Econstudentlog

Nutrition at a Glance

This is a Wiley-Blackwell publication about human nutrition. It is also perhaps the strangest W-B publication I’ve ever read, because of the combination of the following two facts: i. Each chapter is two pages long (the book has 62 chapters). ii. This is an academic text without a single source or reference. The latter of those two points is the main reason why I have not rated the book.

The chapters are denser than you’d think (they have a lot of information considering what you’d expect from two-page chapters), and many chapters ‘come in pairs’ or deal with related stuff; for example there are three main chapters dealing exclusively with proteins – one about the ‘chemistry and digestion’ of proteins, another one about the ‘functions of proteins in the body’, and a third one about the ‘needs and sources’ of proteins. Carbohydrates and fats also get multiple chapters each, and micronutrients get 7 chapters of ‘exclusive coverage’ dealing only with those things. The level of detail is reasonably high (again considering what you’d expect), but of course there’s only so much stuff you can cram into a chapter two pages long. I think in many ways it is a really neat book to have for looking up stuff in this area that you’re wondering about and/or can’t quite remember (‘what was the role of butyric acid in the colon again?’ ‘Which factors affect calcium absorption?’ ‘What are the roles of the various B-vitamins in metabolism? How might I get into trouble if I don’t get enough riboflavin, and what can I do to avoid that situation?’). Quite a few of the things she talks about I don’t really consider it too problematic that she does not source; she knows a lot more about which role folate plays in the regulation of homocysteine levels than I do, and I don’t think there’s a big risk involved in just taking her word for it that those things work the way she says they do. Much of the book covers stuff on a level where I could justify thinking along such lines; many of the chapters are a bit like book versions of short Khan Academy nutrition lectures (perhaps a bit like lectures like this and this, I guess without having watched those lectures), and I figure if I’m okay with watching lectures like those I should be okay with reading a book like this as well, which is a big part of the reason why I didn’t just throw it away the moment I realized that there was actually not a single page of references to be found anywhere.

As I didn’t rate the book because of it, the lack of sourcing of course bothered me. One thing which puzzled me is why she decided to write the book this way – I simply do not understand that decision at all. Given the current state of affairs of nutritional science (and the current state at the time the book was written in 2007), I will say that I think the way she has chosen to write this book is simply flabbergasting to me. Nothing tempts people to disregard your information like not telling them where it comes from. There is not a single sentence in this book with the words ‘[X & Y] found that…’ – she only ever writes ‘a study found that…’, and this is just infuriating. There are various recommendations of daily intake of various substances in the book, but you have no idea who came up with those recommendations or which evidence base they are based on – there’s not even a source indicated in the headings in those cases. I simply don’t understand why she’d write the book that way – the lack of sources makes much of the stuff look deeply suspect, regardless of whether or not it’s actually all of it based on ‘the best available evidence’, and occasionally it seems as if she’s gone out of her way to avoid adding a source even in situations where it would make a lot more sense to add it than to not do that. To add insult to injury, a couple of the reported estimates in the last half of the book were so out of line with other estimates I could find elsewhere that I seriously considered throwing the book away. So, yeah.

But the book has a lot of good stuff as well which presumably a lot of people would benefit from knowing about, so it’s really hard for me to know what to think about it. To take an example of what I’m talking about here, Rees et al. observed in their book – as I have pointed out – that: “Vegans who omit all animal products from their diet often have subclinical vitamin B12 deficiency.” They probably wouldn’t have if they’d read this book. Anemia is a very common condition worldwide, and iron deficiency is estimated to be the most common cause. An estimated 250 million preschool children are vitamin A deficient. We humans need a lot of different stuff to keep going, and the food we eat plays many roles most of us probably haven’t given any thought. Everybody needs to eat, so there aren’t many people who would not benefit from knowing more about how these things work; even people following ‘an ideal balanced diet’ can be at risk of developing deficiency states due to malabsorption syndromes or various disease states which may change nutritional requirements.

