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For nearly three-quarters of obese patients with fibromyalgia, drastic caloric restriction in a small trial led to 30% or greater reduction in symptom severity in only 3 weeks, researchers reported.

The findings support an emerging hypothesis that caloric restriction has its own analgesic effect independent of weight loss, according to Aaron Stubbs, MD, of Michigan Medicine in Ann Arbor, and colleagues .

"Although a larger study with a control group would be the next step in investigating this association, this provides important information for clinicians who counsel patients on alternatives to pharmacologic treatments for pain and other somatic symptoms," the researchers concluded.

A connection between obesity and chronic pain has been well documented, with indicating a 60% increase in prevalence of persistent pain in individuals with obesity compared with the non-obese. That has led to a number of studies seeking to determine whether weight loss would relieve pain in such individuals.

Among these was a Michigan Medicine study with many of the same authors that examined a very low-calorie diet for obese people with pain (not specifically fibromyalgia), finding significant improvement after 12 weeks. Although pain relief was tied to weight loss in that analysis, the relief "was not confined to weight-bearing joints," Stubbs and colleagues noted. That observation suggested that "the benefits seemingly conferred by weight loss cannot be attributed to a reduction in mechanical stress on weight-bearing joints alone; rather, it seems likely that at least a subset of patients experience a global reduction in pain sensitivity."

And if that's the case, then perhaps a low-calorie diet would show benefit long before patients begin to lose significant weight -- hence the current study.

Stubbs and co-authors drew on a weight management program running at the University of Michigan for people with obesity, identifying 195 participants who scored at least 4 on (FSC) at baseline. This score represents at least moderate degrees of pain and pain-related problems. These individuals agreed to a liquid meal replacement diet supplying no more than 800 kcal/day, which was supposed to last 12 weeks. The primary analysis was conducted after 3 weeks, with secondary outcomes assessed in 119 who stayed the 12-week course.

Mean body mass index at baseline was 41.5 (SD 5.2); patients' mean age was 45, and 74% were women. About half also had depressive symptoms.

At week 3, participants had lost an average of about 5.6% of body weight, with mean FSC scores falling from 8.39 at baseline to 4.78 across all participants. Symptom reductions of at least 30% were seen in 72%. There was an ever-so-slight trend toward greater weight loss in patients with very marked improvement (6.0%) and those with moderate improvement (5.5%) compared with those showing little to no improvement (5.2%), which the investigators dismissed as trivial.

Indeed, while weight loss continued over the full 12 weeks, reaching a mean 15% of body weight, further pain relief was not observed among those showing at least moderate improvement at week 3. (Average FSC scores did continue to decline through week 8 in those with little to no initial improvement, dipping below 5.0, and all groups showed a rebound in mean scores after week 8.)

Stubbs and colleagues speculated on potential mechanisms to explain the rapid onset of pain relief. One candidate is that caloric restriction inhibits various inflammatory mediators. Previous studies have found effects on interleukin-10, C-reactive protein, lipopolysaccharide binding protein, and inhibitory factor κB-α, although not consistently. Alternatively, Stubbs' group suggested, caloric restriction might relieve pain "through central alteration of neurotransmission" via action on, for example, neuropeptide Y or opioid receptors.

The study did come with one major limitation, the researchers said: the lack of a control group. "Clearly, randomized controlled clinical trials will be necessary to confirm these results," the team wrote. Stubbs and colleagues also acknowledged that no validated thresholds have been established for FSC score changes that could be considered clinically significant.

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