Bio-thermodynamics and aging

I suspect nature is constrained by basic physics/chemistry/thermodynamics in a variety of interesting ways.

For example, chemical reactions (and thus biological processes) proceed more quickly when they are warm than cold–this is pretty much a tautology, since temperature=movement–and thus it seems reasonable to expect certain biological processes to proceed more slowly in colder places/seasons than in warmer ones.

The Greenland Shark, which lives in very cold waters, lives to be about 300-500 years old. It’s no coincidence:

Temperature is a basic and essential property of any physical system, including living systems. Even modest variations in temperature can have profound effects on organisms, and it has long been thought that as metabolism increases at higher temperatures so should rates of ageing. Here, we review the literature on how temperature affects longevity, ageing and life history traits. From poikilotherms to homeotherms, there is a clear trend for lower temperature being associated with longer lifespans both in wild populations and in laboratory conditions. Many life-extending manipulations in rodents, such as caloric restriction, also decrease core body temperature.

This implies, in turn, that people (or animals) who overeat will tend to die younger, not necessarily due to any particular effects of having extra lumps of fat around, but because they burn hotter and thus faster.

Weighing more may trigger certain physiological changes–like menarchy–to begin earlier due to the beneficial presence of fat–you don’t want to menstruate if you don’t have at least a little weight to spare–which may in turn speed up certain other parts of aging, but there could be an additional effect on aging just from the presence of more cells in the body, each requiring additional metabolic processes to maintain.

Increased human height (due to better nutrition) over the past century could have a similar effect–shorter men do seem to live longer than taller men, eg: 

Observational study of 8,003 American men of Japanese ancestry from the Honolulu Heart Program/Honolulu-Asia Aging Study (HHP/HAAS), a genetically and culturally homogeneous cohort followed for over 40 years. …

A positive association was found between baseline height and all-cause mortality (RR = 1.007; 95% CI 1.003–1.011; P = 0.002) over the follow-up period. Adjustments for possible confounding variables reduced this association only slightly (RR = 1.006; 95% CI 1.002–1.010; P = 0.007). In addition, height was positively associated with all cancer mortality and mortality from cancer unrelated to smoking. A Cox regression model with time-dependent covariates showed that relative risk for baseline height on mortality increased as the population aged. Comparison of genotypes of a longevity-associated single nucleotide polymorphism in FOXO3 showed that the longevity allele was inversely associated with height. This finding was consistent with prior findings in model organisms of aging. Height was also positively associated with fasting blood insulin level, a risk factor for mortality. Regression analysis of fasting insulin level (mIU/L) on height (cm) adjusting for the age both data were collected yielded a regression coefficient of 0.26 (95% CI 0.10–0.42; P = 0.001).

The more of you there is, the more of you there is to age.

Interesting: lots of data on human height.

But there’s another possibility involving internal temperature–since internal body temperature requires calories to maintain, people who “run hot” (that is, are naturally warmer) may burn more calories and tend to be thinner than people who tend to run cool, who may burn fewer calories and thus tend to weigh more. Eg, low body temperature linked to obesity in new study: 

A new study has found that obese people (BMI >30) have lower body temperature during the day than normal weight people. The obese people had an average body temperature that was .63 degrees F cooler than normal weight people. The researchers calculated that this lower body temperature—which reflects a lower metabolic rate—would result in a body fat accumulation of approximately 160 grams per month, or four to five pounds a year, enough for the creeping weight gain many people experience.

There’s an interesting discussion in the link on thyroid issues that cause people to run cold and thus gain weight, and how some people lose weight with thyroid treatment.

On the other hand, this study found the opposite, and maybe the whole thing just washes out to women and men having different internal temperatures?

Obese people are–according to one study–more likely to suffer mood or mental disorders, which could also be triggered by an underlying health problem. They also suffer faster functional decline in old age:

Women had a higher prevalence of reported functional decline than men at the upper range of BMI categories (31.4% vs 14.3% for BMI > or =40). Women (odds ratio (OR) = 2.61, 95% confidence interval (CI) = 1.39-4.95) and men (OR = 3.32, 95% CI = 1.29-8.46) exhibited increased risk for any functional decline at BMI of 35 or greater. Weight loss of 10 pounds and weight gain of 20 pounds were also risk factors for any functional decline.

Note that gaining weight and losing weight were also related to decline, probably due to health problems that caused the weight fluctuations in the first place.

Of course, general physical decline and mental decline go hand-in-hand. Whether obesity causes declining health, declining health causes obesity, or some underlying third factor, like biological aging underlies both, I don’t know.

Anyway, I know this thought is a bit disjointed; it’s mostly just food for thought.