Can Prebiotics Improve Cognition?

Some of the earlier data (just from 2012!) demonstrated galactooligosaccharide (2% in drinking water) could delay the disease onset of amyotrophic lateral sclerosis in mouse models⁴. Since then more sophisticated behavioral work has been conducted identifying benefits of GOS.

Starting with human data, Schmidt et al.,⁵ found that subjects consuming GOS demonstrated improved outcomes related to stress. In a double-blind randomized placebo-controlled design, male and female participants were instructed to take 5.5 g/day of GOS, FOS, or maltodextrin every day for 3 weeks. Before and after supplementation participants were tested on a battery of cognitive tests to assess emotional processing and salivary cortisol was collected upon waking to quantify the salivary cortisol waking response. For controls, salivary cortisol peaked 30 minutes after waking and declined thereafter. This held true for all groups but those who consumed GOS demonstrated a less drastic rise and fall in cortisol. After performing a task to assess attention toward emotionally positive or negative words, those fed GOS shifted their attention from negative toward positive stimuli.

Rats fed GOS in drinking water (~6.25 g/L) and injected with lipopolysaccharide (an endotoxin that can induce anxiety-like behavior) demonstrated less anxious behavior than those not fed GOS, and performed closer to controls⁶.

Rats fed GOS (0.3-0.4 g/mouse/day) were more exploratory, less anxious, more prosocial, and showed less depression-like behavior than controls. They showed these behaviors when fed both GOS and FOS, in addition to rats fed the combination showing greater resilience to chronic social stress⁷.

The benefits of GOS extend to cognition and are not restricted to emotional states.

Although less documented than their potential anxiolytic effects, GOS may also promote memory related function.

One study looked at the capacity of GOS to reduce neuro-inflammation related cognitive deficits post-surgery⁸. They found adult rats recovering from surgery displayed improved recognition memory when provided GOS in drinking water (15 g/L), though still not to the level of non-surgery controls.

A later study further investigated the impact of dietary GOS on cognition and found GOS (3% GOS in drinking water) improved attentional set shifting performance in adult male Sprague Dawley rats⁹.

In mouse models of Alzheimer’s Disease, mice fed FOS (25 or 50 g/kg in chow) were resilient to induced cognitive deficits¹⁰. In a water maze they were able to reach performance levels of controls. Though effective here, other research shows that FOS is either just as effective or less-so than GOS.

Burokas and colleagues looked at both GOS and FOS, alone or in combination at 0.3-0.4 g/mouse/day. Across behavioral tasks assessing anxiety, depression, sociability, and cognition they found very little evidence that FOS was as effective as GOS. In most cases, rats fed FOS performed no better than controls, with the exception of a forced swim test designed to assess depression-like behavior.

Remember the above study by Schmidt et al.,⁵ detailing the positive effects on salivary cortisol and attention when human subjects consumed 5.5 g/day of GOS? Unfortunately, those results did not extend to the same dose of FOS.

We ourselves found that oligofructose improved short-term recognition memory in pigs and increased their exploratory behavior, but this did not extend to long-term memory¹¹.

Finally, there have been few studies investigating the role of FOS on cognition, and the results are mixed! This by no means closes the door on FOS as a potentially new and innovative way to alter behavior, it’s simply less researched.

Just as exciting as GOS and FOS, is emerging research on oligosaccharides present at high levels in human milk. Of those studied, both sialyllactose and 2’-fucosyllactose have seen substantial research. Both these molecules contain residues found in glycoproteins and glycolipids the brain^12,13, those being sialic acid and fucose.

What makes sialyllactose distinct is its N-acetylneuraminic acid (NeuAc) residue, a type of sialic acid. Sialic acid plays a big role in functions like synaptic transmission, plasticity and neurogenesis¹⁴. Sialic acid is commonly found as part of gangliosides in the brain. In human milk nearly 2/3^rds of the sialic acid comes from oligosaccharides¹²!

Sakai¹⁵ directly compared forms of sialic acid to sialyllactose to understand if the oligosaccharide as a whole was beneficial, or sialic acid itself. Using two different water mazes to test spatial navigation in adult rats, they fed 1% sialyllactose or two forms of sialic acid in chow for 2 weeks. The authors noted that those fed sialyllactose or one of the forms of sialic acid (glycosylated N-acetylneuraminic acid) tended to complete the tasks more quickly, but they were hardly better than controls.

