Beige is the New Brown - Implications for CR and Cold Exposure
This is another one of those slightly complicated stories based on a new study  and related evidence. But I think it will be worth it to follow along for anyone practicing CR, either without or without CE. So I urge you to bear with me.
Rapamycin extends lifespan in mice , and was once considered a promising anti-aging therapy, possibly acting as a CR-mimetic. Like one of the effects of CR, rapamycin inhibits the anabolic protein complex called 'mTOR'. In fact, the 'R' in 'mTOR' stands for 'Rapamycin', as in "mammalian Target Of Rapamycin" (mTOR).
But a Michael points out here, Rapamycin has some nasty side effects. One of the biggies is by turning off mTOR, rapamycin results in immune system suppression as shown in the diagram below. Rapamycin is much worse in this regard than the immunosuppressive effects of CR that operates at least in part by the same mechanism, i.e. downregulation of mTOR by reducing circulating insulin and IGF-1. In fact rapamycin is used as an immunosuppressant drug given to organ transplant recipients to prevent organ rejection by turning down the vigilance of the immune system.
That side-effect alone is enough reason to avoid taking rapamycin, unless you're an organ transplant recipient. Unfortunately for transplantees, immunosuppression isn't the only adverse side effect of rapamycin. It also reduces insulin sensitivity and impairs both glucose and lipid metabolism, leading to hyperlipidemia, metabolic syndrome and diabetes. But apparently the mechanism that causes these additional negative consequences of rapamycin has not been well understood...
This new study  sheds some light on that mystery, and also has some really interesting implication for fans of CR and CE.
What they did in  was feed mice either an ad lib control diet or ad lib control diet + rapamycin. Then the tested their metabolism and brown fat characteristics both after thermoneutral housing for several weeks, or after several weeks of cold exposure.
The important study findings for our purposes were:
- Cold exposure & "simulated" cold exposure via β3-adrenergic (catecholamine) receptor activation causes the browning of White Adipose Tissue (WAT) - turning it "beige" by adding mitochondria to WAT and upregulating UPC-1 expression so the WAT turns "beige" and starts burning calories, improving both glucose and lipid metabolism in the cold-exposed control mice.
- This happens through the CE → PKA → mTOR pathway discussed here, and graphically depicted below:
- But by suppressing mTOR activity, rapamycin prevented the cold-induced browning of WAT, and as a result blocked the improvements in glucose and lipid metabolism normally associated with cold exposure, as depicted below:
- Interestingly, rapamycin treatment didn't prevent the cold-induced increase of thermogenesis via expression of UCP-1 in existing BAT tissue, it just prevented WAT from turning brown/beige, apparently by suppressing mitochondrial biogenesis in WAT cells.
- Nevertheless, the existing BAT wasn't enough, and the rapamycin-treated mice weren't able to maintain their body temperature like the control mice could when exposed to extreme cold (4 °C).
So what does all this mean, especially for people?
The most interesting thing is that rodents (and baby humans) have both true BAT cells (cells that were always brown, ever since they were created) and beige cells (cells that were born as WAT but have been turned brown through the addition of mitochondria and other cellular machinery). In contrast, it appears adult humans may have only beige cells , and no true BAT:
To our surprise, nearly all the human BAT abundantly expressed beige cell-selective genes, but the expression of classical brown fat-selective genes were nearly undetectable.
Beige or brown - so what? In fact,  found it doesn't make much difference thermogenically:
When stimulated by such external cues [including cold exposure - DP], beige adipocytes express UCP1 protein at a similar level to classical brown adipocytes and exhibit UCP1-dependent thermogenic capacity.
So what difference does the pedigree of human thermogenic fat cells make? Perhaps plenty of difference. In fact, this might explain several puzzling mysteries associated with cold exposure, and perhaps even CR.
First, from  the "browning" of white fat to beige can at least in theory happen to any normal white adipose cells if it receives the right signals, e.g. as a result of cold exposure. And, unlike true BAT cells which tend to form localized, homogeneous deposits/pads, beige cells are usually found mixed amongst white fat cells. Finally,  points out that "currently available devices do not have enough sensitivity and resolution to detect UCP1-positive adipocytes (i.e. BAT cells) that sporadically reside in subcutaneous WAT and other adipose depots."
