Considering that obesity is a disease of energy imbalance that promotes storage over expenditure, it is not surprising that treatment revolves around reversing this equation to promote weight loss. However, it is questionable whether weight loss is sufficient to reverse all metabolic changes induced by obesity.

A great example of this is the Look AHEAD trial, in which over 5000 overweight-obese patients with type-2 diabetes were followed for an average of 10 years while undergoing an intensive lifestyle intervention that promoted weight loss through a decreased caloric intake and increased physical activity. Despite losing significantly more weight than a control group, resulting in greater reductions in all nearly all cardiovascular risk factors, there was no difference between the two groups in cardiovascular morbidity and mortality.

Another related issue is that formerly obese individuals are prone to regaining their lost weight, leading to a rebound of cardiometabolic risk factors. Anyone can cut calories for a couple months and drop the fat, but keeping the weight off is where the struggle occurs. This is the sole reason for why the National Weight Control Registry was formed – to determine the commonalities among individuals who do successfully maintain weight loss.

This begs the question, why has research into the biological mechanisms surrounding weight regain not been conducted? Well, it has. The issue is the selection bias of choosing already obese patients. Genetic, epigenetic, and behavioral predispositions that led to obesity in the first place may influence weight regain and be responsible for some of the differences observed between formerly obese individuals.

Fortunately, Schmitz et al from the Max Planck Institute for Metabolism Research and the University of Leipzig, Germany circumvented this issue by analyzing the effect of caloric restriction in diet-induced obese mice that were genetically identical (male wild type C57BL/6 mice). Don’t worry, the rodent data was supplemented by a related experiment in morbidly obese humans undergoing bariatric surgery.

Making mice fat and skinny

The experiment in mice lasted 32 weeks (fig 1). When the mice were 4 weeks old, they were fed a normal low-fat diet (LFD) or a high-fat diet (HFD) designed to induce obesity for 18 weeks. Some of the HFD mice were then subjected to a 40% caloric restriction diet that maintained the vitamin and mineral and amino acid content of the other diets for 6 weeks. Thus, these mice were only missing out on calories and not nutrients. Finally, after the 6 weeks of caloric restriction, this group of mice was provided unrestricted access to an LFD or pair-fed to the LFD control group (meaning that they were provided a rationed amount of food equivalent to the amount eaten by the LFD control group).

We thus have 4 experimental groups: mice that consumed an LFD throughout the 32 weeks (LFD control), mice that consumed an HFD only (HFD), mice that were weight-reduced and fed an unrestricted LFD diet (WR ad lib), and mice that were WR and pair-fed to the LFD control group (WR pair). We also have two timepoints for comparisons to make: after weight loss via caloric restriction (TP1; week 28) and after the weight maintenance phase (TP2; week 36).

As expected, the HFD established severe obesity, leading to a 50% increase in bodyweight, 4-fold increase in body fat, and a slight increase in lean mass compared to the lean, normal LFD-fed control mice (fig 2). Additionally, leptin was significantly increased with the HFD, indicating an establishment of leptin resistance commonly seen among obese humans. However, these parameters normalized after a caloric restriction at TP1.

Bodyweight, body fat, and leptin remained similar to the control group in the WR pair-fed mice (fig 3). However, the WR ad lib mice displayed a significant increase in bodyweight and body fat mass, as well as a trend for increased leptin concentrations.

It turns out that the WR ad lib mice showed a daily increase in caloric intake of up to 16% compared to the LFD control group, and this increase persisted throughout the entire 8-week period between TP1 and TP2 (fig 4). Thus, hyperphagia may be the primary cause for weight regain after weight loss. Interestingly, changes in the mRNA expression of established hypothalamic regulators of food intake, namely Proopiomelanocortin (Pomc) and Agouti-related peptide (Agrp), were not different between groups at TP2, suggesting that these molecular drivers of food intake were not the primary reason for observed hyperphagia.

To address the other side of the energy balance equation, the researchers also investigated energy expenditure among the mice (fig 5). At TP1, the WR mice showed an 11% decrease in total energy expenditure (TEE) during the light (inactive; sleeping) phase when compared to the LFD control group, consistent with the reduction in resting energy expenditure observed in humans after dieting and weight loss (the so-called metabolic adaptation). However, this reduction did not persist after refeeding in either the WR group. There was also no difference in TEE during the dark (active) phase between the groups at TP1 or TP2.

Looking at activity levels, whereas HFD fed mice showed a significant reduction in physical activity throughout the experiment, the WR pair and WR ad lib mice displayed a similar level of activity compared to LFD controls. These results further confirm the pivotal role of hyperphagia in weight regain.

Glucose homeostasis

Supporting the known risk obesity imposes for type-2 diabetes in humans, HFD feeding significantly impaired glucose tolerance (fig 6). However, WR went above and beyond restoring glucose tolerance and actually significantly increased it compared to the LFD control mice. Moreover, this marked improvement was maintained throughout refeeding in both WR pair and WR ad lib.

In contrast to glucose tolerance, systemic insulin sensitivity remained comparable to the HFD mice after WR. However, whereas insulin sensitivity of WR ad lib mice was similar to that of HFD-fed mice after refeeding (TP2), insulin sensitivity was improved in WR pair mice to the level of the LFD group. Thus, although glucose tolerance improves rapidly upon weight loss, the improvement of systemic insulin sensitivity requires long-term maintenance of weight reduction.

