Ligands are small signaling molecules that are highly specific and only bind to their respective receptors to initiate particular responses. Ligands could be small proteins, hormones, neuropeptides, neurotransmitters, or even structural components of bacteria such as LPS.

The limbic system is composed of four main brain structures: the hypothalamus, the amygdala, the thalamus, and the hippocampus. It helps regulate behavioral and emotional responses, especially survival-related responses, such as eating behavior. The limbic system and the prefrontal cortex have a reciprocal relationship. The automatic and primitive reactions mediated by the limbic system depend highly on the emotion, reasoning, and decision-making functions mediated by the prefrontal cortex and vice versa.

Lipopolysaccharides (LPS), also called endotoxins, are large molecules composed of lipids (fat) and polysaccharides (sugar) found on the outer walls of gram-negative bacteria. They are potentially toxic and have been implicated in many diseases or infections.

Action where gut peptides influence nearby target cells’ function by binding on their receptors. For instance, short-chain fatty acids (SCFAs) nourish intestinal cells and strengthen intestinal barrier integrity.

Loneliness is a subjective emotional state characterized by feelings of isolation or disconnection, regardless of one’s level of social interaction. It arises from a perceived gap between desired and actual social connections, resulting in a sense of emptiness and dissatisfaction with one’s social relationships.

Low-density lipoprotein (LDL) cholesterol can build up in arteries, leading to plaque formation and an increased risk of heart disease. It is often referred to as “bad cholesterol.”

Luminal contents refer to the various components in the lumen. These components include food substrates, compounds produced during digestion, toxic chemicals, microbes, fluids, and digestive enzymes.

A macro theory is a broad, large-scale theoretical framework aiming to explain a wide range of phenomena. They generally propose a number of basic principles with a high level of abstraction that are than used to generate hypotheses and create mini theories that apply those principles to explaining specific phenomena. Although such frameworks exist in all sciences, the term “macro theory” is almost exclusively used in social sciences and psychology.

Macronutrients are nutrients required in large amounts by the body for energy and proper functioning. They include carbohydrates (energy), proteins (growth and repair), and fats (energy storage and insulation).

Macrophages are large white blood cells that engulf and digest cellular debris, foreign substances, microbes, and other harmful invaders. They play a key role in immune responses by presenting antigens to other immune cells.