Why the Low Weight Group Exhibits Worse Outcomes in Some Epidemiological Studies
Today's open access paper outlines results from an epidemiological study, and is a fairly standard examination of the relationship between being overweight and outcomes such as increased risk of age-related disease and mortality. Being a modern study, it uses waist circumference rather than body mass index, as the most problematic fat tissue held by overweight individuals is the visceral fat in the abdomen. Visceral fat tissue is metabolically active, generating chronic inflammation via a range of mechanisms that include mimicking the signaling of infected cells, generating more senescent cells, and an increased burden of cell debris that triggers an innate immune reaction. This chronic inflammation in turn accelerates all of the common pathologies of aging.
Despite all of the evidence for visceral fat to be a bad thing, the low weight tertile often performs more poorly than one might expect in this sort of epidemiological study. The current consensus on the reasons for this outcome is that the lowest weight tertile includes people who have serious chronic issues and an outsized risk of mortality and disease. That is why they do not put on weight like most of the rest of the population. In most epidemiological databases it is somewhere between hard to impossible to distinguish exactly why it is that someone is of normal to low normal weight and thus exclude the problematic portions of the study population.
The present study focused on the association of changes in waist circumference (WC), waist-to-height ratio (WHtR), and weight-adjusted-waist index (WWI) with multimorbidity among older Chinese adults. Our results showed that rising changes in WC, WHtR, and WWI were associated with an increased risk of multimorbidity. Compared with participants in the persistently low group of WC, WHtR, those in the gain group and the persistently high group of WC, WHtR had significantly higher multimorbidity risk. Moreover, compared with the persistently low WWI group, the WWI loss group was correlated with a lower risk of multimorbidity.
The association between adiposity and multimorbidity has been extensively explored in epidemiological studies. Although studies have consistently demonstrated that obesity is positively associated with multimorbidity, the studies on the relationship between obesity indicators change and multimorbidity are limited and still controversial.
In this study, we first explored the association between WC, WHtR, and WWI change patterns and multimorbidity. WC and WHtR are considered to be important anthropometric indicators of abdominal obesity. Previous studies have suggested that WC and WHtR can reflect body fat percentage accurately and play an important role in predicting some chronic diseases, such as cardiovascular disease and metabolic syndrome. The pathway may explain that abdominal obesity significantly increased plasma triglycerides, low density lipoproteins, and very low density lipoproteins, which have been shown to increase the risk of adverse outcomes such as cardiovascular disease, diabetes, hypertension, and kidney diseases. In addition, people with abdominal obesity tend to have excess visceral fat, which can lead to high doses of adipokines from the portal vein to the liver and other body tissues, causing a variety of chronic diseases. Previous studies have highlighted that visceral adipose tissue produces large amounts of interleukin-6 (IL-6), which promotes the secretion of acute-phase proteins such as C-reactive protein (CRP), and thus the levels of IL-6 and CRP are significantly increased in individuals with abdominal obesity.
Therefore, WWI loss may reduce inflammation and thus the risk of multimorbidity, which could reasonably explain our results. Moreover, the persistently low WWI group included those who had always been underweight, and underweight older adults were prone to malnutrition, which is associated with some noninfectious chronic diseases. This may explain why the risk of multimorbidity was higher in the persistently low WWI group than in the WWI loss group.
From my research a larger waist circumference and body fat up to a point is protective of death but not co-morbidities.
When old, the fat as a metabolic reserve keeps you alive in health emergencies but not necessarily optimally healthy.