Nutrient Metabolism

Structures, Functions, and Genes

Martin Kohlmeier

• Publisher: Academic Press
• Year: 2015
• ISBN:
  9780123877888
• Edition: 2
• Topic: Technik & Maschinenbau
• Subtopic: Lebensmittelwissenschaft

Nutrient Metabolism, Second Edition, provides a comprehensive overview of the supply and use of nutrients in the human body and how the body regulates intake. Chapters detail the principles determining digestion and absorption of food ingredients and how these compounds and their metabolites get into the brain, cross the placenta and pass through the kidneys. Each nutrient's coverage contains a nutritional summary that describes its function, its food sources, dietary requirements, potential health risks if deficient, and impact of excessive intake. This handbook contains the latest information on the scope of structures, processes, genes and cofactors involved in maintaining a healthy balance of nutrient supplies. Of interest to a wide range of professionals because nutrient issues connect to so many audiences, the book contains a useful link to dietary supplements.

• Latest research findings on health and clinical effects of nutrients and of interventions affecting nutrient supply or metabolism
• Each nutrient covered contains a nutritional summary describing its function, food sources, dietary requirements, potential health risks if deficient, and impact of excessive intake.
• Nutrient information immediately accessible--from source to effect--in one volume

Chapter 1

Chemical Senses

Chemical senses are essential to preserving the human existence. The ability to detect odors from foods is an important appetite stimulant and often initiates the secretion of digestive juices. Aversive reactions to foul odors provide some protection against ingestion of unsafe foods. In this chapter, the anatomical structure, molecular mechanisms, and variation in olfaction (smell) are described and explained. The ability of healthy people to distinguish a wide range of tastes (gustation) is also explained in detail, describing taste buds, innervation, taste-enhancing secretions, the molecular basis of taste signaling, salty taste, sweet taste, bitter taste, sour taste, meaty taste (umami), savory taste (kokumi), fat taste, calcium taste, other taste qualities, and variation in taste sensitivity. Intestinal sensing and chemesthesis also play a role in digestion, and this is explained in this chapter.

Keywords

Smell; chemical senses; olfactory nerve; odors; olfaction; smell; taste buds; innervation; taste-enhancing secretions; the molecular basis of taste signaling; salty taste; sweet taste; bitter taste; sour taste; meaty taste (umami); kokumi; fat taste; calcium taste; mechanoreceptors; nociceptors; intestinal sensing; chemesthesis; digestion

Chapter Outline

Smell 1

Taste 4

Intestinal Sensing 16

Physical Sensing and Chemesthesis 20

Smell

The ability to detect odors from foods is an important appetite stimulant and often initiates the secretion of digestive juices. Aversive reactions to foul or otherwise disagreeable odors provide some protection against ingestion of unsafe foods. Impaired ability to smell, which becomes more common with advancing age, increases the risk of inadequate food intake and food poisoning.

Abbreviation

VNO vomeronasal organ

Anatomical Structure

Humans detect odors by olfactory epithelia covering about 10 cm² at the top of the nasal passage and transmitted through perforations in the skull along cranial nerve 1 (the olfactory nerve) to the limbic, cortical, hippocampal, and hypothalamic regions. A layer of mucus from specialized secretory cells, which constitute the Bowman’s glands, separates the lumen of the nasal passage from the epithelial surface (Purves et al., 2001). The actual detecting units are receptor neurons (bipolar nerve cells), and supporting cells surround them. Basal cells give rise to new receptor neurons that reach maturity within 30–120 days (Costanzo and Graziadei, 1983). The receptor neurons extend cilia (microvilli) into the mucus on the epithelial surface. Odors can reach this surface both from the nostrils (sniffing) and from the pharynx (retronasal space, during chewing and swallowing). Odorants present in the epithelial mucus layer can bind to specific receptors on the cilia, trigger a cyclic adenosine monophosphate (cAMP)–mediated signaling cascade inside the receptor cell and thereby initiate depolarization. Action potentials propagate along the olfactory cell axons to mitral and tufted cells in the glomeruli of the olfactory bulb.

Each glomerulus appears to have a characteristic, limited molecular receptive range and thereby contributes to odor discrimination. Such odor-specific coordination may be related, in part, to input from lateral dendritic connections, which modulate the output of mitral/tufted cells to higher brain regions. Olfactory receptor cells with evoked activity survive better than nonstimulated cells, pointing to use-based selection as an important principle in the organization and maintenance of the olfactory system. The vomeronasal organ (VNO, Jacobson’s organ) is a walled-off cluster of chemosensory cells at the anterior septum of the nose with a narrow opening that is easily missed (Smith et al., 2001). Many adults have only one identifiable VNO, and many have none at all. The irregular presence, the high proportion of nonfunctional pseudogenes coding for associated receptors, and other reasons have led some to believe that the VNO is a vestigial organ without functional significance (Kouros-Mehr et al., 2001). Others still consider the possibility of functional importance (Meredith, 2001). The VNO in rodents detects pheromones, a group of odorant molecules released by the animals to signal sexual and social states. Nasal irritation by odorants such as acetic acid (vinegar) is transmitted by stimulation of the trigeminus nerve (cranial nerve V; Rauchfuss et al., 1987).

