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Taste & development: the genesis of sweet preference
298
which the developing human is exposed and which may be involved cesses related to both normal and pathological states.
in pro-
IAN D. MORRIS
Taste &Development: the Genesis of Sweet Preference J.M. Weiffenbach (ed.) (435 pages) DHEW Publication No. (NIH) 77-1068, U.S. Department of Health, Education and Welfare, Public Health Service, National Institutes of Health, Bethesda, MD, 1977 US$ 9.Anyone with even a passing acquaintance with children in Western society knows of their predilection for sweet things. Undoubtedly they enjoy eating sweets, but unfortunately this does them little physical good, assuming that other sources of carbohydrate are available, and may do them harm both now and later. Sugar intake has been associated with some certainty with dental caries and may contribute to obesity, atherosclerosis, degenerative heart disease and diabetes mellitus. The conference which gave rise to the book Taste and Development was born of a concern for this unfortunate paradox that something so universally attractive should be so harmful. It is regrettable for those actively engaged in taste research that 3 yr elapsed between the conference in 1974 and the publication of the book in 1977, though this probably matters little to the more general reader. The 29 chapters range widely from the anatomy and electrophysiology of taste perception, through developmental topics, to the relationship of taste to ingestion with a concurrent concern for the separation of the hedonic from the more mundane physico-chemical properties of taste. In view of the title of the book the reader can perhaps be forgiven for expecting rather more than half of it to be devoted to the ontogeny of taste. Even so, that half contains a great deal of fascinating information about the taste-related responses of babies and children. Differential responses to sweet substances can evidently be demonstrated in babies only a day or two old. Steiner reports on some experiments in which he filmed babies’ responses to sweet, sour or bitter substances touched on to the tongue. When untrained observers were asked to interpret the babies’ facial expressions in terms of what they thought the babies were tasting they were able to do so with great accuracy. In other words, the neonatal responses were adult-like, had a reliable communicative role and could be interpreted as indicating a preference for sweet. Even anencephalic babies possessed the appropriate repertoire of expressions. The chapters by Lipsitt and Crook describe some very detailed studies of babies’ sucking responses in a situation in which each suck can be rewarded with a small volume of fluid.
299
With increasing sweetness there are orderly changes in the temporal patterning of sucking and increases in heart rate also. Up to concentrations of 10% sucrose, the sweeter the fluid the higher the overall sucking rate. Paradoxically sucking rate within bursts of sucks slows down, and the overall effect is achieved by having longer bursts and shorter pauses between bursts. Lipsitt is tempted to suggest an interpretation of the changes in hedonic terms as a ‘savouring’ of the sweet fluid. Before agreeing I would like to know what is happening concurrently to strength of sucking. The developmental origins of sweet preference have begun to be traced back even before the time of normal birth. Lipsitt quotes studies by Eckstein in which premature babies were found to respond differently to sweet, sour, salt and bitter substances. Anatomical investigations reviewed by Mistretta and Bradley indicate that the equipment for tasting is present even earlier. Both human and sheep fetuses possess morphologically mature taste buds long before birth and in the lamb these can be shown to respond electro-physiologically to chemicals as early as mid-gestation. The above studies are extremely valuable in providing descriptions of perceptual competence and behaviour at different stages during development. The next step is to study the emergence of differences in taste preferences, in particular sweet preference, and to ask whether these relate to environmental factors. One would have thought that this type of research would have enjoyed high priority in view of the evident desire to prevent overindulgence of sweet preference. Yet there is no mention of any such human study: the area apparently awaits investigation. Two chapters by Henderson and Smith and by Galef on developing rats are important in this context, not so much because of the results presented, as in the questions posed and the style of investigation. Galef, for instance, attempts to find out why weanling rats prefer their parents’ habitual diet rather than other equally nutritious but differently flavoured food. He concludes from an elegant series of experiments that one of the major factors is some quality of the mother’s milk. A minor disappointment was the absence of any serious attempt to discuss the adaptive significance of sweet preference. If its consequences are as harmful as many would suggest, why has not selection pressure removed the trait? Yet it is still with us, and, moreover, apparently universally so. Several speculative explanations spring to mind. The benefits of sweet preference may outweigh the disadvantages even now, or more likely, may have done so until comparatively recently in terms of evolutionary history. Sweet preference may be an evolutionary relic. After all, refined sugar has become readily available only in the last 200 yr. The switch to harmful consequences may outweigh the disadvanatges even now, or more likely, may have done which natural selection could act. Furthermore, the disadvantages of sweet preference may be neutral with respect to natural selection: they are unlikely to have much effect on reproductive success. Bolstered up by modern medicine and dentistry, sweet preference is likely to remain a universal concomitant of human existence.
