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HomeA Focus'Protective effect of obesity' against dementia challenged in 2 studies

'Protective effect of obesity' against dementia challenged in 2 studies

ObeseTwo new studies appear to refute some earlier research suggesting obesity might protect against dementia. A large UK study over 15 years found that long-term, being obese was associated with increased risks of dementia. The second study that linked weight loss over time and an eventual dementia diagnosis, points to the symptoms of developing dementia contributing to weight loss.

Healthy older adults who have been obese for years may be at higher risk of developing dementia than their peers who aren’t overweight, Reuters Health reports research from the UK suggests.

The study team followed two groups of dementia-free adults aged 65 to 74 years for up to 15 years. One group, considered healthy, included 257,523 non-smokers who didn’t have cancer, heart failure or multiple chronic health problems; another group of 161,927 adults, deemed unhealthy, did smoke or have serious chronic medical issues.

Over the first decade of the study, healthy people who were obese or overweight were less likely to develop dementia than healthy people at a normal weight, the study found. But after that, obesity was associated with a 17% higher risk of dementia and being heavier no longer appeared to be protective.

“When we looked long-term, being obese was definitely associated with increased risks of dementia,” said senior study author David Melzer of the University of Exeter in the UK in the report. People with obesity often have other health problems like diabetes and high blood pressure that can independently increase the risk of dementia, previous research has found. But results regarding the connection between obesity and dementia have been mixed, with some previous studies suggesting that this excess weight might actually be protective.

The report says in the current study, 9,774 people in the “healthy” group were diagnosed with dementia. Slightly more than half of the dementia patients had lost at least 2.5kg during the decade prior to their diagnosis. Weight loss prior to the dementia diagnosis might mask the connection between obesity and cognitive decline, Melzer is quoted in the report as saying.

Alzheimer’s disease, the main cause of dementia, can develop slowly over up to 20 years before people get diagnosed, Melzer noted. “The same is true of damage to the arteries in the brain, which also contributes to dementia,” Melzer said. “This slow development of dementia makes it difficult to separate real risk factors from the effects of the disease.”

Interestingly, obesity was associated with a lower short-term and long-term risk of dementia for the unhealthy group in the study, the report says. A total of 6,070 individuals in the unhealthy group developed dementia.

“In general, losing weight, being more physically active, and getting blood pressures and cholesterol levels under control should make a big difference for dementia risk, plus risks of diabetes and heart disease,” Melzer said.

The report says the study wasn’t designed to prove whether or how obesity might directly cause dementia in later years. Another limitation is that researchers lacked data to examine the connection between obesity and specific forms of dementia like Alzheimer’s disease, the authors note.

The report says in a separate study in the same journal, researchers led by Alexander Allen of the London School of Hygiene and Tropical Medicine also examined the connection between overweight and dementia, and also cast doubt on the idea that obesity is protective. The researchers analysed the link between excess belly fat in middle age and the risk of death from dementia over the next 40 years in about 19,000 male civil servants participating in a long-term health study. They found that weight loss over 30 years, starting in middle age, was associated with an increased risk of dementia in old age. Having excess fat in old age, however, was tied to a lower risk of dementia.

While that may appear to suggest a protective effect of extra weight, in fact, the strongest connection, between weight loss over time and an eventual dementia diagnosis, points to the symptoms of developing dementia contributing to the weight loss, Allen and colleagues are quoted in the report as saying. “These effects may reflect changes in appetite or other aspects of behavior that result in reduced energy intake,” they note. “Thus, claims from previous studies that underweight increases the risk of dementia may be an artefact of the effects of reverse causality.”

The report says the Allen didn’t respond to requests for comment. “Regular weight checks could provide an easily measured marker for risk of frailty and subsequent detection of dementia,” Allen and colleagues write. “Whether this could allow early interventions to improve dementia outcomes could also merit further investigation.”

Abstract 1
Background: Overweight or obesity at ages <65 years associates with increased dementia incidence, but at ≥65 years estimates are paradoxical. Weight loss before dementia diagnosis, plus smoking and diseases causing weight loss may confound associations.
Objective: To estimate weight loss before dementia diagnosis, plus short and longer-term body mass index associations with incident dementia in 65–74 year olds within primary care populations in England.
Methods: We studied dementia diagnosis free subjects: 257,523 non-smokers without baseline cancer, heart failure or multi-morbidity (group A) plus 161,927 with these confounders (group B), followed ≤14.9 years. Competing hazard models accounted for mortality.
Results: In group A, 9,774 were diagnosed with dementia and in those with repeat weight measures, 54% lost ≥2.5 kg during 10 years pre-diagnosis. During <10 years obesity (≥30.0 kg/m2) or overweight (25.0 to <30.0) were inversely associated with incident dementia (versus 22.5 to <25.0). However, from 10 to 14.9 years, obesity was associated with increased dementia incidence (hazard ratio [HR] 1.17; 95% CI: 1.03–1.32). Overweight protective associations disappeared in longer-term analyses (HR, 1.01; 95% CI: 0.90–1.13). In group B, (n = 6,070 with incident dementia), obesity was associated with lower dementia risks in the short and longer-term.
Conclusions: In 65–74 year olds (free of smoking, cancer, heart failure or multi-morbidity at baseline) obesity associates with higher longer-term incidence of dementia. Paradoxical associations were present short-term and in those with likely confounders. Reports of protective effects of obesity or overweight on dementia risk in older groups may reflect biases, especially weight loss before dementia diagnosis.

Kirsty Bowman, Madhav Thambisetty, George A Kuchel, Luigi Ferrucci, David Melzer

Abstract 2
Aims and objectives: To examine the hypothesis that obesity is protective for dementia, we compared the associations of death from dementia with body weight and body mass index (BMI) in both middle and old age.
Design: Height and weight were measured in a prospective study of 19,019 middle-aged men in the Whitehall study in 1967–70 and in 6,158 surviving participants at resurvey in 1997. Cox regression was used to examine the associations of death from dementia over a 40-year period with weight or BMI measured by health professionals in middle and old age adjusting for age, smoking habits, employment grade and marital status.
Setting: Central government employees in London, UK.
Main outcomes measure: Feath due to dementia in 320 participants.
Results: Body weight measured in middle age was weakly inversely associated with death from dementia (hazard ratio 0.98 [95%CI: 0.97–0.99] per kg), but neither height nor BMI were related to risk of dementia. In contrast, body weight in old age was more strongly inversely related to deaths from dementia (0.96; [0.95–0.98] per kg) as was BMI (0.92 [0.86–0.97] per kg/m2). Weight loss over the 30 years between baseline and resurvey was associated with a higher risk of death from dementia, with an adjusted HR per kg/30 years of 1.04 [95%CI: 1.02–1.08] and the association with loss of BMI was even stronger (adjusted HR of 1.10 [1.03–1.19]) per kg/m2 decrease.
Conclusions: The stronger inverse associations of deaths from dementia with BMI in old age, compared with middle age, together with strong positive associations of loss of BMI or body weight between middle and old age casts doubt on previous suggestions that obesity protects against death from dementia.

Alexander N Allen, Robert Clarke, Martin Shipley, David A Leon

[link url=""]Reuters Health material[/link]
[link url=""]Age and Ageing abstract 1[/link]
[link url=""]Age and Ageing abstract 2[/link]

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