During this prolonged period of time at home, what seems to be on our mind is food. To acknowledge that ‘I am hungry’, is absolutely necessary to provide our body with enough energy and nutrients for essential metabolic processes, yet at the same time it can lead to one to consume more or less food than their body requires. This dynamic homeostatic process of balancing energy intake and output is the subject of much research and is complex. In this post I’m going to explore the regulation of feeding behaviour and hunger. However, before commencing it is important to make the distinction between hunger and appetite:
Hunger: the sensation that represents your body’s physiological need to eat
Appetite: a psychological desire to eat (dependent on memory and associations)
Hunger and appetite combine to form the feeling of satiety [the satisfied feeling of being full after eating]. The interplay of theses three notions inform the utterance of ‘I am hungry’ and the action which follows. There are 5 main ways the body detects when we need to eat, and head for the snack cupboard:
1.Neural Signals from the Digestive Tract
The contents of the gut are evaluated by vagal nerve fibres, for short term control of energy homeostasis. Distention of the stomach activates mechanoreceptors, increased activation if larger distention, which activates signals sent up the vagus nerve to the hypothalamus in the brain. Research has also shown that the sensitivity of these mechanoreceptors operates with a circadian rhythm: in mice there was 3 times less sensitivity when feed during dark cycles- indicating the operation of feeding cycles. There is also chemosensitive vagus enervation, which can be triggered by varying pH, osmolarity and neurotransmitters. This vagal enervation can be influenced by the gut microbiome directly ( inflamation or stress) or indirectly ( release of neuroactive molecules), and is an area of interest in the psychobiotic revolution. These bacteria affect our mood, feeding habits among many other things and the development of probiotics could potentially help those suffering from eating disorders.
2.Blood-borne signals relating to body energy stores
- Increased blood glucose levels, increases the rate of glucose catabolism (Catabolism is what happens when you digest food and the molecules break down in the body for use as energy or to make more molecules) which increase the activation of glucose receptors in the brain. These depress the desire to eat
- Increased Amino Acid concentration ( from protien catabolism) , depress the desire to eat
- Increased fatty acids and leptin ( a peptide hormone released by fat tissue) inhibit hunger
The opposites for each of these are true( ie decreased Amino Acid concentration = increased desire to eat)
Hormones are chemical messengers that circulate in your blood which have effects on target organs such as the brain ( usually the hypothalamus when concerning energy homeostatis). Some important hormones that are involved in the hunger are:
- Insulin :Released by the pancreas, insulin convert glucose to glycogen to be stored in liver and muscle cells. Increased levels of insulin lead to decreased levels of hunger
- Leptin :Individuals with larger amounts of body fat release more Leptin from their adipose (fat) cells and intestinal cells. Dubbed the ‘satiety hormone’ it regulates long term food intake and energy expenditure by inhibiting hunger.3
- Adrenaline :The concentration of adrenaline increases when fasting, stimulating hunger as it induces increased rate of metabolism by inducing fat burning
- Cholecystokinin : An intestinal hormone which depresses hunger
- Ghrelin :Hormone produced and released by the stomach dubbed the ‘hunger hormone’ as it stimulates appetite and promotes fat intake
4. Body Temperature
Increasing body temperature lowers food intake. Metabolic processes such as digestion release vast amounts of energy in heat leading the body to overheat.
Food visuals play an important role in appetite and satiety, as the context and presentation of food that we detect with our senses have psychological effects on our desire to eat them. Neurotransmitters Dopamine and Seratonin allow us to develop relationships with food which can enhance or inhibit prior physiological mechanisms. An example of this is found in a 2015 study of football fans who were given lemon-lime sorbet. Fans whose team had won reported their sorbet was more sweet than sour, whilst the losing team’s fans reported it was more sour. The former group had high levels of Serotonin ( a neurotransmitter which contributes to happiness and well being )and the latter high levels of Noradrenaline ( associated with stress responses).
Extra note: Neuropeptide Y(NPY)
This is a 36 amino acid peptide chain synthesised by neurons which is heavily related to feelings of hunger. It does A LOT of things but some of interest are that it stimulates the growth of fat tissue, and increases food intake. In obese mice, a gene mutation resulted in chronically high levels of NPY. NYP is really interesting, as it plays a lot of roles, so maybe I’ll do an article on it in the future
N.Marieb, E., 2004. Human Anatomy & Physiology. 6th ed.