Why do I get hungry? The biological mechanisms that cause you to feel ‘hungry’ and ‘full’

Have you ever wondered how we experience the sensations of hunger and fullness? Much nutritional content takes a prescriptive approach, providing us with various strategies or techniques to manage our feelings. I’ve always been motivated to understand the “whys” . Countless articles provide “how” to respond to food or your feelings around food – take an intuitive approach, follow a diet plan, instead of eating go for a walk, cut out a meal…. Although I don’t underestimate the usefulness of these strategies within certain contexts, having a greater understanding of what may be causing one to feel a certain way can be helpful too. In fact, in some situations it can potentially alleviate the necessity for one to even need a strategy and in other situations it can assist in reducing the level of unknown and thus discomfort.

I will provide a grossly simplified summarised understanding of the biological processes that contribute to the feelings experienced in relation to food intake, in the hope that it may either help you in some way, and if not, you may find it interesting!

1. Definitions

I think that having an awareness of the acute differences between the meanings of words we use to describe our own feelings can be helpful in then being able to accurately identify them. As well as this being empowering, it enables us to then respond to those feelings in a more appropriate manner.

Hunger: a drive for food or food seeking behaviour, due to a demand for energy.

Appetite: an integrated response to the sight, smell, taste or thought of food that initiates the eating process.

Satiety: inhibition of further intake of food or meal – the feeling of fullness and satisfaction and reduced interest in food after a meal.

Satiation: termination of a meal

Liking: the level of pleasure or reward from food

Wanting: the motivation or drive to seek out food

2. Short term nutrient related signals

As soon as food begins to be digested, the presence of these products (sugars, fatty acids, amino acids, peptides) in the gut begins a series of processes which result in communication with the brain. Within the small intestine the nutrients activate receptors, either by their physical presence (mechanoreceptors) or by their chemical properties (chemoreceptors). Signals then travel via the gut-brain axis either via the vagus nerve (as nerve fibres), or as hormones (in the blood). The presence of these digested food products stimulates the cells of the small intestine to release hormones such as cholecystokinin (CCK), glucagon-like peptide 1 (GLP1) and peptide YY (PYY), which all signal to the brain to reduce hunger and reduce food intake. During a fasting period, on the other hand, when the stomach is emptier, there is the secretion of the hormone ghrelin, and this stimulates a sensation of appetite.

How quickly and how greatly one feels satiated in the short term depends on a wide range of factors; for the purpose of this article, I only focus on the responses to the presence of one single meal. I therefore exclude stress, exercise, smoking, previous meal, sleep etc. all examples of factors that can influence the signalling processes for the sensations of hunger and fullness.

The macronutrient composition of a meal has been shown to affect satiety levels: the below graphs show the results from normal weight subjects, fed isocaloric meals, of either high-protein (carbohydrate (CHO) 17%, fat 18%, protein 65%), high-fat (CHO 17%, fat 66%, protein 17%) or high carbohydrate (CHO 65%, fat 18%, protein 17%). Subjects rated the greatest reduction in hunger from the high-protein meal, followed by the high fat meal and then the high carbohydrate meal. The measured levels of PYY (a hormone which stimulates satiety) followed the same pattern.

Graphs: responses to isocaloric meals of different macronutrient ratios

The amount and type of fibre has also been associated with differing satiety levels. For instance, whether fibre is soluble, insoluble or non-digestible may all influence satiety ratings. Soluble fibre, with viscous properties, has been associated with slowing the rate of gastric emptying and increasing sensations of satiety. Non-digestible fibres such as non-digestible plant polysaccharides (cellulose and lignin in plants) are associated with the production of certain compounds (through their fermentation in the colon) that then result in the production of satiety hormone. Thus, the type of carbohydrate and quantity and type of fibre within it, will affect the level of satiety experienced. Hence why carbohydrate quality matters. Different protein types have also been shown to produce different satiety responses.