300
The few criticisms above are relatively minor, however, and to some extent reflect the development of the subject rather than the content of the book. It is valuable as a statement of progress to 1974 and, while few will wish to read it cover to cover, many from a variety of disciplines will find something in it to interest them. JAMES
Handbook
of Clinical Neurology
Volume
L. SMART
30:
Congenital Malformations of the Brain and Skull, Part 1 P.J. Vinken and G.W. Bruyn (eds.) (xvi + 706 pages) North-Holland Publishing Company, Amsterdam, 1977 DFl. 280.--; US$ll4.50 The processes of normal central nervous system development form an essential background to all aspects of perinatal medicine since maintenance of the structural and functional integrity of the central nervous system is a major goal of medical care over this period of life. Developmental neurobiology is thus one of the central subjects of concern to all whose work relates to the fetus or infant. As we become more aware of the complex development of functional inter-relationships between the central nervous system and virtually all other systems in the embryo and fetus we may well look towards congenital malformations of the central nervous system to provide us with further insight into the control mechanisms involved. Three early chapters in this volume of the well-known Handbook provide invaluable background data and a vast wealth of references to enable the clinician or medical scientist from another field to find his way into this fascinating world. O’Rahilly and Gardner discuss the developmental anatomy and histology of the human central nervous system in a well ordered manner that gives a succinct description with tables and diagrams of developmental stages, but links in essential biochemical and functional aspects of central nervous organization. Within a space of a few pages the reader can see sequential diagrams of the early growth of the central nervous system and know which stage corresponds to the time that reflex movements are first elicited, learn that there is measurable morphological asymmetry of the brain as early as 29 wk gestational age, indicating a built in capacity for lateralized hemispheric function, and know of the timing of brain growth spurts and the mode of neuronal migration in the cerebral cortex. Functional maturation and development of monoamine neuron systems are among the many other topics covered in this stimulating chapter. The article by Bengt Kallen on errors in the differentiation of the central nervous system reviews specific aspects of normal differentiation processes and the susceptibility of the various stages to maldevelopment. Both classical
which the developing human is exposed and which may be involved cesses related to both normal and pathological states.
in pro-
IAN D. MORRIS
Taste &Development: the Genesis of Sweet Preference J.M. Weiffenbach (ed.) (435 pages) DHEW Publication No. (NIH) 77-1068, U.S. Department of Health, Education and Welfare, Public Health Service, National Institutes of Health, Bethesda, MD, 1977 US$ 9.Anyone with even a passing acquaintance with children in Western society knows of their predilection for sweet things. Undoubtedly they enjoy eating sweets, but unfortunately this does them little physical good, assuming that other sources of carbohydrate are available, and may do them harm both now and later. Sugar intake has been associated with some certainty with dental caries and may contribute to obesity, atherosclerosis, degenerative heart disease and diabetes mellitus. The conference which gave rise to the book Taste and Development was born of a concern for this unfortunate paradox that something so universally attractive should be so harmful. It is regrettable for those actively engaged in taste research that 3 yr elapsed between the conference in 1974 and the publication of the book in 1977, though this probably matters little to the more general reader. The 29 chapters range widely from the anatomy and electrophysiology of taste perception, through developmental topics, to the relationship of taste to ingestion with a concurrent concern for the separation of the hedonic from the more mundane physico-chemical properties of taste. In view of the title of the book the reader can perhaps be forgiven for expecting rather more than half of it to be devoted to the ontogeny of taste. Even so, that half contains a great deal of fascinating information about the taste-related responses of babies and children. Differential responses to sweet substances can evidently be demonstrated in babies only a day or two old. Steiner reports on some experiments in which he filmed babies’ responses to sweet, sour or bitter substances touched on to the tongue. When untrained observers were asked to interpret the babies’ facial expressions in terms of what they thought the babies were tasting they were able to do so with great accuracy. In other words, the neonatal responses were adult-like, had a reliable communicative role and could be interpreted as indicating a preference for sweet. Even anencephalic babies possessed the appropriate repertoire of expressions. The chapters by Lipsitt and Crook describe some very detailed studies of babies’ sucking responses in a situation in which each suck can be rewarded with a small volume of fluid.