In other words, CE turns WAT cells to thermogenic 'beige' cells in adult humans, and these are what we call human BAT. And these beige cells may be forming anywhere in the body where WAT is deposited, not just in the neck, upper chest and upper back regions where existing PET & thermal imaging technology can detect BAT or BAT-related thermogenic activity.
This could point to an answer to the mystery of how such small amounts of measurable BAT tissue (i.e. only a couple ounces in BAT+ people) can possibly account for the dramatic improvement in glucose clearance (discussed here) and increase metabolic rate (~200kcal/day, as discussed here) that is observed in people chronically exposed to cold. If beige adipose cells are more numerous than commonly believed in cold-exposed people and distributed around the body rather than concentrated only in detectable pockets of BAT, they could be contributing a lot more to thermogenesis and calorie-expenditure than seems possible based on the small amount of BAT that existing technology can detect around the neck region. This could also explain Michael's now infamous "jiggling pecs" study , which found the BAT deposits near the neck account for only a small fraction of human thermogenesis in response to cold. Perhaps the neck BAT is just the tip of the iceberg... I still think sarcolipin-induced thermogenesis in skeletal muscles is the more likely explanation for where all the calories are going, but thermogenic "beige" BAT cells may be more numerous and distributed than previously believed, and therefore playing a bigger role in human cold-induced thermogenesis than anyone realizes.
Which brings me to the second set of puzzles that this study may solve.
Ever wonder why detectable human BAT tissue (I'm going to continue to use BAT, even those human BAT is really beige, not brown) has a sweet spot when it comes to BMI? On the one hand, very thin people, like anorexics and still-quite-thin recovered anorexics, have zero detectable BAT, as discussed here. But on the other hand, it is the leaner people in the normal/overweight range who have greater amounts of BAT than the really fat people, as discussed here. The upper end isn't too hard to explain via two possible mechanisms. First, obese people have more thermal insulation and so probably need less BAT than learner people to stay warm. Second, reverse causality. I.e people with BAT burn more calories, and hence remain thinner than people without BAT. So in the heavyweight range, BAT → greater thinness rather than greater thinness → BAT.
But why don't really skinny folks have any BAT, when they are in dire need of more thermogenesis? Recall, Speakman found that BAT was the only tissue which was increased (doubled no less!) in mass in CRed mice relative to controls , as discussed here. So why do skinny mice have lots of BAT, but skinny humans have none?
This study  may explain the paradox. If human BAT (really beige adipose tissue) is generated only through the 'browning' of white adipose tissue, it's no wonder it's entirely lacking in people who have extremely low levels of (white) body fat. Anorexics, and by implication, hard-core CR practitioners don't have enough WAT to convert into appreciable amounts of BAT, even when chronically cold exposed. In other words, unlike mice, humans have to have a bit of fat on their bones in order for it to be converted into BAT!
And the strong apparent linkage between BAT and glucose control could explain why, in Luigi Fontana's study of human CR practitioners, those of us who were the most severely CRed paradoxically exhibited impaired glucose clearance in response to a glucose tolerance test as I discussed here. Since we have so little white fat and since we have such low Insulin/IGF-1 levels which suppresses mTOR and therefore suppressing browning of what little WAT we may have, seriously CRed humans just don't have enough BAT to help clear a large glucose load from our circulatory system during an OGTT.
But getting back to the study at hand  on rapamycin and cold exposure. It seems pretty well established that rapamycin extends lifespan in mice, perhaps by about 10% , by shutting down mTOR as shown in the diagram above. In striking contrast, rapamycin is pretty toxic to humans due to it's side effects as Michael discussed here, including compromised immune system and increased risk of metabolic syndrome / diabetes.