I’ve added some ‘sample observations’ from the book, as well as a few comments, below:

“There is a continuous turnover of protein in the body, which in healthy adults exhibits a balance between synthesis and breakdown, and amounts to 3–6 g/kg body weight per day. During growth there is an excess of synthesis over breakdown, and in wasting conditions (e.g. starvation, cancer and after surgery or trauma), breakdown exceeds synthesis. Protein synthesis is regulated principally by insulin, and catabolism by glucocorticoids. […] The body is unable to make nine of the amino acids used in protein synthesis […] Lack of any one of these will limit the synthesis of protein, even if all the other required amino acids are present in adequate amounts. […] In addition, there are a number of other amino acids that can be synthesised in the body under normal conditions, given the necessary supply of precursor molecules. In the absence of these precursors, the amino acids become ‘conditionally indispensable […] Protein synthesis is an energy-demanding process; it has been calculated that the energy requirement is 4.2 kJ (1 kcal)/g of protein synthesised. Protein synthesis occurs more rapidly after a meal than in the fasting state, due to the greater supply of amino acids. On average the energy used in protein synthesis accounts for 12% of the basal metabolic rate. […] The digestibility of proteins from animal sources is much greater than that from plant sources. Digestibility for egg is given as 97%. […] Poor digestibility, of between 60 and 80%, is found in legumes and cereals with tough cell walls, particularly when uncooked, and is a factor in diets that are low in protein. […] Worldwide, the availability of plant proteins is relatively consistent, at about 50 g/person/day. However, the availability of animal protein sources varies widely, from <5 to 50 g/head/day, highest in most Western countries. […] Inadequate protein intakes rarely occur alone, and are generally found within a wider picture of undernutrition. Insufficient intakes of energy cause protein to be used for energy, and make it unavailable for tissue maintenance or growth.”

“The most rapid period of brain growth occurs from mid-gestation to 18 months after birth. At birth the brain accounts for 10% of the body weight; an adult brain weighs about 1.4 kg, and comprises 2% of body weight. Different components of the brain grow at different rates and have ‘critical periods’ when growth is most rapid and vulnerable to adverse influence.”

‘Alcohol and folic acid’ would probably be two factors most people would know about in this context. But there’s of course a lot more to developing a human brain than these variables. Generalized undernutrition can lead to smaller brain and less extensive neural networks, long-chain polyunsaturated fatty acids are quite important for brain development, you need copper for myelin synthesis but too much of it may be toxic, iodine deficiency leads to cretinism, severe iron deficiency may lead to long-term reductions in cognitive performance whereas too much of it again may lead to toxicity, excess vitamin A intake may be teratogenic, pyridoxine deficiency may lead to seizures/neurologic symptoms, … Incidentally the arrow doesn’t just go from food intake to brain performance; the brain is also helping you figure out what to eat: “neurotransmitters acting within the brain are thought to regulate preferences for particular macronutrients. Serotonin may influence the balance between carbohydrate and protein intakes. Noradrenaline and opiates are also believed to have a role. […] Disturbances of neurotransmitter release, whether of endogenous (e.g. in disorders of brain function) or exogenous origin (e.g. by drugs), are likely therefore to affect food intake.”

Some more stuff about childhood growth from a few chapters on related matters:

