The epigenome is a set of chemical modifications that affect how genes are expressed. These modifications can be influenced by environmental and lifestyle factors, such as diet, exercise, sleep, and stress, throughout the lifespan. One of the common pathways that mediates the effects of epigenetics on health is chronic inflammation, which is involved in many diseases and can be reduced by adopting healthy lifestyle habits. The human microbiome is also relevant in understanding the physiological mechanisms by which lifestyle, in particular nutrition, affects health. The microbiome is the collection of microorganisms that live in and on the human body. The microbiome has a key role in modulating the immune system, metabolism, and brain health. The most diverse and influential part of the microbiome is the gut microbiome, which consists of trillions of bacteria, viruses, fungi, and other microbes that reside in the gastrointestinal tract. A healthy and balanced gut microbiome is associated with many benefits for physical and mental health, while an imbalance or dysbiosis can lead to many chronic conditions. The gut microbiome communicates with the brain through the gut–brain axis, which is a complex network of nerves, hormones, and neurotransmitters that influences mood, cognition, and behaviour.