This discrepancy might at least in part be explained by the facts that a) mice can have both true BAT and beige adipose tissue and b) the mice in the rapamycin longevity studies (like virtually all rodent studies) were housed at temperatures that are chilly for mice. While rapamycin may have shut down the mice's ability to turn white fat into beige in , presumably the mice still likely had substantial true BAT deposits as a result of their cool housing conditions. And so the rapamycin-treated mice could benefit from the BAT-induced improvements in insulin sensitivity / glucose control, as well as improvements in immune system performance, and live a long time, since absent the negative side effects, mTOR suppression really is beneficial for longevity after all. The current study  supports this conjecture, since they found the thermogenic capacity of true BAT was not impaired by rapamycin treatment:
However, the respiratory capacity of BAT was 5-10 fold greater ... and was only
mildly impaired by rapamycin, suggesting that significant capacity for BAT mediated nonshivering
thermogenesis might still remain in cold-challenged, rapamycin-treated animals.
In short shutting down mTOR (via rapamycin or CR) may not be so detrimental in mice because mice can maintain native BAT and BAT-thermogenesis even in the absence of mTOR activity, although not enough thermogenic capacity to keep them warm when subjected to the extreme 4°C cold challenge used in this study! In contrast, adult humans don't have much (if any) native, true BAT, but only beige adipose tissue, and they can't even produce any of the beige fat from WAT if mTOR is shut down by CR or rapamycin. if BAT is as beneficial as I argue it is, and is indeed critical for CR to work it's magic (as I've argued here, here and here) it's no wonder rapamycin (and, heaven forbid CR...) is more toxic in humans than rodents, because only in humans does rapamycin (or severe CR) entirely prevent BAT formation.
Finally, this rapamycin study might explain one more seeming anomaly in the CR literature. Study  found that mice strains that retain the most fat when subjected to CR live the longest. Conversely, mice strains that lose the most fat have their lives cut short, rather than extended, by CR. Could it be that this correlation between CR lifespan benefits and the ability to retain some fat when CRed results from the chubbier CRed mice's ability to turn some of their remaining white fat to beneficial beige fat in the cool housing conditions of these lifespan experiments? Obviously this idea is quite speculative, but an intriguing possibility nonetheless...
Takeaway messages based on these studies:
- Humans shouldn't take rapamycin unless you've had an organ transplant - in case you didn't know that already.
- Rodents (and baby humans) have two types of thermogenic fat tissue - true BAT and "beige" adipose tissue which is white fat cells that has been "browned" - i.e. converted to a BAT-like profile by cold exposure or pharmacological means. But adult humans have only beige adipose tissue.
- If you don't eat enough to have a little fat on your bones, you won't have sufficient WAT that can be browned, so you won't generate any BAT, even if you beat yourself up with extreme cold exposure.
- (Speculative) CR, like rapamycin, extends lifespan in rodents at least if they are are cold-exposed, but CR may not work in humans for the following reason. Both CR and rapamycin greatly depress mTOR activity in mice and men. Mice can still have BAT without mTOR activity, but mTOR activity is obligatory if humans are to have any BAT at all. If BAT, or more generally, the metabolic milieu created by BAT and CE, is a critical adjunct to CR as I've argued here, here and here, that means that in humans, no mTOR → no BAT → no life extension.
Fortunately, cold exposure can activate mTOR via the PKA pathway, as discussed here, so there is still hope for CR benefits in humans despite humans being so different from mice with respect to BAT formation and prevalence. But if this model is correct, for humans to benefit from CR they need to practice cold exposure and eat enough to allow mTOR to "do it's thing" of turning WAT into BAT.
For those of us playing around with cold exposure, it seems like that third point can't be stressed enough. And for anyone practicing CR, the last point, while speculative, seems well worth considering.
 Diabetes. 2016 Feb 8. pii: db150502. [Epub ahead of print]
Rapamycin blocks induction of the thermogenic program in white adipose tissue.
Tran CM(1), Mukherjee S(1), Ye L(2), Frederick DW(1), Kissig M(3), Davis JG(1),
Lamming DW(4), Seale P(3), Baur JA(5).
Rapamycin extends lifespan in mice, yet paradoxically causes lipid dysregulation
and glucose intolerance through mechanisms that remain incompletely understood.
