Organ Specific Resting Metabolic Rates and Diet-Induced "Metabolic Damage". Plus: At Rest Heart, Liver & Kidney Consume 83x More Energy/kg Organ Mass Than Muscle

No, your muscles are not the primary gas guzzler in your body.

The problems arising as a consequence of a diet-induced reduction of the metabolic rate are among the recurring themes here at the SuppVersity. For a good reason, as I would say. After all, they are the #1 reason for weight loss plateaus and the yoyo effect. Although the notion that "calories count" is not very popular these days there is no debating that an energy deficit is a necessary prerequisite for weight loss. The problem however is that you cannot determine your energy balance with a calculator, a body fat caliper and a scale. There are way too many other factors involved - the amount, macro- and micronutrient composition, timing, frequency, volume, texture and palatability of the ood you eat, stress, hormonal factors, etc - all of which will affect the amount of energy you expend and subvert the results of over-simplistic calories-in vs. calories-out calculations.

What are the most notorious gas guzzlers in our body?

Things would actually already be complex enough, if we focused solely on that "input" <> "output" recursion, but unfortunately, even the notion of a "global" (=valid for the whole body) metabolic rate is nothing we can really rely on. If we wanted to have a somewhat more accurate estimate of our basal energy requirements, i.e. the amount of energy we need if we don't move all day (which is basically what the average Westerner does, these days ;-), we would have to know the individual energy requirements of all our major organs and add them up, using a formula like this:

The more you eat, the more you burn. You can find more evidence that men & women are no bomb-calorimeters here

240x brain mass in kg

+ 440x heart mass in kg
+ 200x liver mass in kg
+ 440x kidney mass in kg

+ 13x skeletal muscle mass in kg
+ 4.5x adipose tissue mass in kg

+ 12x residual mass in kg

This formula, which was developed based on studies of Elia et al. in 1998, assumes that the metabolic activity of an organ increases linearly with its mass and that the specific metabolic rates (ki-values, i.e. 240 for the brain, 440 for the heart, etc.) are accurate. In the average, normal weight non-dieting individual these values are constant and have been confirmed lately in a set of experiments that were conducted by Wang et al. (see figure 1)

Figure 1: ki-Values of adipose tissue, skeletal muscle, liver, brain, heart, kidneys, residual volume from the Wang studies; all values expressed relative to the reference values from Elia (1998)

These studies, which were published subsequently in 2010, 2011 and 2012, also reported that there are distinct trends for decreasing ki-values and thus lower resting energy expenditures at identical organ masses in both obese / lean and older / younger individuals - an effect which can be explained by either lower cellularity or lower specific metabolic rates of the respective organs and tissues. In light of the fact that the "organ weight x ki-value"-calculations are very accurate and that

"there is only a small and nonsignificant difference between REEm [measured resting energy experience] and REEc [the energy experience calculated based on ki-values and organ masses] of about 13 to 80 kcal/day" (Müller. 2013b)

it should be obvious that both aging and already being obese put you at a higher risk of weight gain in a society where energy dense foods and large portion sizes are the rule, not the exception.

Is there something like organ specific metabolic damage?

A couple of recent studies have investigated the effects of weight loss and regain on organ-specific energy expenditure in order to find out if this may be the, or at least one of the underlying reason for the reduced resting energy expenditure in formerly obese individuals (Müller. 2013a; Bosy-Westphal. 2009 & 2013). These studies support the idea of a fall in the organ size and weight and the corresponding ki-values of high metabolic rate organs (heart, kidney, liver) with weight loss. Bosy-Westphal (2009), for example report a -136kcal/day reduction in resting energy expenditure (REEm = measured) with 4-6% loss of liver, heart and kidney mass in obese women after 9.5kg body weight loss (2.6% fat free mass).

