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Nutritional physiology
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Summary of the lectures and articles of nutritional physiology (first year course in Msc Nutrition and Health)
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Nutritional physiologyLecture 0: Introduction
Email: hap30306@wur.nl
Course learning objectives:
- Describe and explain general digestive functions and main metabolic pathways
- Predict settings of metabolism based on nutritional input and physiological ouwtput
- Estimate changes in energetic efficiencies in relation to changing conditions
- Judge physiological conditions in terms of constrains for homeostatic control
Week 1: Digestive system
Lecture 1: Mass slow of nutrients (through the body and its cells)
Nutritional physiology: mass flow
Nutritional input and physiological output should be in balance, metabolism makes sure that there is
a balance.
- Nutritional input
o Total daily intake (kJ)
o Meal size, pattern
o Nutrient composition
- Physiological output
o Physiological status (cycling, walking etc.)
o Health status
o Environment
- Metabolism (continuously switching, depending on nutrient, body composition, physical
activity etc.)
o Δ Metabolic pathways
o Δ Energetic efficiencies
o Δ Body composition - Waste products
Physiological output can be different: f.e. different between young students vs elderly
Physiological output of elderly and metabolism is running different compared to young people,
meaning that the metabolism is different.
Dietary components: nutrients
- Macronutrients = carbohydrates, proteins, fat
- How do they move through the body?
o Translocation (position of macronutrients)
o Transformation (of the nutrients)
- How are these movements regulated?
o Neural system
o Endocrine system
, - What are physiological options/benefits for the body?
- How do nutrients leave the body?
o Feces the nutrients that you cannot digest and absorb leave the body through
feces
o Breathing: CO2 major compound by which nutrients leave the body because fully
oxidation gives CO2 and H2O, which are the waste products.
What happens after eating?
- Hardly any influence on the fate of the food after swallowing
o Digestion and absorption
o Storage
o Utilisation (converting)
The body regulates this.
- Increase interest in feeding strategies aimed to influence the fate of the food. Maybe there
are compounds which influence metabolism.
So, nutritional input influences metabolism and metabolism influences nutritional input.
Furthermore, metabolism influences physiological output and physiological output influences
metabolism.
So nutritional input metabolism physiological output
Post prandial phase = after food intake (processed and metabolised)
Post absorptive phase = after absorption; fasting state (basal state). In this case, you use the internal
storages, so different pathways are used during post absorptive phase.
Interindividual differences = everyone has another digestion and absorption.
Digestion fermentation
Compounds digested into monomers, which can be absorbed. These monomers can provide energy.
Microbial fermentations provide energy, but no amino acids. Everything that cannot be absorbed is
waste.
Microbial fermentation may provide energy (SCFAs), but fermentation will never produce amino
acids.
,A lot of energy is needed to get a compound broken down. F.e. to break down peptides in a non-
enzymic way, 10-12 hours in 12 mol/L HCI at 105 degrees is needed. So, this is not efficient. When
peptides are broken down in the body, 1-2 hours at 37 degrees is needed with the help of enzymes.
Why does the digest system not digest itself? It would if you don’t regulate this.
- Activity restricted to presence of food.
- Regulation (local, distal, proximal)
- Enzymes stored in cells as inactive pro-enzymes (called zymogens); when they are released,
they get activated and then they are activated to digest compounds.
- Non-digestible mucus coats the walls.
- High replacement rate (turnover) of mucosal cells; the GI-system has a high turnover,
meaning that if there is damage, they will be replaced.
All have influence on metabolic settings. F.e. high enzyme production can affect the metabolism of
some specific compounds.
Settings of metabolism:
- Digestion to and absorption of monomers
- Intermediary metabolism = all conversions of monomers
o Interconversions of absorbed monomers (f.e. glucose can be stored as triglycerides)
o Anabolic reactions (e.g. protein metabolism) = build up (mostly costs energy)
Synthesis of body constituents
o Catabolic reactions (energy/ATP/heat) = breaking down (release energy)
Release of energy from food or body constituents
o Unavoidable waste products
- Time scale of events: post prandial phase post absorptive phase
- Physiological adaption
Stored as protein, glycogen or neutral fat.
, In the end, everything (amino acids, glucose or lipids) will be made into acetyl-CoA, which is used to
generate ATP.
Meal feeding:
- Post prandial phase (digestion – absorption – storage): input > needs
o Especially anabolic or catabolic disposal (mostly anabolic because you store all the
nutrients, always catabolic as well)
o From the intestines glucose is absorbed and released in circulation and fuels the
muscles and the brain (brain only relies on glucose). If you have too much glucose, it
will be stored as triacylglycerol in the adipose tissue (fat). Besides this, following a
meal, you have more protein synthesis (out of aa)
- Post absorptive phase (utilisation): Input < needs (turnover, interconversion, oxidation). Post
absorptive phase: needs are higher than your input.
o Mobilisation of storages is needed to provide energy.
o Muscle mass declines over the night because you break down proteins for the
morning. This energy is needed for cellular processes.
o Triacylglycerol can be converted into glycerol or fatty acids, which can be converted
into glucose. Amino acids can also be used for glucose. In the liver, ketone bodies can
be made, to help the brain.
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