Whole body energy balance can be influenced by beige/brite adipocytes, which are
inducible by cold and other stimuli via β-adrenergic signaling in white adipose
depots. Induction of beige adipocytes is considered a promising strategy to
combat obesity because of their ability to metabolize glucose and lipids,
dissipating the resulting energy as heat through uncoupling protein 1 (UCP1).
Here, we report that rapamycin blocks the ability of β-adrenergic signaling to
induce beige adipocytes and expression of thermogenic genes in white adipose
depots. Rapamycin enhanced transcriptional negative feedback on the β3-adrenergic
receptor. However, thermogenic gene expression remained impaired even when the
receptor was bypassed with a cell-permeable cAMP analogue, revealing the
existence of a second inhibitory mechanism. Accordingly, rapamycin-treated mice
are cold-intolerant, failing to maintain body temperature and weight when shifted
to 4 °: C. Adipocyte-specific deletion of the mTORC1 subunit Raptor recapitulated
the block in beta-adrenergic signaling. Our findings demonstrate a positive role
for mTORC1 in the recruitment of beige adipocytes and suggest that inhibition of
β-adrenergic signaling by rapamycin may contribute to its physiological effects.
© 2016 by the American Diabetes Association. Readers may use this article as long
as the work is properly cited, the use is educational and not for profit, and the
work is not altered.
PMCID: PMC4806661 [Available on 2017-04-01]
 PLoS One. 2012;7(11):e49452. doi: 10.1371/journal.pone.0049452. Epub 2012 Nov 16.
Human BAT possesses molecular signatures that resemble beige/brite cells.
Sharp LZ(1), Shinoda K, Ohno H, Scheel DW, Tomoda E, Ruiz L, Hu H, Wang L,
Pavlova Z, Gilsanz V, Kajimura S.
(1)UCSF Diabetes Center and Department of Cell and Tissue Biology, University of
California San Francisco, San Francisco, California, USA.
Brown adipose tissue (BAT) dissipates chemical energy and generates heat to
protect animals from cold and obesity. Rodents possess two types of UCP-1
positive brown adipocytes arising from distinct developmental lineages:
"classical" brown adipocytes develop during the prenatal stage whereas "beige" or
"brite" cells that reside in white adipose tissue (WAT) develop during the
postnatal stage in response to chronic cold or PPARγ agonists. Beige cells'
inducible characteristics make them a promising therapeutic target for obesity
treatment, however, the relevance of this cell type in humans remains unknown. In
the present study, we determined the gene signatures that were unique to
classical brown adipocytes and to beige cells induced by a specific PPARγ agonist
rosiglitazone in mice. Subsequently we applied the transcriptional data to humans
and examined the molecular signatures of human BAT isolated from multiple adipose
depots. To our surprise, nearly all the human BAT abundantly expressed beige
cell-selective genes, but the expression of classical brown fat-selective genes
were nearly undetectable. Interestingly, expression of known brown fat-selective
genes such as PRDM16 was strongly correlated with that of the newly identified
beige cell-selective genes, but not with that of classical brown fat-selective
genes. Furthermore, histological analyses showed that a new beige cell marker,
CITED1, was selectively expressed in the UCP1-positive beige cells as well as in
human BAT. These data indicate that human BAT may be primary composed of
 J Clin Invest. 2015 Feb;125(2):478-86. doi: 10.1172/JCI78362. Epub 2015 Feb 2.
Brown and beige fat in humans: thermogenic adipocytes that control energy and
Sidossis L, Kajimura S.
Brown adipose tissue (BAT), a specialized fat that dissipates energy to produce
heat, plays an important role in the regulation of energy balance. Two types of
thermogenic adipocytes with distinct developmental and anatomical features exist
in rodents and humans: classical brown adipocytes and beige (also referred to as
brite) adipocytes. While classical brown adipocytes are located mainly in
dedicated BAT depots of rodents and infants, beige adipocytes sporadically reside
with white adipocytes and emerge in response to certain environmental cues, such
as chronic cold exposure, a process often referred to as "browning" of white
adipose tissue. Recent studies indicate the existence of beige adipocytes in
adult humans, making this cell type an attractive therapeutic target for obesity
and obesity-related diseases, including type 2 diabetes. This Review aims to
cover recent progress in our understanding of the anatomical, developmental, and
functional characteristics of brown and beige adipocytes and discuss emerging
questions, with a special emphasis on adult human BAT.