Figure 2: Difference between measured and calculated energy expenditure in MJ/day at baseline, after weight loss and regain in  47 obese men and women who lost 12kg (weight stable) and 9kg (weight regainers) in a study by Bosy-Westphal et al. from 2013

"In addition, the effect of weight loss and weight regain over a longer follow-up period of 6 months had been studied in 47 obese males and females (Bosy-Westphal. 2013). There were considerable differences between weight-reduced/weight-stable individuals compared with weight regainers. Over a period of 6 months, weight-reduced/weight-stable individuals had lost 12 kg body weight, the weight change-associated changes in the REEm - REEc values were 33 and 45 kcal/day, with initial weight loss and with long-term follow-up (i.e. between 12 weeks and 6 months). By contrast, weight regainers regained 6.3 kg body weight after an initial loss of about 9 kg. The corresponding data on the weight changeassociated changes in the REEm - REEc values were 69 and 10 kcal/day, respectively. Individual data for the group ‘regainers’ at basal before and after weight loss, as well after weight regain, are shown in [figure 2]. The changes in the REEm- REEc values argue for changes in specific metabolic rates with weight changes." (Müller. 2013b)

What? Ok, I have to admit that this paragraph from Müller's 2013 review of the literature is not actually easy to understand. So let's take a look at the data in figure 2 again. The main message here is that the weight loss narrows the natural spectrum of REEs down to the minimal requirements of your organs. In other words, the body is running on low fumes and is thus particularly prone to weight regain which can - but does not have to - lead to an increase in the per pound organ weight energy expenditure that would then become obvious in the form of a larger difference between the measured (REEm) and calculated (REEc) resting energy expenditure (green and red circles in figure 2). With the pre-post weight regain difference being 69 vs. 10, it is yet unfortunately more common that the initial organ energy expenditure is not being restored (red circle in figure 2) and the energy expenditure remains low although people regain a lot if not all of their weight.

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Suggested read: "Do Chronic Energy Deficits Make Athletes Fat? The Longer & More Severe You Starve, the Fatter You Are. Irrespective of What the Calories-in-VS-Calories-Out Formula May Say" | read more

What can you do with this information? Not that much, I have to admit. If anything the major contribution of non-muscle tissue to the diet induced reduction of the resting metabolic rate should remind you that it may be at least equally important to spare the mass of the organs in your splachnic bed as it is to maintain as much lean muscle tissue as possible when your dieting.

I don't know if you remember the recent study about citrulline and it's effect on the maintenance of muscle and visceral tissue mass (see figure 1 in the respective article) or previous SuppVersity posts on other non-essential amino acids, such as glutamine or arginine? All of them are primarily "organ food" and an adequate provision of these conditionally essential amino acids should be considered as important as the provision of the purportedly muscle-protecting BCAAs if you want to keep the loss of organ mass at a minimum and your resting metabolic rate up. Whether and how you can influence the individual metabolic rate, of these organs is yet a totally different question to which no one has found a definitive answer, yet.

References:

• Bosy-Westphal A, Kossel E, Goele K,et al. Contribution of individual organ mass loss to weight-loss associated decline in resting energy expenditure. Am J Clin Nutr 2009; 90:993–1001.
• Bosy-Westphal A, Schautz B, Lagerpusch M,et al.Effect of weight loss and regain on adipose tissue distribution, composition of lean mass and resting energy expenditure in young overweight and obese adults. Int J Obes 2013.
• Elia M. Organ and tissue contribution to metabolic rate. In: Kinney J, Tucker HN, editors. Energy metabolism: tissue determinants and cellular corollaries. New York: Raven Press; 1992. pp. 61–79
• Müller MJ, Bosy-Westphal A. Adaptive thermogenesis with weight loss in humans. Obesity 2013a; 21:218–228.
• Müller MJ, Wang Z, Heymsfield SB, Schautz B, Bosy-Westphal A. Advances in the understanding of specific metabolic rates of major organs and tissues in humans. Curr Opin Clin Nutr Metab Care. 2013b Sep;16(5):501-8.
• Wang Z, Ying Z, Bosy-Westphal A,et al.Specific metabolic rates of major organs and tissues across adulthood: evaluation by mechanistic model of resting energy expenditure. Am J Clin Nutr 2010; 92:1369–1377.
• Wang Z, Ying Z, Bosy-Westphal A,et al.Evaluation of specific metabolic rates of major organs and tissues: comparison between men and women. Am J Hum Biol 2011; 23:333–338.
• Wang Z, Ying Z, Bosy-Westphal A,et al.Evaluation of specific metabolic rates of major organs and tissues: comparison between nonobese and obese women. Obesity 2012; 20:95–100.