One major challenge in determining the different food responses that different macronutrients elicit is due to the food matrix: the concept that the physical and chemical arrangement of the molecules making up the food, will undoubtedly impact the way in which the food is digested and absorbed within the body. For example, even though a whole apple and juiced apple may be exactly the same ingredients, they will elicit a different food response and thus satiety response. This is exactly why the extent of processing, such as heating, or pulsing or grinding, will also affect the food matrix and thus its response in the body and satiety effect.

3. Energy stores: long term mechanisms

In order to ensure that the body has adequate energy to survive and thrive, it cleverly communicates information about its energy status over the long-term. There are two key hormones involved in this: leptin and insulin. Leptin communicates information about the fat stores and is positively correlated to the levels stored. Insulin, communicates the level of carbohydrate stores, from glycogen in the liver. Both leptin and insulin inhibit the part of the brain that stimulates hunger (NPY/AgRP) and activate the part of the brain that stimulates satiety (POMC/CART).

4. Emotional and cognitive mechanisms hedonic

There are also a set of signals that contribute to food intake based on the reward of food. The drivers are whether one ‘likes’ (the pleasure of food) or ‘wants’ food (the drive to consume food). A set of receptors (cannabinoid for ‘liking’ and dopaminergic for ‘wanting’) are hypothesised to be activated in response to certain foods which are palatable. Activation of these receptors then results in a further drive to increase the intake of these foods (evidence from both animals and humans support this). For example rats exposed to either a sucrose (sugar) or highly palatable diet, had an elevated dopamine release as well as a reduction in dopamine receptor expression, supporting the idea of ‘food addiction’ to explain over-eating.

The cannabinoid and dopaminergic pathways play a role in modulating a variety of other processes (mood, memory, motivation, pain, pleasure) and other factors (emotion, stress, arousal, metabolic status) can therefore interact with them.

Key points to be aware of:

1. Short term signals interact with long term signals: an example of this is long term intake of a high sugar diet which has been associated with reduced insulin signalling and insulin resistance.

2. The short and long term mechanisms described function effectively in a “healthy” individual who is in a state of balanced energy. Resistance of hormones occurs in diseased states.

3. All of the signals described use common pathways (gut-brain axis) so there may be cross-talk, making the whole process pretty complex.

So what?

By gaining a better understanding of the complex processes that take place in the body, it provides an opportunity to be impressed by our incredible bodies and perhaps offer a little more self-compassion to ourselves when we become frustrated with “feelings”. Whilst I recognise that at times (or continually) strategies around food and nutrition are certainly necessary, having a greater awareness of why one may or may not experience certain feelings around food may be beneficial in enabling a deeper connection between mind and body and thus being able to choose to mindfully respond, rather than react or be submerged in subconscious thoughts. Just a thought.

Glossary:

Hormones: a chemical messenger that travels in the blood.

NPY/AgRP: neuropeptide Y/agouti-related protein.

POMC/CART: proopiomelanocortin/ cocaine and amphetamine regulated transcript.

Gut-brain axis: includes the vagus nerve, sympathetic, endocrine and immune links, as well as the gut microbiota.

Glycogen: storage of carbohydrate (branched chains of glucose in the liver and muscles.

Vagus nerve: the main component of the parasympathetic nervous system, which signals bidirectionally from the digestive system and organs to the brain (involved in control of mood, immune response, digestion, and heart rate).

Insulin resistance: results in reduced disposal of nutrients such as glucose into tissues.

References:

Breit S. et al (2018). Vagus nerve as modulator of the brain-gut axis in psychiatric and inflammatory disorders. Front psychiatry.

Batterham R. et al. (2006). Critical role for peptide YY in protein-mediated satiation and body-weight regulation. Cell.

Slavin J. et al (2007). Dietary fibre and satiety. British nutrition foundation.

Sylvie T. et al (2011). Food matrix impact on macronutrients nutritional properties. Food hydrocolloids.

Frayn K. (2010). Metabolic regulation, a human perspective.

Yu J. (2015). Metabolic vs hedonic obesity. Obesity reviews.