p and Animal size —
In five minutes a human will burn around 350 joules (J) of energy per kilogram of body mass. A kilogram of mice will burn 3,000 J in the same time, and a 4,000- kg African elephant will burn just 200 J per kilogram. On a gram-for-gram basis, large animals burn less energy and require less food than small ones. The exact relationship between metabolic rate and body mass, and the mechanistic basis for it, remains an open question. Elephants are some 200,000 times greater in mass than mice, but require only 15,000 times more energy each day.
In Nature April 1 2010 p 753 Kolokotrones et at. show that the standard power equation that has been used for decades to describe the relationship may not be appropriate for mammals.
They use the deviation from this equation to test one of the most prominent explanations for the relationship.
In Paris in the 1830s, Sarrus and Rameaux suggested that, because the heat produced by an animal as a by-product of metabolism must be lost through the body surface, the rate at which it produces heat (met¬abolic rate) should be proportional not to its body mass, but to the surface area over which the heat is lost. Rubner then demonstrated empirically in 1883 that the basal metabolic rate of dogs is proportional to their body sur¬face area.
In 1916, Krogh suggested that the relationship between body mass and metabolic rate is best described by a power function, so that metabolic rate is not proportional to mass, but to mass raised to some power p. When p is equal to one, the relationship is a straight line and metabolic rate is said to scale isometrically (in proportion) with mass. When p does not equal one, the relationship is curved, and meta¬bolic rate is said to scale allometrically.
Krogh also suggested that for endotherms (birds and mammals) p is close to the value of 2/3 suggested by Rubner’s ‘surface law’, whereas for ectotherms, such as insects, fish, amphibians and reptiles, the p value is closer to 1.
However, in 1932 Kleiber, Brody and Proctor showed that metabolic rate was on a scale with a p value close to ¾. ( Kleiber’s law).
Kolokotrones et al. however demonstrate that a stand¬ard power equation may not be appropriate for describing the relationship between basal metabolic rate and body mass in mammals. They show that the value of p increases with body size: metabolic rate increases more rapidly with mass for large mammals than for small ones. They show that the relationship between mass and metabolic rate has convex curvature on a logarithmic scale, and is therefore not a pure power law, even after accounting for body temperature.
These results extend our understanding of a very complicated topic .
Size, surface are , heat production and loss, rate of movement, mass, types of muscles and more.
White 2010 There is no single p Nature vol 464 p 691
Kolokotrones et al 2010 Curvature in metabolic scaling . Nature vol 464 pp 753-756
Pengelly and Morris (Scottish Medical Journal 2009) have reviewed the relative merits of measurements of body mass index and weight distribution.
It has been accepted for many years that being overweight or obese, as indicated by a body mass index (BM I) of 25 or over for the former and 30 or over for the latter, is associated with impairment of long term health and prognosis. The World Health Organisation (WHO) has indicated that, in Caucasians, waist measurements of 94cm or more in men, and 80cm or more in women have similar adverse effects on health, with increased risks at 102 cm or more in men and 88 cm in women.
The role of waist-hip ratio (W/H) and whether it represents a better index than waist (W) measurement alone is being debated; many papers favour waist measurement alone. But two papers in 2005 discussing 27,098 subjects, 12,461 of whom had myocardial infarction and 14,637 controls, come down firmly in favour of W/H and were followed by a Lancet Editorial entitled Farewell to Body Mass Index?
Life assurance companies at medical examination usually request height and weight measurements (and therefore BMI). Most ask for waist measurements and a few hip measurements in addition (and therefore W/H).
The authors have reviewed the data in 816 consecutive subjects for life assurance examination in whom BMls, Ws and W/Hs were all recorded. In these the evidence supports the use of W as the best indicator of risk in men (634 cases), but not in the relatively small number of women (182 cases) in whom H appeared better.
We believe that BMI, W and W/H should be recorded in every subject at life assurance examination so that the insurance companies in the long term will be able to reach valid conclusions about their individual and collective value.
Pengelly and Morris Body mass index and weight distribution . Scottish Medical Journal 2009 vol 54 p 17-21
Waist Circumference Review
In December 2008, the World Health Organization (WHO) convened a consultation to discuss cut-points for waist circumference (WC).
There are large differences in body composition in men and women, with women having more body fat. Fat distribution also differs with gender, with men having a relatively more central distribution of fat. These differences begin early in life and become more apparent in puberty due to changes in sex hormone levels. In both, men and women, waist and waist-to-hip ratio increase with age. A large portion of this increase is due to gains in body weight, but the increases observed are larger than those that would be predicted from increases in the body mass index alone, and increases in WC are seen with aging in the absence of weight gain. The current practice of using separate waist cut-points by gender is appropriate. Although WC increases with age, so does the risk of many chronic diseases. An evaluation of the need for age-specific waist cut-points in adults would need to consider disease risk.
Current waist circumference (WC) and waist-to-hip ratio (WHR) cutoffs have been identified from studies of predominantly European-derived populations. However, these cutoffs may not be appropriate for other ethnic groups. Studies investigating ethnic-specific cutoffs were identified among Aboriginal, Asian, African (Sub-Saharan), African-American, Hispanic, Middle Eastern, Pacific Islander and South American populations. The majority of studies recommending ethnic-specific cutoffs was for Asian populations. Few studies recommended cutoffs in Aboriginal, African (Sub-Saharan), Pacific Islanders and South American populations. All studies were cross-sectional, and the overwhelming majority of studies used receiver operating characteristic curves. The studies used a number of methods for assessing WC and WHR, and a variety of outcome measures, making cross-study comparison difficult. There is possible evidence that Asians should have a lower WC cutoff than Europeans. The evidence is insufficient for specific cutoffs for African-American, Hispanic and Middle Eastern populations but some studies indicate current cutoffs for Europeans may be appropriate, whereas there is insufficient evidence for the other ethnic groups. Future studies are needed to address the methodological limitations of the current literature.