By definition, colicky babies cry excessively and inconsolably. But why? As I note in this evidence-based overview of colic, some babies may suffer from specific physical ailments, like allergies, migraine, or gastroesophageal reflux disease.
But there is also evidence that some infants are "wired up" a little differently.
They may react negatively to care-giving maneuvers that non-colicky babies regard as neutral or pleasant.
They may be more prone to negative emotions.
They may lack the daily hormonal rhythms that help them wind down and sleep at night.
And they may have a greater sensitivity to pain.
Individual differences of temperament have been observed during the first few days postpartum (Tsuchiya 2011), and to some degree, you can predict which newborns will become colicky by seeing how they respond to being undressed, handled, or put down. The babies who are most upset by these maneuvers are most likely to develop colic (St James-Roberts et al 2003).
So maybe colicky babies are simply less tolerant of disruptions and transitions.
Consistent with this idea, several studies have found that colic symptoms improve when parents are instructed to stimulate their babies less (Lucassen et al 1998).
Another possibility concerns a baby's circadian rhythms -- in particular, the cyclic, daily production of the hormones like melatonin.
If babies don't produce enough melatonin at night, it could interfere with their ability to wind down and sleep well. This, in turn, could worsen their moods, and make them more intolerant of pain and discomfort (Leuchter et al 2013; Cohen et al 2012).
It also appears that colicky babies have more trouble calming down once they begin crying.
In one controlled study, researchers found that normal and colicky babies had the same frequency of crying bouts. What distinguished colicky babies was that they colic cried longer (Barr et al 1992).
And colicky babies seem to react differently to care-giving maneuvers that are meant to soothe them.
You can see this in an experiment that tested the effects of sugar on babies.
In everyday life, babies don't encounter sugar by itself. But milk contains natural sugars, and there is evidence that the sweetness of milk is intrinsically calming to infants.
When Ronald Barr and his colleagues gave 6-week old babies a sugar solution to taste, the researchers discovered that all babies -- those with colic and those without -- responded to the sugar by calming down.
But the calming effect lasted longer for normal infants. Babies with colic were more likely to resume crying two minutes later (Barr et al 1999).
Why these differences? Perhaps, Barr speculates, something is wrong with the system that rewards the brain with endogenous opioids -- natural, self-produced painkillers.
In normal babies, the sugar is a signal for the brain to release these feel-good drugs. In colicky babies, this response is impaired (Barr 1999). According to this idea, colic eventually improves because the opioid release system matures.
Another clue comes from research on the effects of carrying and holding infants -- and responding quickly to their cries.
In a couple of experiments, assigned Western parents to care for the babies the way that traditional hunter-gatherers do: Hold or carry the baby at least 80% of the time. If your baby cries, respond within seconds by feeding or soothing the infant.
What happened when parents used this approach? It reduced crying in normal babies, but not in babies with who had been diagnosed with colic (Hunziger and Barr 1986; Barr et al 1991). The same attempts to soothe didn't have the same effect.
Finally, there is the theory that babies have more pain receptors in their intestines -- making them more sensitive to pain.
In recent years, researchers have amassed compelling evidence that babies with colic have a different mix of bacteria in their large intestines.
Compared with non-colicky infants, they are more likely to have high concentrations of the type of bacteria that can cause inflammation and excess gas. They may also have lower concentrations of the "good," probiotic bacteria.
This alone might explain the crankiness of colicky infants: They might have low grade inflammation of the gut.
But researchers also speculate that the imbalance of gut flora might also activate nerve receptors in the intestines, making babies more sensitive to abdominal pain (Pärtty and Kalliomäki 2017; O'Mahoney et al 2016).
If this is the cause of an infant's problems, it's possible that physician-supervised doses of the probiotic bacteria, Lactobacillus reuteri, could help. But the research on this subject is mixed (Pärtty and Kalliomäki 2017). In some studies of breastfed babies, probiotic treatment helped substantially. In other studies, it made little difference.
More studies are needed to understand why probiotics don't always work. One likely factor is that it depends on an individual's pre-existing mix of bacteria (Pärtty and Kalliomäki 2017). This may vary according to local differences in diet, and other environmental factors. Simply adding probiotics might not crowd out enough of the troublesome bacteria -- not for some babies.
In addition, it's important to understand that probiotics therapy isn't safe for babies with impaired immune systems. You shouldn't attempt probiotics therapy without the approval of your baby's doctor.
For more information about the causes of colic, see this overview.
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O'Mahony SM, Dinan TG, Cryan JF. 2016. The gut microbiota as a key regulator of visceral pain. Pain 58(1):S19–S28.
Pärtty A and Kalliomäki M. 2017. Infant colic is still a mysterious disorder of the microbiota-gut-brain axis. Acta Paediatr. 106(4):528-529.
St James-Roberts I, Goodwin J, Peter B, Adams D, and Hunt S. 2003. Individual differences in responsivity to a neurobehavioural examination predict crying patterns of 1-week-old infants at home Developmental Medicine & Child Neurology 45(6):400-407.
Tsuchiya H. 2011. Emergence of temperament in the neonate: neonates who cry longer during their first bath still cry longer at their next bathings. Infant Behav Dev. 34(4):627-31.
content last modified 10/2017
For references cited in my other articles about colic, click here.