Impairment of the sense of smell becomes more common with advancing age. Partially this reflects a general decline in neuronal survival and function. Indeed, diminished odor recognition both correlates with cognitive decline in nondemented older people (Swan and Carmelli, 2002) and may indicate the progression of Alzheimer’s dementia. Trauma is another possible cause for a diminished sense of smell. The olfactory nerve is the cranial nerve that is most often damaged by a fracture of the skull base (Kruse and Awasthi, 1998).

Molecular Mechanisms of Olfaction

Healthy individuals can detect several hundred specific odors (and possibly thousands). Humans are able to distinguish aliphatic ketones (2-butanone to 2-decanone) and acetic esters (ethyl acetate to n-octyl acetate) based on carbon chain length (Laska and Hubener, 2001). However, as part of mixtures, only a maximum of three or four individual components can be identified. For some compounds, the addition of a second odorant is sufficient to obliterate detection of the first (Jinks and Laing, 2001).

An extensive family of transmembrane receptors recognizes volatile compounds. Some of these genes are related to major histocompatibility complex (MHC) genes and share their ability to bind and respond adaptively to diverse molecules. The human genome contains more than a thousand genes related to olfactory receptors. However, most of these are pseudogenes and not expressed (Younger et al., 2001; Zhang and Firestein, 2002). So far, only a few odors have been linked to particular receptors (Touhara, 2001).

Several gene products, other than receptors, have been linked to olfaction, but a comprehensive understanding of the entire process is still lacking. Both catecholamines and cholecystokinin (CCK) are involved in the olfactory signaling cascade. The odorant-binding protein is a member of the alpha-2-microglobulin superfamily in nasal epithelium. Additional odorant-binding proteins are expressed in olfactory neurons (Raming et al., 1993).

It may be of interest to note that cyanide, which is both a strong odorant and a potent inhalatory toxin, is specifically detoxified in nasal mucosa by the enzyme rhodanese (Lewis et al., 1991). Since this enzyme is otherwise mainly expressed in the liver, its presence in the nasal mucosa may confer some protection while sniffing at unknown food sources.

Variation in Smell Sensitivity

The ability of individuals to detect low concentrations of particular odorants varies greatly between individuals and tends to decline with age. The inability to detect a particular odor, such as butylmercaptan (the overpowering foul odorant in skunk secretion), has been referred to as smell-blindness (analogous to color blindness, where sensitivity to one color is lost, but others are still detected).

Androstenone: About 50% of people are not able to detect the odor of androstenone, even at high concentrations; 15% are moderately sensitive, and 35% are very sensitive (0.2 ppb in air). The sensitivity appears to be inducible by prior exposure.

Asparagus: One in 10 adults detects with high sensitivity a particular odor associated with asparagus consumption (Lison et al., 1980). The odorant substance may be methanethiol. Originally, it had been thought that some people excreted this compound more effectively than others, but there seems to be little variation in that respect.

Musk: As many as 7% of Caucasians appear to be unable to detect musklike odors.

References

1. Costanzo RM, Graziadei PP. A quantitative analysis of changes in the olfactory epithelium following bulbectomy in hamster. J Comp Neurol. 1983;215:370–381.

2. Jinks A, Laing DG. The analysis of odor mixtures by humans: evidence for a configurational process. Physiol Behav. 2001;72:51–63.

3. Kouros-Mehr H, Pintchovski S, Melnyk J, et al. Identification of non-functional human VNO receptor genes provides evidence for vestigiality of the human VNO. Chem Senses. 2001;26:1167–1174.

4. Kruse JJ, Awasthi D. Skull-base trauma: neurosurgical perspective. J Cranio-Maxillofac Trauma. 1998;4:8–14.

5. Laska M, Hubener E. Olfactory discrimination ability for homologous series of aliphatic ketones and acetic esters. Behav Brain Res. 2001;119:193–201.

6. Lewis JL, Rhoades CE, Gervasi PG, Griffith WC, Dahl AR. The cyanide-metabolizing enzyme rhodanese in human nasal respiratory mucosa. Toxicol Appl Pharmacol. 1991;108:114–120.

7. Lison M, Blondheim SH, Melmed RN. A polymorphism of the ability to smell urinary metabolites of asparagus. Br Med J. 1980;281:1676–1678.

8. Meredith M. Human vomeronasal organ function: a critical review of best and worst cases. Chem Senses. 2001;26:433–445.

9. Purves D, Augustine GJ, Fitzpatrick D, Katz LC, LaMantia AS, McNamara JO, eds. Neuroscience. second ed. Sunderland, MA: Sinauer Associates; 2001.

10. Raming K, Krieger J, Strotmann J, et al. Cloning and expression of odorant receptors. Nature. 1993;361:353–356.

11. Rauchfuss A, Hiller E, Leitner H, Wollmer W, Reaktion des M. Tensor tympani—ausgelöst durch nasal applizierte Trigeminusreizstoffe. Laryngol Rhinol Otol. 1987;66:131–132.

12. Smith TD, Buttery TA, Bhatnagar KE, Burrows AM, Mooney ME, Siegel MI. Anatomical position of the vomeronasal organ in postnatal humans. Ann Anat. 2001;183:475–47...