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Summary HAP-30306 Nutritional Physiology
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Summary of Nutritional Physiology, including information of the practica
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NP01 Intro to course & digestive systemMass flow of the digestive system which should be in balance:
- Nutritional input
o Total daily intake (kJ), meal size, pattern, nutrient composition
- Physiological output
o Physiological status, health status (inflammation --> energy expenditure increases),
environment (oxygen availability, temperature)
- Metabolism (is adjusted to have a balance between input and output)
o Δ Metabolic pathways, Δ Energetic efficiencies, Δ body composition – waste products (if
there is not a balance)
o The settings of metabolism can be influenced by environmental conditions, nutritional
input and physiological output
- Post prandial phase: nutrients are used to generate storage
- Post absorptive phase: storage used to produce energy
Foods move through the body by:
o Translocation (mouth to stomach to intestine)
o Transformation (modifications in macronutrients e.g. broken down)
It is regulated by the neural and endocrine (hormone) system.
The GI consists out of: mouth, esophagus, stomach, small intestine (duodenum, jejunum, ileum), colon
(ascending, transverse, descending), rectum and anus
The ingested bolus is broken down to monomers which are absorbed. In the colon bacteria ferment the
food, which may give energy (short chain fatty acids) to the host. Microbial fermentation may provide
energy, but no amino acids. Amino acids are always derived from food.
The digestive system does nog digest itself as enzymes are only active in the presence of food. The
enzymes are regulated (local, distal, proximal) and are are stored as inactive pro-enzymes (zymogens).
Additionally, non-digestible mucus coats the walls and there is a high replacement rate (turnover) of
mucosal cells, as they have constant damage.
The digestion and absorption of monomers, time scale of events and physiological adaptation have
influence on the metabolic setting (how the body deals with the food). Also the intermediary
metabolism has influence of this via interconversions of absorbed monomers, unavoidable waste
products, anabolic reactions (synthesis) and catabolic reactions (break down).
In the post prandial phase, input > needs.
Glucose is taken up and goes via the portal vain to the liver. Glucose is stored in the liver and muscle as
glycogen. The brain can only use glucose. Glucose can be transferred to triaglycerol. Lipids do not go via
portal vain, they go immediately to the fat tissues. Energy is mainly stored as fat or carbohydrates.
The thermogenic effect is the produce heat after eating = diet-induced thermogenesis (DIT). The post
prandial metabolism (PPM) increases heat production (5 - 20% of ingested ME). The effects depend on
ingested nutrients and metabolic settings: proteins > carbohydrates > fat
The ingested ME (on label) is not corrected for energetic costs of PPM.
Under specific conditions, consequences of PPM might urge to change
metabolic settings.
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,In the post absorptive phase (fasting state), input < need. There is
mobilization of nutrients/energy via turnover, interconversion, oxidation.
TG in adipose tissue are broken down in glycerol and fatty acids. Fatty acids
go to liver where it is oxidized releasing ATP. Fatty acids go also to the
muscle, to provide them with nutrients and ATP. With long time fasting, the
fatty acids can be turned over in ketone bodies for muscles. Muscles can not
only rely on fatty acids oxidation, it needs glucose or ketone bodies as well.
Glycerol is used to be converted in glucose. Glucose is needed to provide the
brain of energy. In the kidneys, amino acids may be converted into glucose to
get enough glucose for the brain. Ketone bodies can also be used for the
brain, but only if glucose levels are really low.
Homeostasis is the ability of an organism to counteract (within limits of the metabolic scope) factors
that disturb vital functions. We want to remain in homeostasis, so a stable level of glucose, pH,
temperature and oxygen level.
The levels of glucose, etc. fluctuate around a set point, but the fluctuations should be in a normal range.
Conflict of interest may occur: the metabolism increases temperature, but the body regulation wants 37
degrees constant.
There should be an energy balance, energy intake = energy expenditure.
Nutritional paradox: Food and body are no natural allies. The composition of food is related to their
natural function, not to our nutritional requirements. So, the requirements are not the same as the food
that we eat. Some of the constituents can be used as nutrients, others cannot, or are even toxic.
Paracelsus: only the dose permits something not be poisonous. (e.g. amino acids are needed, but toxic
at high concentrations).
Food and body are synthesized from the same monomeric components. The transfer of monomers from
food to body, requires digestion to absorbable (water soluble) components. However, protective
devices are needed against autodigestion. The motor behaviour (muscle contraction) and secretory
behaviour enhance digestion and absorption.
Amylases digest glucose, lipase digests lipids and pepsin & trypsin digest protein.
Water is mainly absorbed in the colon.
Accessory digestive organs enhance the digestion:
- Teeth, tongue, salivary glands
- Exocrine pancreas
- Liver, gall bladder
- Villi (increase area) in the small intestine
The mucosal surface consists out of:
- Longitudinal foldings: plicae circularis
- Villi
o contain one layer of cells: the entrocytes. These can
be absorptive entrocytes and mucus secreting goblet cells. The villi have lymphatic
drainage (drains lacteal which contains lymph) and venous drainage (with blood).
- Microvilli are foldings of the cell membrane, also indicated as the brush border. In this cell
membrane there are enzymes which are present for the carbohydrates.
2
,The GI tract contains 4 layers (side of food to side of body)
- Mucosa secretion and absorption
- Submucosa vascular layer (support)
- Muscularis segmental contractions
o Especially in the stomach you have huge muscles, as it needs to grind. The muscularis is
smaller in the small and large intestine
- Serosa protective layer
Gut retention time is the time between presence of food in the lumen and the absorption in the blood.
Not everything is immediately taken up in the blood, but it is regulated (in the enterocyte).
Gut stimuli evoke digestive responses via the enteric and the central nervous system.
Stimuli sensors enteric nervous system effectors
The link between the sensors and the enteric nervous system are the brain and spinal cord
3
, GI-tract communication:
- Endocrine via hormones (via blood)
- Neurocrine via neurons can make muscles contract (also in brain)
o Via neurons (CNS) releasing messengers, neurotransmitters
- Paracrine: locally (local cell-to-cell communication)
Segmentation: circular muscles contract to move the bolus and to break down in smaller pieces.
Neurons sense that a bolus is present. This triggers a response by which the muscles contract or relax.
It enhances the movements of the bolus in the small intestine.
The backside of the muscle containing a bolus it is contracted; the fore side is relaxed.
Monomers are absorbed by:
- Simple diffusion
- Facilitated diffusion (transporter needed, but no ATP is needed, e.g. fructose)
- Active transport (e.g. glucose, ATP is needed)
MMC = migrating motor complex
There is motor activity firstly in stomach, then in the duodenum and than in small intestine. During the
motor activity there is gastric secretion followed by gallbladder contraction. The pancreatic secretion
and biliary secretion go into the small intestine.
This happens consequently, time after time. It always takes place to get rid of food in e.g. the stomach.
It empties the organ
MMC in humans:
- Phase I: 40 min rest
- Phase II: 40 min start peristalsis
- Phase III: 10 min max peristalsis
The MMC prevents of bacterial stasis and final cleaning of non-digestible parts.
Transit time of food in the organs:
- Stomach 1-5 hours
- Small intestine 1.5 hours
- Large intestine 1-2 days
Integrated response to a meal: mass flow of nutrients & regulation
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