuse more highly textured foods (42).

Potential Risks of Early Introduction of Solids

Allergies

It has long been proposed that early introduction of solids increases the risk of allergies later in life (43) however, evidence is accumulating that early introduction of solids may decrease the risk of food allergies (44). In the LEAP study, 640 infants with eczema, egg allergy or both were randomized to avoid peanuts or to consume a minimum amount of peanut containing foods. At 5 years of age, the proportion of children who had peanut allergy as assessed by oral food challenge was substantially lower in the peanut consumption group (45). In a follow-up study a year later, the findings were unchanged (46). In the EAT study 1,303 exclusively breastfed infants were introduced to six allergenic foods (peanut, cooked egg, cow's milk, sesame, whitefish and wheat) at 4 months, or were exclusively breast fed through 6 months. At 36 months, 2.4% of the early exposure group was allergic to one or more food as compared to 7.3% of the group who were introduced to solids after 6 months, and the early exposure group had significantly lower rates of peanut, egg, and milk allergy (47).

These findings suggest there is a window for oral tolerance during early infancy that begins to close somewhere around 6 months of age. This comports with animal models suggesting tolerance is dependent upon exposure to exogenous proteins coupled with development of a healthy intestinal microbiome during a critical early window in life (48).

Obesity

There is little evidence early introduction into an infant's diet increases their subsequent risk of obesity (49). In one trial, 165 formula fed infants were randomized to be fed solid foods beginning at 3 months as compared to 6 months of age. Weights were identical at 3, 6, and 12 months of age (50). In a Scandinavian study, 100 breast-fed infants were randomized to receive complementary foods beginning at 4 months or to continue to exclusively breast feed through 6 months. Weights at 9, 12, and 36 months were identical in the two groups (51). In the EAT study described above, exclusively breastfed infants in the UK were introduced to six allergenic foods at 4 months, or were exclusively breast fed through 6 months and the weights of the two groups were identical at 3, 12, and 36 months (47). The majority of studies show no association between introduction of solids at 4 months and an increased risk of obesity later in childhood. There are several retrospective observational studies suggesting there may be an increased risk of obesity when solids are introduced before 4 months however, it is important to consider the possibility of reverse causality as rapid weight gain and size at 6 months are independent risk factors for future obesity (52). Moreover, it is important to remember that the most common reason parents cite for starting solids early is because they perceive the baby as being hungry much of the time, whether this is right or wrong (15–17).

The Intestinal Microbiome

During exclusive breast or formula feeding, the infant's intestinal microbiome contains a pre-ponderance of bifidobacteria and enterobacteriacae with smaller numbers of streptocci, lachnospiracaie, lactobacilli, and clostridial species. With the introduction of solids into the diet, bifidiobacteriae, enterobacterial, lactobacilli and clostridial species decline and the fiber fermenters lachnospiracaie, bacteriodes, and ruminococcace increase. There is also a fairly marked increase in the diversity of the fecal microbiome with the introduction of solids, and these changes appear independent of geographic location, mode of delivery, and whether the infant is breast and/or formula fed (52). Moreover, low microbial diversity early in life has been associated with an increased incidence of infantile colic, eczema, asthma and type 1 diabetes (53).

Conclusions

There appears to be a sensitive period in the first several months of life when infants readily accept a wide variety of tastes and this period overlaps with a critical window for oral tolerance. As a result, it makes sense to expose infants to a wide variety of flavors while mother is pregnant, while mother is nursing and beginning at an early age. There also appears to be a sensitive period between 4 and 9 months when infants are most receptive to different food textures.

There remains debate as to when it is best to start introducing solid foods into an infant's diet however, the available evidence suggests provided the water and food supply are free of contamination, and the infant is provided adequate nutrition, there are no clear contraindications to feeding infants complementary foods at any age. There is emerging evidence that introduction of solid foods into an infant's diet by 4 months may increase their willingness to eat a variety of fruits and vegetables later in life, decrease their risk of having feeding problems later in life, and decrease their risk of developing food allergies, and the early introduction of solid foods into an infant's diet does not appear to increase their risk of obesity later in childhood.

Author Contributions

SB is solely responsible for this manuscript. He conceived the paper, he solely wrote and edited the manuscript, and he has read and approved the final manuscript.

Conflict of Interest

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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