299
With increasing sweetness there are orderly changes in the temporal patterning of sucking and increases in heart rate also. Up to concentrations of 10% sucrose, the sweeter the fluid the higher the overall sucking rate. Paradoxically sucking rate within bursts of sucks slows down, and the overall effect is achieved by having longer bursts and shorter pauses between bursts. Lipsitt is tempted to suggest an interpretation of the changes in hedonic terms as a ‘savouring’ of the sweet fluid. Before agreeing I would like to know what is happening concurrently to strength of sucking. The developmental origins of sweet preference have begun to be traced back even before the time of normal birth. Lipsitt quotes studies by Eckstein in which premature babies were found to respond differently to sweet, sour, salt and bitter substances. Anatomical investigations reviewed by Mistretta and Bradley indicate that the equipment for tasting is present even earlier. Both human and sheep fetuses possess morphologically mature taste buds long before birth and in the lamb these can be shown to respond electro-physiologically to chemicals as early as mid-gestation. The above studies are extremely valuable in providing descriptions of perceptual competence and behaviour at different stages during development. The next step is to study the emergence of differences in taste preferences, in particular sweet preference, and to ask whether these relate to environmental factors. One would have thought that this type of research would have enjoyed high priority in view of the evident desire to prevent overindulgence of sweet preference. Yet there is no mention of any such human study: the area apparently awaits investigation. Two chapters by Henderson and Smith and by Galef on developing rats are important in this context, not so much because of the results presented, as in the questions posed and the style of investigation. Galef, for instance, attempts to find out why weanling rats prefer their parents’ habitual diet rather than other equally nutritious but differently flavoured food. He concludes from an elegant series of experiments that one of the major factors is some quality of the mother’s milk. A minor disappointment was the absence of any serious attempt to discuss the adaptive significance of sweet preference. If its consequences are as harmful as many would suggest, why has not selection pressure removed the trait? Yet it is still with us, and, moreover, apparently universally so. Several speculative explanations spring to mind. The benefits of sweet preference may outweigh the disadvantages even now, or more likely, may have done so until comparatively recently in terms of evolutionary history. Sweet preference may be an evolutionary relic. After all, refined sugar has become readily available only in the last 200 yr. The switch to harmful consequences may outweigh the disadvanatges even now, or more likely, may have done which natural selection could act. Furthermore, the disadvantages of sweet preference may be neutral with respect to natural selection: they are unlikely to have much effect on reproductive success. Bolstered up by modern medicine and dentistry, sweet preference is likely to remain a universal concomitant of human existence.
300
The few criticisms above are relatively minor, however, and to some extent reflect the development of the subject rather than the content of the book. It is valuable as a statement of progress to 1974 and, while few will wish to read it cover to cover, many from a variety of disciplines will find something in it to interest them. JAMES
Handbook
of Clinical Neurology
Volume
L. SMART
30:
Congenital Malformations of the Brain and Skull, Part 1 P.J. Vinken and G.W. Bruyn (eds.) (xvi + 706 pages) North-Holland Publishing Company, Amsterdam, 1977 DFl. 280.--; US$ll4.50 The processes of normal central nervous system development form an essential background to all aspects of perinatal medicine since maintenance of the structural and functional integrity of the central nervous system is a major goal of medical care over this period of life. Developmental neurobiology is thus one of the central subjects of concern to all whose work relates to the fetus or infant. As we become more aware of the complex development of functional inter-relationships between the central nervous system and virtually all other systems in the embryo and fetus we may well look towards congenital malformations of the central nervous system to provide us with further insight into the control mechanisms involved. Three early chapters in this volume of the well-known Handbook provide invaluable background data and a vast wealth of references to enable the clinician or medical scientist from another field to find his way into this fascinating world. O’Rahilly and Gardner discuss the developmental anatomy and histology of the human central nervous system in a well ordered manner that gives a succinct description with tables and diagrams of developmental stages, but links in essential biochemical and functional aspects of central nervous organization. Within a space of a few pages the reader can see sequential diagrams of the early growth of the central nervous system and know which stage corresponds to the time that reflex movements are first elicited, learn that there is measurable morphological asymmetry of the brain as early as 29 wk gestational age, indicating a built in capacity for lateralized hemispheric function, and know of the timing of brain growth spurts and the mode of neuronal migration in the cerebral cortex. Functional maturation and development of monoamine neuron systems are among the many other topics covered in this stimulating chapter. The article by Bengt Kallen on errors in the differentiation of the central nervous system reviews specific aspects of normal differentiation processes and the susceptibility of the various stages to maldevelopment. Both classical