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Cognitive And Autonomic Determinants of Energy Balance

Denis Richard, PhD, Professor, Université Laval, QC, Canada

Denis Richard, PhD, Professor, Université Laval, QC, Canada

While I’m here at the 10th Canadian Obesity Network Summer School (Boot Camp), in the Canadian Rockies, it is perhaps of interest to note that one of the founding faculty of this school, Denis Richard from Laval University, has just published a paper in Nature Reviews Endocrinology, which nicely reviews the complex neurobiology of energy balance.

The paper focuses largely on the “energy out” part of energy homeostasis, which is partly determined by the themogenesis of brown adipose tissue and mediated by the sympathetic nervous system.

Thus, several areas of the brain work together in complex neuronal networks involving a host of neuronal systems including the opioid, endocannabinoid and melanocortin systems, that not only control  appetite and eating behaviour but also thermogenesis.

These neuronal systems, in turn receive inputs from a wide range of peripheral organs including the gut, liver and adipose tissue via hormonal and neuronal pathways that signal energy stores and nutritional status.

The paper also discusses how some of these findings may be relevant to the development of novel treatments for obesity.

For researchers and students: the paper includes a number of excellent graphics that nicely illustrate these systems.

@DrSharma
Kananaskis, AB

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Limbic Effects On Physical Activity And Obesity

sharma-obesity-running-mouseThe amygdala is a part of the so-called limbic system that performs a primary role in the processing of memory, decision-making, and emotional reactions. The amygdala has also been implicated in a variety of mental health problems including anxiety, binge drinking and post-traumatic stress syndrome.

A study by Xu and colleagues, published in the Journal of Clinical Investigation now shows that in mice, activity of the estrogen receptor–α (ERα) in the medial amygdala may have a profound influence on the development of obesity – an effect, which appears to me largely mediated through effects on physical activity.

Building on previous work showing that ERα activity in the brain prevents obesity in both males and female rats, the researchers used a series of complex experiments to demonstrate that specific deletion of the ERα gene from SIM1 neurons, which are highly expressed in the medial amygdala, cause a marked decrease in physical activity and weight gain in both male and female mice fed with regular chow, without any increase in food intake. In addition, this deletion caused increased susceptibility to diet-induced obesity in males but not in females.

Deletion of the ERα receptor also blunted the body weight-lowering effects of a glucagon-like peptide-1-estrogen (GLP-1-estrogen) conjugate.

In contrast, over-expression or stimulation of SIM1 neurons increased physical activity in mice and protected them from diet-induced obesity.

These findings point to a novel mechanism of neuronal control of physical activity, which in turn appears to have important effects on the susceptibility to weight gain.

@DrSharma
Edmonton, AB

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How Your Gut Tastes What You Eat

sharma-obesity-guthormones2If you thought that the only senses that determine the palatability of food are your sense of taste and smell, you may be wrong.

It turns out that we have a rather sophisticated sensing mechanism in our gut that senses the composition of our diet and interacts with the brain to regulate our appetite and food intake.

Just how exactly this gut “nutrient-sensing” system works, is reviewed by Sophie Hamr and colleagues from the University of Toronto in a paper published in Current Diabetes Reports.

As the authors point out,

“…the gastrointestinal (GI) tract is anatomically positioned to provide initial feedback following a meal via detection of incoming nutrients and relaying signals to the brain and peripheral tissues to prevent excess energy intake and circulating nutrients…..This, coupled with the vast neural and humoral connectivity of the gut to other important sites of energy regulation, such as the brain, allows the gut to effectively relay information to the rest of the body about the size and composition of an incoming meal.”

Each nutrient (fats, carbohydrates, protein) interacts with specific sensory and signal transduction mechanisms in the gut.

Animal studies show that exposing the gut to certain nutrients (for e.g. by tube feeding) can stimulate or suppress feeding behaviour making animals chose or avoid certain foods. Often these effects can persist for days or even weeks, well beyond the time course of a single meal.

Furthermore, these effects appear to be largely dependent on the presence of specific nutrients rather than on the actual nutritional or energy state of the animal.

“…these evidences lend notion for the intestine to sense specific nutrients (i.e., lipid and carbohydrate) at specific concentrations, rather than calories, in an effort to drive further food consumption.”

The authors point out that changes in how the gut senses nutrients may well explain how bariatric surgery works to reduce appetite and change food preferences.

No doubt, a better understanding these mechanisms and the molecular mechanisms involved could lead to novel dietary or pharmacological interventions to prevent or treat obesity.