“The fastest rate of growth is in the first 6 months of life, with a doubling of birth weight, and slows towards 12 months, to achieve about three times birth weight. Body weight only doubles between the ages of 1 and 5 years. Standard growth charts are useful to check that growth is progressing appropriately […] During the years of [school-age] childhood, mean growth is relatively constant, and averages 2.5 kg and 6 cm per year. During puberty, on average: • girls increase by 20 cm (height) and 20 kg (weight); • boys increase by 30 cm (height) and 30 kg (weight). These increases represent 40% of eventual adult weight. Growth is vulnerable to faltering if nutritional intakes do not keep pace with the demands. […] Body fat percentage levels increase rapidly in the first months of life, but start to fall after the first year. […] There is a further increase from about the age of 5 years (adiposity rebound), which may start earlier in larger, fatter children […] In boys, the fat content starts to fall during the pubertal growth spurt, but in girls it continues to increase, resulting in the average 10% fat content differential between the sexes seen in adults. […] Growth in infants and young children, usually recorded as weight, should progress along a centile line on standard growth charts. Reasons for centile crossing (moving from one centile line to another) need to be established. […] Infants of diabetic mothers, who are often born very large (>4.5 kg), may exhibit ‘catch-down’ growth during the first year of life. Once removed from the oversupply of nutrients in the womb, their growth rate slows. […] An infant undernourished in the womb may show ‘catch-up’ growth. This should be an increase in lean body mass, rather than fat; the latter is linked to risk of later disease.”

“It is now recognised that vitamin D is synthesised in the skin by the action of ultraviolet light on a precursor, and could strictly be termed a hormone rather than a vitamin. Further, niacin [vitamin B3] can be made in the body from the amino acid tryptophan, so a separate supply may not be needed if protein intakes are adequate. However, in both of these cases, there are situations where synthesis is insufficient, and so a dietary need remains.”

“Inadequate intakes of macronutrients will most obviously be reflected in disturbed growth in children and body weight changes in adults. […] Undernutrition in the elderly is poorly reported, but is believed to be widespread […] The consequences of undernutrition can manifest both in the short and long term, and may have intergenerational effects, through poor pregnancy outcome and low birth weight. […] Worldwide, deficiencies of iron, vitamin A and iodine affect the greatest numbers of people. […] Several other micronutrients may become deficient when diets lack specific food groups. These include: • vitamin B12, when vegan diets are consumed; • calcium, when dairy products are excluded from the diet; • riboflavin, when diets are low in green vegetables and dairy products.”

“The typical increase in weight (in the UK) during pregnancy is 11–16 kg but varies widely. Gains in the second and third trimesters should average 0.4 kg/week for normal weight women, less (0.3 kg/ week) for overweight women and more (0.5 kg/week) for women who are underweight. […] The extra energy costs of pregnancy are estimated at 310 MJ (77 000 kcal) […] The mother’s nutritional status is unlikely to affect the volume or the macronutrient content of her milk for the first few weeks of lactation. However, poorly nourished women will not be able to sustain the same level of nutrients for prolonged periods. The fat content of the milk correlates with the mother’s levels of body fat, and the pattern of fatty acids secreted in the milk partly reflects those in the mother’s dietary intake. Neither the fat-soluble vitamin content nor the mineral content of the milk fluctuates with maternal dietary intake.”

ATP is the fundamental molecule that on breakdown to ADP provides energy for contracting muscle. ATP stores are very limited and require continual replenishment; the amount stored would fuel only about two seconds of exercise. […] CHO [Carbohydrate] stores in muscle (300–800 g) and liver (80 g) are limited; fats stored mainly in subcutaneous tissue are found in very much greater amounts (minimum 5 kg in males, more in females). • Lipids are considerably more energy dense than CHOs. Metabolism of one gram of fat will deliver considerably more ATP molecules than one gram of CHO; however, more oxygen is required to metabolise fats, and fats cannot be metabolised anaerobically […] An important principle is that carbohydrates are the preferred energy source particularly for more intense and prolonged exercise […] Most athletes already consume sufficient protein in a mixed diet […] With the exception of energy intake the evidence that normal dietary supplements enhance sporting performance is poor.”

I found the two chapters about nutrition and sport interesting in a way, but mostly because they helped me figure out what happens in normal people – a book like this (which I have considered reading in the past, but has never gotten around to actually reading) is probably better at elucidating relevant mechanisms in my case.

“Consumers generally believe that foods produced organically, often by more traditional agricultural methods, have superior nutritional quality. This is not currently supported by the scientific literature, in which studies find no difference in nutrient content between organic and non-organic produce; there is also no information about impact on human health.”

June 21, 2014 - Posted by | books, health, medicine

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