 Eur J Nucl Med Mol Imaging. 2016 Mar 19. [Epub ahead of print]
Human brown adipose tissue [(15)O]O2 PET imaging in the presence and absence of
U Din M(1,)(2), Raiko J(1,)(2), Saari T(1,)(2), Kudomi N(3), Tolvanen T(1,)(2),
Oikonen V(1,)(2), Teuho J(1,)(2), Sipilä HT(1,)(2), Savisto N(1,)(2), Parkkola
R(4), Nuutila P(1,)(2), Virtanen KA(5,)(6).
PURPOSE: Brown adipose tissue (BAT) is considered a potential target for
combatting obesity, as it produces heat instead of ATP in cellular respiration
due to uncoupling protein-1 (UCP-1) in mitochondria. However, BAT-specific
thermogenic capacity, in comparison to whole-body thermogenesis during cold
stimulus, is still controversial. In our present study, we aimed to determine
human BAT oxygen consumption with [(15)O]O2 positron emission tomography (PET)
imaging. Further, we explored whether BAT-specific energy expenditure (EE) is
associated with BAT blood flow, non-esterified fatty acid (NEFA) uptake, and
METHODS: Seven healthy study subjects were studied at two different scanning
sessions, 1) at room temperature (RT) and 2) with acute cold exposure.
Radiotracers [(15)O]O2, [(15)O]H2O, and [(18)F]FTHA were given for the
measurements of BAT oxygen consumption, blood flow, and NEFA uptake,
respectively, with PET-CT. Indirect calorimetry was performed to assess
differences in whole-body EE between RT and cold.
RESULTS: BAT-specific EE and oxygen consumption was higher during cold stimulus
(approx. 50 %); similarly, whole-body EE was higher during cold stimulus (range
2-47 %). However, there was no association in BAT-specific EE and whole-body EE.
BAT-specific EE was found to be a minor contributor in cold induced whole-body
thermogenesis (almost 1 % of total whole-body elevation in EE). Certain deep
muscles in the cervico-thoracic region made a major contribution to this
cold-induced thermogenesis (CIT) without any visual signs or individual
perception of shivering. Moreover, BAT-specific EE associated with BAT blood flow
and NEFA uptake both at RT and during cold stimulus.
CONCLUSION: Our study suggests that BAT is a minor and deep muscles are a major
contributor to CIT. In BAT, both in RT and during cold, cellular respiration is
linked with circulatory NEFA uptake.
 Nature. 2009 Jul 16;460(7253):392-5. doi: 10.1038/nature08221. Epub 2009 Jul 8.
Rapamycin fed late in life extends lifespan in genetically heterogeneous mice.
Harrison DE(1), Strong R, Sharp ZD, Nelson JF, Astle CM, Flurkey K, Nadon NL,
Wilkinson JE, Frenkel K, Carter CS, Pahor M, Javors MA, Fernandez E, Miller RA.
(1)The Jackson Laboratory, Bar Harbor, Maine 04609, USA. email@example.com
Nature. 2009 Jul 16;460(7253):331-2.
Inhibition of the TOR signalling pathway by genetic or pharmacological
intervention extends lifespan in invertebrates, including yeast, nematodes and
fruitflies; however, whether inhibition of mTOR signalling can extend lifespan in
a mammalian species was unknown. Here we report that rapamycin, an inhibitor of
the mTOR pathway, extends median and maximal lifespan of both male and female
mice when fed beginning at 600 days of age. On the basis of age at 90% mortality,
rapamycin led to an increase of 14% for females and 9% for males. The effect was
seen at three independent test sites in genetically heterogeneous mice, chosen to
avoid genotype-specific effects on disease susceptibility. Disease patterns of
rapamycin-treated mice did not differ from those of control mice. In a separate
study, rapamycin fed to mice beginning at 270 days of age also increased survival
in both males and females, based on an interim analysis conducted near the median
survival point. Rapamycin may extend lifespan by postponing death from cancer, by
retarding mechanisms of ageing, or both. To our knowledge, these are the first
results to demonstrate a role for mTOR signalling in the regulation of mammalian
lifespan, as well as pharmacological extension of lifespan in both genders. These
findings have implications for further development of interventions targeting
mTOR for the treatment and prevention of age-related diseases.
