One of the things that turns up in large sets of data on weight and mortality - by which I really mean amount of fat tissue and mortality - is that both maintaining excess fat tissue and later the loss of that fat tissue are associated with increased mortality. This is because visceral fat tissue causes chronic inflammation and other forms of metabolic dysregulation. The more of it you have, the worse off you are over the long term: it is actively causing harm that accumulates to significantly raise the risk of all of the common age-related disease. Later in life, the progression and treatment of many of these age-related conditions, such as cancer, are accompanied by involuntary weight loss. There are many reasons for this ranging from simple loss of appetite to disease mechanisms that impact the normal operation of metabolism in pathological ways. If you pick out a group of people who are sharply losing weight, especially older people, the mortality rate for that group will tend to be higher than for those who maintain their weight. This is because the losing group contains a larger number of individuals who are suffering the later stages of age-related disease.
This does not mean, as some have said in the past, that it is good to be overweight. You can't lump this data together and make that claim. Involuntary weight loss is so very joined at the hip to high mortality risk that it distorts the picture, and most of the good data sources for large numbers of people make no distinction as to how or why weight changes occur. Any number of people in the world want to be told that is is fine to be overweight and nothing bad is going to happen as a result: there is always a market for comforting lies. Even a moderate level of excess fat tissue has a significant impact on the future risk of incurring all of the common age-related diseases, however. If you want the best odds of living a healthy life for as long as possible, then don't allow yourself to become fat. It is a choice, and one that you can avoid or reverse with sufficient exercise of willpower.
Unlike involuntary weight loss, deliberately setting out to lose your excess fat tissue is a good thing and produces benefits. You are cutting out a source of damage to your health, and that makes a difference over the long-term to your mortality risk. That shows up in epidemiological data, as demonstrated here.
Advanced age and obesity are risk factors for disability, morbidity, and mortality. Weight loss interventions in overweight and obese older adults positively affect several strong risk factors for mortality. Yet, many observational studies in middle-aged and older adults report an association between weight loss and increased mortality. Difficulty reconciling these contradictory findings (the so-called "obesity paradox"), coupled with the strong negative prognostic implication of rapid involuntary weight loss with advanced age, has led to a reluctance to recommend weight loss in older adults. Attempts to refine observational analyses to avoid confounding (i.e. distinguishing between intentional and unintentional weight loss, and restricting populations to those without co-morbid conditions or non-smokers) typically reveal no increase, and perhaps some decrease, in mortality risk with intentional weight loss.
Although results from a randomized controlled trial (RCT) of weight loss would theoretically resolve these issues, such a trial would require a large sample size over a long duration to detect clinically meaningful differences in mortality. In light of the high prevalence of obesity, its negative impact on health and quality of life, and the discrepancy between the proven risk factor improvements of short-term intentional weight loss and the inverse association of weight loss with increased all-cause mortality frequently seen in observational studies, we conducted a meta-analysis to estimate the effect of interventions which included intentional weight loss on all-cause mortality in overweight and obese adults. We hypothesized that intentional weight loss would be associated with reduced all-cause mortality. Further, as weight loss in older persons is a cause of clinical concern that may lead health care providers to recommend against weight loss for obese, older adults, we sought to examine the effects in a subset of trials with a mean baseline age of at least 55 years.
Trials enrolled 17,186 participants (53% female, mean age at randomization = 52 years). Mean body mass indices ranged from 30-46 kg/m2, follow-up times ranged from 18 months to 12.6 years (mean: 27 months), and average weight loss in reported trials was 5.5±4.0 kg. A total of 264 deaths were reported in weight loss groups and 310 in non-weight loss groups. The weight loss groups experienced a 15% lower all-cause mortality risk. There was no evidence for heterogeneity of effect.