Adolescence: Biological development

Adolescence

A period of physical, psychological and social transition between childhood and adulthood.

Changes during adolescence

• Rapidly changing psychosocial factors e.g. social environment, expectations of others

• Cognitive development

• Strong, increased orientation towards peers

• Ongoing brain maturation

• Changes in the gut microbiome

Risk for mental health problems

Kessler et al. (2006)

• Age of onset for mood disorders, anxiety, psychotic disorders and substance use disorder all within adolescence

• Mean age 17

Global mental health burden

Specific disorders

• Anxiety and depressive disorders make up 42.9% of all adolescent mental disorders

Brain development

A protracted process taking place over the first 24 years of one’s life.

• Different regions develop at different rates, allowing the individual to respond flexibly to changing environments and challenges

White matter

Increases during adolescence

• During development the brain’s axons become increasingly ensheathed

• Signaling becomes faster and more reliable

grey matter

Decreases during adolescence

• During development, the brain’s synapses are being pruned, use it or lose it principle

• More efficienct, fine-tuned circuits

Emotion & reward systems grow early so…

Teens feel things strongly and are drawn to rewards (incl. peers)

Control systems (PFC) mature later so…

Planning and stopping impulses are still developing

Mismatch = common behaviours so…

More risk-taking, big emotions and uneven self-control, especially with friends or under stress

Still wiring up (pruning and myelination) so…

Skills improve fast, but can be inconsistent day to day

Shaw et al (2007) - Longitudinal fMRI (ages 7-18), ADHD vs typically developing chn

• Children with ADHD show a later peak of cortical thickness, especially in PFC

• Reactive delay in brain maturation as a neurodevelopmental marker of ADHD

• Early demonstration of timing differences in structural development

Romer et al (2023) - ABCD cohort, 9000+ participants, baseline MRI + 2 year follow up

• Differences in structural brain metrics might predict mental health trajectories

• Thinner baseline cortex (in sensorimotor, temporal and other regions) predicted greater decreases in internalising symptoms over time

• ‘Relatively advanced maturation’ may support resilience or faster symptom recovery

WHO recommends for nutrition

• Reduction of sugar

• Reduction of salt intake

• Supplementation of iron and iodine

Microbiome Gut-brain axis

• Two-way communication between gut and brain, sending signals through nerves, hormones and immune molecules

• Microbes make NTs and short-chain fatty acids influencing mood, stress and cognition

• Stress axis and other endocrine pathways allow the brain to affect gut activity and microbiota balance

• Emotional states or chronic stress alter gut movement, inflammation and microbial composition

Gut-microbiome axis during development

During adolescence, brain, hormones and microbiota are co-developing

• This is a critical period where gut-brain interactions can shape long-term emotional and cognitive health

Psychobiotics studies

Ingestion of prebiotics (organic substances supporting the growth of commensal bacteria) or probiotics (live bacteria) over several weeks, rather than as a single dose

Strengths of psychobiotics research

• Enables causal inferences about the contributions of bacteria to psycho- and physiological processes

• Viable for investigation as adjunct therapies alongside mainstream antidepressants and anxiolytics

Limitations of psychobiotics research

• Limited or no information on dose-response functions, effects on longevity, side-effects, time-course and potential moderators (e.g. diet, genotype, sex)

Diet is the one thing we change everyday…

As every meal reshapes our gut ecosystem and the signals it sends to the brain

Kitchen table interventions

Simple dietary choices can alter microbial balance and metabolites

Microbial messengers

Diet-driven microbes produce short-chain fatty acids, neurotransmitters and immune signals that influence mood and cognition

brain and wellbeing effects

Evidence links Mediterranean-style and high-fibre diets with reduced anxiety, improved mood and sharper cognition

Plasticity matters

Adolescence and other developmental stages may be sensitive to dietary modulation of the gut-brain axis

Wibowo et al (2021)

• Microbiome differs as a function of lifestyle

• Western diets had significantly levels of PC1 bacteria than Eastern diets

• 
  

Psychobiotics

Live bacteria, prebiotics or dietary components that positively influence mood and cognition through the gut-brain axis

• Shown to reduce anxiety and improve emotion regulation among adults

• Still an emerging field - effects may depend on individual microbiome composition

• Future: Personalised nutrition and psychobiotic interventions for mental wellbeing

Johnstone et al. (2021) - Prebiotics intervention in young women

• 80 pts, aged 17-25 yrs

• Comprehensive behavioural and psychological assessment

• Nutrition diary, stool sample for microbiome sequencing

• MRI imaging of key emotion regulation brain regions

Gut-microbiome and brain

• Gut microbes produce NTs and short-chain fatty acids influencing mood, stress and cognition

• Diet is a powerful, daily way to influence the gut microbiome and, in turn, brain function

• Evidence shows prebiotics and Mediterranean-style diets may reduce anxiety and enhance emotion regulation

Hen or egg question in MGBA research

Does an imbalance in the gut cause changes in the brain - or do brain changes alter the gut microbiome?