@DrSharma
Edmonton, AB

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Post-Weight Loss Fat Gain in US Rangers

army-rangersAnd finally, to conclude this week’s discussion of evidence to support the notion that weight cycling predicts weight (fat) gain especially in normal weight individuals, I turn back to the paper by Dulloo and colleagues published in Obesity Reviews, which quotes these interesting findings in US Rangers:

“…U.S. Army Ranger School where about 12% of weight loss was observed following 8–9 weeks of training in a multi-stressor environment that includes energy deficit. Nindl et al. reported that at week 5 in the post-training recovery phase, body weight had overshot by 5 kg, reflected primarily in large gains in fat mass, and that all the 10 subjects in that study had higher fat mass than before weight lost. Similarly, in another 8 weeks of U.S. Army Ranger training course that consisted of four repeated cycles of restricted energy intake and refeeding, Friedl et al. showed that more weight was regained than was lost after 5 weeks of recovery following training cessation, with substantial fat overshooting (∼4 kg on average) representing an absolute increase of 40% in body fat compared with pre-training levels. From the data obtained in a parallel group of subjects, they showed that hyperphagia peaked at ∼4 weeks post-training, thereby suggesting that hyperphagia was likely persisting over the last week of refeeding, during which body fat had already exceeded baseline levels.”

Obviously, association (even in a prospective cohort) does not prove causality or, for that matter, provide insights into the physiological mechanisms underlying this observation.

All we can conclude, is that these observations in US Rangers (and the other studies cited in Dulloo’s article) are consistent with the notion that weight loss in normal weight individuals can be followed by significant weight gain, often overshooting initial weight.

Incidentally, these findings are also consistent with observational studies in women recovering from anorexia nervosa, famine, cancer survivors and other situations resulting in significant weight loss in normal weight individuals.

Certainly enough evidence to consider a work of caution against “recreational” weight loss, especially in individuals of normal weight.

@DrSharma
Edmonton, AB

ResearchBlogging.orgDulloo AG, Jacquet J, Montani JP, & Schutz Y (2015). How dieting makes the lean fatter: from a perspective of body composition autoregulation through adipostats and proteinstats awaiting discovery. Obesity reviews : an official journal of the International Association for the Study of Obesity, 16 Suppl 1, 25-35 PMID: 25614201

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Did Dieting Make You Fat? Blame Your ‘Proteinstat’

Skeletal muscle

Skeletal muscle

Yesterday, I posted on the intriguing finding (now documented in 15 prospective studies) that dieting can make you fat – especially if you start out with a normal weight.

In the paper by Dulloo and colleagues published in Obesity Reviews, the authors attribute part of this effect to the so far elusive “proteinstat” – a system, similar but different from the “adipostat” – that is designed to protect your lean body mass.

As the paper nicely delineates, the problem with post-dieting weight regain is that the fat comes back first but that the drive to eat does not cease till you have also regained the lost lean body mass (muscle).

It appears as though there are two complimentary biological systems that regulate weight regain.

The better known system is the “adipostat” that worries about protecting and restoring fat mass – the neuroendocrine players include leptin and perhaps other signals derived from fat tissue that signal fat stores to the brain. This system works (primarily through dropping metabolic rate but also through effects on appetite) to very quickly and effectively restore the depleted fat mass after dieting.

The less known system is the “proteinstat”, that apparenty worries about restoring lean body mass. The system works slower than the “adipostat” but continues its activity (often reaching its peak) even after all the lost fat has been regained and you are back to your original weight. In fact, it continuous working (primarily through appetite and cravings) till lean body mass is restored, even if this means gaining even more fat in the process.

In their careful reanalysis of starvation studies, Dulloo and colleagues also come up with an explanation why this process of “weight overshoot” results in more gain the skinnier the individual is to begin with.

“…the lower the initial adiposity, the greater the proportion of energy mobilized as body protein (referred to as P-ratio) during weight loss. The steep part of the negative exponential curve lies between 8–20% body fat, and a shift from the upper to the lower values in this range, generally considered to reflect a ‘normal’range of adiposity for men living in affluent societies, results in 2.5- to 3-fold increase in the P-ratio; the latter constitutes a proxy of the fraction of weight that is lost as FFM since protein belongs to the FFM compartment. This extremely high sensitivity of the P-ratio with regard to the initial body composition emphasizes the critical importance of even small differences in the initial percentage body fat in dictating the individual’s energy-partitioning characteristic and, hence, the pattern of lean and fat tissue deposition during weight loss and subsequent
weight regain, in turn, determining the extent of fat overshooting.”

In other words, lean dieters are far more susceptible to mobilising energy (and thus losing mass) from their muscle than from their fat stores, resulting in a much greater likelihood of overshooting their original weight.

Eventually, as these dieters get fatter with every diet cycle, they get less and less susceptible to this effect, which matches well with the finding that dieting is a far better predictor of long-term weight gain in people with lower fat percentages than in those who already have overweight or obesity).

As for exactly how the “proteinstat” works, much remains unclear. Early work focussed on the notion that certain amino acids may serve as signals of protein stores, however, now work is focussing on the far more plausible theory that some of the over 100 molecules now known to be secreted by skeletal muscle (myokines) may play a role in this system.

Certainly a topic that will be interesting to watch develop over the coming years.

@DrSharma
Calgary, AB

ResearchBlogging.orgDulloo AG, Jacquet J, Montani JP, & Schutz Y (2015). How dieting makes the lean fatter: from a perspective of body composition autoregulation through adipostats and proteinstats awaiting discovery. Obesity reviews : an official journal of the International Association for the Study of Obesity, 16 Suppl 1, 25-35 PMID: 25614201

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