 Aging Cell. 2011 Aug;10(4):629-39. doi: 10.1111/j.1474-9726.2011.00702.x. Epub
2011 Apr 25.
Fat maintenance is a predictor of the murine lifespan response to dietary
Liao CY(1), Rikke BA, Johnson TE, Gelfond JA, Diaz V, Nelson JF.
(1)Department of Physiology, University of Texas Health Science Center, San
Antonio, TX 78229, USA.
Dietary restriction (DR), one of the most robust life-extending manipulations, is
usually associated with reduced adiposity. This reduction is hypothesized to be
important in the life-extending effect of DR, because excess adiposity is
associated with metabolic and age-related disease. Previously, we described
remarkable variation in the lifespan response of 41 recombinant inbred strains of
mice to DR, ranging from life extension to life shortening. Here, we used this
variation to determine the relationship of lifespan modulation under DR to fat
loss. Across strains, DR life extension correlated inversely with fat reduction,
measured at midlife (males, r= -0.41, P<0.05, n=38 strains; females, r= -0.63,
P<0.001, n=33 strains) and later ages. Thus, strains with the least reduction in
fat were more likely to show life extension, and those with the greatest
reduction were more likely to have shortened lifespan. We identified two
significant quantitative trait loci (QTLs) affecting fat mass under DR in males
but none for lifespan, precluding the confirmation of these loci as coordinate
modulators of adiposity and longevity. Our data also provide evidence for a QTL
previously shown to affect fuel efficiency under DR. In summary, the data do not
support an important role for fat reduction in life extension by DR. They suggest
instead that factors associated with maintaining adiposity are important for
survival and life extension under DR.
© 2011 The Authors. Aging Cell © 2011 Blackwell Publishing Ltd/Anatomical Society
of Great Britain and Ireland.
 Mech Ageing Dev. 2005 Jun-Jul;126(6-7):783-93. Epub 2005 Mar 16.
Energy expenditure of calorically restricted rats is higher than predicted from
their altered body composition.
Selman C(1), Phillips T, Staib JL, Duncan JS, Leeuwenburgh C, Speakman JR.
(1)University of Florida, Department of Aging and Geriatric Research, College of
Medicine, Gainesville, 32608, USA. firstname.lastname@example.org
Debate exists over the impact of caloric restriction (CR) on the level of energy
expenditure. At the whole animal level, CR decreases metabolic rates but in
parallel body mass also declines. The question arises whether the reduction in
metabolism is greater, smaller or not different from the expectation based on
body mass change alone. Answers to this question depend on how metabolic rate is
normalized and it has recently been suggested that this issue can only be
resolved through detailed morphological investigation. Added to this issue is the
problem of how appropriate the resting energy expenditure is to characterize
metabolic events relating to aging phenomena. We measured the daily energy
demands of young and old rats under ad libitum (AD) food intake or 40% CR, using
the doubly labeled water (DLW) method and made detailed morphological examination
of individuals, including 21 different body components. Whole body energy demands
of CR rats were lower than AD rats, but the extent of this difference was much
less than expected from the degree of caloric restriction, consistent with other
studies using the DLW method on CR animals. Using multiple regression and
multivariate data reduction methods we built two empirical predictive models of
the association between daily energy demands and body composition using the ad
lib animals. We then predicted the expected energy expenditures of the CR animals
based on their altered morphology and compared these predictions to the observed
daily energy demands. Independent of how we constructed the prediction, young and
old rats under CR expended 30 and 50% more energy, respectively, than the
prediction from their altered body composition. This effect is consistent with
recent intra-specific observations of positive associations between energy
metabolism and lifespan and theoretical ideas about mechanisms underpinning the
relationship between oxygen consumption and reactive oxygen species production in
There will never be peace in the world while there are animals in our bellies.