To further investigate the observed discrepancy between systemic glucose tolerance and systemic insulin sensitivity, the researchers measured fasting insulin concentrations and glucose-stimulated insulin secretion (GSIS) to test whether the dynamic regulation of insulin release might be altered (fig 7). Whereas the HFD led to a significant increase in fasting insulin, WR normalized it and refeeding did not affect it.

Compared to LFD control mice, HFD-fed mice showed significant increases in insulin secretion during the GSIS test at both TP1 and TP2, consistent with the compensatory hyperinsulinemia that accompanies massive insulin resistance on the road to type-2 diabetes. This abnormality persisted in the WR mice at TP1 and only showed a modest improvement at TP2. Notably, the HFD, WR pair, and WR ad lib mice all showed significantly increased pancreatic beta-cell mass after re-feeding when compared to the LFD mice (fig 8).

Collectively, all the above support human data showing that diet-induced obesity leads to systemic insulin resistance and compensatory hyperinsulinemia that result in increased pancreatic beta-cell mass. This study adds to the picture by further showing that increased glucose tolerance associated with weight loss is likely the result of continued hyperinsulinemia, and that weight loss must be maintained for systemic insulin sensitivity to begin to show improvement. Even then, the pancreatic beta-cells are still enlarged, making hyperinsulinemia a persistent metabolic trait of being formerly obese.

Tissue-specific insulin resistance and inflammation

The researchers went on to assess activation of the insulin signaling cascade in liver and white adipose tissue (WAT) in the different groups of mice, as these two organs are at the heart of type-2 diabetes pathophysiology.

Insulin signaling was all but completely abolished in both the liver and WAT with the HFD (fig 9). Hepatic insulin sensitivity was significantly increased with pair-feeding after WR, no longer being different from the LFD control group. However, the WR ad lib mice still displayed a degree of impairment. In contrast to both these results in the liver, both WR mice groups showed an increase in WAT insulin signaling compared to the HFD, but this was still significantly less than that of the LFD control mice.

Obesity and insulin resistance are associated with the development of a chronic, low-grade inflammatory state, leading the researchers to next explore fat cell morphology and macrophage infiltration (a pro-inflammatory sign). Both the HFD and WR mice showed a 20-fold elevation in crown-like structures indicative of inflammation compared to the LFD control mice (fig 10). Moreover, a return to normal levels was not achieved after the weight maintenance period in either the WR pair or WR ad lib mice.

HFD-feeding robustly induced expression of several pro-inflammatory genes in the liver and WAT compared to LFD-feeding, indicating the development of obesity-associated inflammation in these organs. Interestingly, whereas WR efficiently reduced expression of the liver-genes, inflammatory gene expression was unaltered in WAT of WR mice compared to HFD-fed animals. Moreover, this WAT inflammatory gene expression persisted throughout TP2, suggesting that obesity-associated inflammatory gene expression changes were largely irreversible in WAT but not in the liver.

Corroborating with humans

In order to verify the rodent findings in humans, the researchers recruited 55 morbidly obese individuals (baseline BMI of 53.6) who underwent bariatric surgery and followed them for 12-months post-operation. All participants demonstrated marked weight loss and improvements in insulin sensitivity and inflammation (figure 11).

However, 23 of the 55 participants did not show a significant change in macrophage infiltration of their fat tissue. Accordingly, this group of individuals displayed unaltered inflammatory gene expression in their fat cells, consistent with the data in mice. By contrast, the other 32 participants who did show a reduction in macrophage infiltration also showed a reduction in inflammatory gene expression.

This inflammation appeared to potentiate insulin resistance within the fat cells, as insulin-stimulated glucose uptake in isolated inflammatory fat cells increased only by 37% following weight loss, compared to a ∼115% in the noninflammatory fat cells (fig 12).

This distinction in obese individuals, who either respond with or without a reduction of inflammatory fat cells to extensive weight, suggests that a substantial proportion of obese human patients display a striking dichotomy between hepatic and fat cell inflammation and insulin sensitivity upon long-term weight reduction. This confirms the rodent data and also suggests that there are additional factors that regulate adipose tissue inflammation in humans compared to in mice (as not all the obese humans had this problem). This could be where genetics or other environmental factors come into play.

Obesity is not 100% reversible

If this study has you take away one key point, it is that preventing obesity is critical. Once you become obese, you can lose weight and regain a large portion of your health in the process. However, you must then fight against hyperphagia for who knows how long in order to prevent weight regain. If you make it past that, then you are in a gamble as to whether the inflammation within your fat cells disappears or stays. If the former, then you may be out of the woods. If the latter, then it is quite possible that the struggle to keep the weight off will be a lifelong battle.

Let me reiterate that. This study identified a subgroup of extremely obese and insulin-resistant individuals (∼42%), in whom extensive weight loss and significant improvements of insulin sensitivity after bariatric surgery were not associated with a significant reduction in fat cell inflammation. That said, the robust insulin resistance observed within the fat cells was not observed on a systemic level. Thus, future studies will have to investigate the long-term physiological consequences distinguishing both subgroups of patients.

So clearly weight loss when obese solves a lot of problems… just not all of them.