A similar line of research was conducted by Oliveros et al.,¹⁶ comparing sialic acid (Neu5Ac) directly to 6’-sialyllactose. Not only did Oliveros et al.,¹⁶ control for the form of sialic acid, they controlled the dose as compared to rodent milk. They measured the amount of sialic acid in milk across lactation and provided experimental groups equimolar amounts of sialic acid (Neu5Ac) or 6’-SL. Rat pups were reared by foster mothers whose pups were weaned at 16 days postnatal, such that foster mothers were used at a time that sialic acid in their milk was at its lowest. The experimental groups were still reared by mothers and consumed milk. Rather than give a static dose, the experimenters matched daily sialic acid to their foster mothers' sialic acid content from their last litter. Thus, the daily amount of sialic acid and 6’-SL were calculated based on body weight. Ultimately, animals were given the same concentration and dynamic dose of sialic acid as if they consumed rat milk!

They found no difference in recognition memory at weaning age, however at 1 year of age they observed that both groups provided sialic acid or 6’-sialyllactose performed better than controls in recognition memory tasks. Thus, making the case for the use of longitudinal designs and follow-ups after developmental periods.

Lastly, Tarr et al.,¹⁷ investigated the capacity of sialyllactose to prevent stress. They fed 6-8 wk old mice 5% (50 g/kg) 3’- or 6’-SL for 2.5 weeks. In the last week, mice were exposed to a social stressor via the introduction of an aggressive male intruder mouse for 2 hrs per day. Rodents fed both 3’-SL and 6’-SL demonstrated behavior like non-stressed controls, demonstrating a protective effect of sialyllactose.

Last but certainly not least, we come to 2’-fucosyllactose (2’-FL). 2’-FL is perhaps one of the most well researched human milk oligosaccharides of them all. It is maybe a controversial molecule, as not all lactating mothers secrete 2’-FL in their milk²², raising questions about its biological relevance. Regardless, research on its potential to promote cognition has shown impressive results.

Like sialyllactose, 2’-FL contains a residue, fucose, found in the synapse and believed to be important for cognitive function²³. Work in the 80’s demonstrated the need for fucosylation of glycoproteins for hippocampal function and performance in behavioral tasks^23,24. Yet, these alone do not provide much evidence that dietary 2’-fucosyllactose has a health benefit related to the brain.

Recently in early 2020 some of the only known clinical data relating 2’-FL to infant cognition was published²⁵. Both the number of feedings per day and concentration of 2’-FL in milk were positively related to infant cognitive development at 24 months of age, as measured by the Bayley Scales of Infant Development III. Curiously, there was no association found at 6 months of age, again suggesting the importance of longitudinal study designs and follow-ups.

This finding is well supported by a few but well-designed studies in rodents. In the first, both young adult mice and rats were provided oral gavages of either 0.312% or 0.625% 2’-FL for 5 or 12 weeks to provide ~350 mg/kg body weight per day²⁶. Supplemented mice performed better on not just one test, but place learning, associative learning, and working memory tasks!

A follow-up study by the same group provided critical evidence linking potential mechanisms to the cognition-promoting effects of dietary 2’-FL²⁷. The authors investigated the role of the vagus nerve in 2’-FL's cognitive-promoting effects. The study was conducted using a 2x2 design wherein rats were vagotomized (or not) and fed 2’-FL (or not) and tested on a 1:1 ratio lever pressing task. The goal was to test the ability of the rat to learn the association between pressing a lever once to receive a food reward. All rats were able to learn this task, but not better than those fed 2’-FL with an intact vagus nerve. Essentially, to maximally reap the benefit of consuming 2’-FL, the vagus nerve is a necessary component. This gave critical evidence to the hypothesis that prebiotics might have a direct effect on the brain, rather than indirect, through the production and circulation of microbial metabolites. Why the vagus nerve is required, we still don’t know.

Why care about human milk oligosaccharides (HMO) if you’re not a parent or an infant? There is more to these oligosaccharides than meets the eye, and their potential to shape even adult brain function is beginning to emerge. Consider work done by Wu et al.,²⁸ on 2’-FL and neurodegeneration, for example. In a rodent model of stroke, they found that oral post-treatment of 2'FL was able to improve locomotor activity. 2’-FL supplemented rats still performed worse than controls, but their performance was markedly better than those stroke-model rodents who did not receive supplementation.

This is one example of what is likely to come: more research on the potential of prebiotics to improve cognition in healthy, injured, and diseased conditions.

• Several prebiotics improve cognition
• Dose response studies haven’t been conducted
• One prebiotic is not necessarily better than another
• Experimental clinical research with longitudinal designs are desperately needed

If you’re an optimist, you might be ready to jump on the bandwagon and get involved in a hot area of research. If you’re a skeptic, you might want to wait and consider future work done to identify the right doses, structures, and compositions to support the results seen in preclinical models.

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