Young hunter hypothesis: Why gaining weight with age increases longevity

The Daily Mail UK recently reported on a paper published in the journal Medical Hypotheses. Endocrinologists from Israel proposed an interesting hypotheses that JFS readers may find intriguing. Called the 'young hunter hypothesis,' it is supported by the clinical research to date on set-point and the natural weight gain during healthy aging seen among humans throughout history. As the researchers explain, the weight and fat gained in adulthood, particularly around the abdomen, has evolutionary roots and is a major driving force behind human survival and longevity.

They opened explaining how set-point biologically determines our weights, which are largely beyond our control, regardless of what we eat or do:

According to the set-point hypothesis, the brain continuously adjusts our metabolism and subconsciously manipulates our behavior with an extraordinary degree of control, in order to maintain a target weight. Thus, despite a wide variation in food intake and energy expenditure, the weight of the individual remains relatively stable.

They referenced the renowned research of doctors Michael Rosenbaum, Rudolph Leibel and Jules Hirsch at the Laboratory of Human Behavior and Metabolism, Rockefeller University in New York. Rigorous clinical tests at Rockefeller found that at our natural body weights, fat and lean people eat and expend the same amount of calories per unit of lean body mass. For a naturally fat person to try and maintain a lower body weight than is natural for his/her, however, it means continual hunger and eating fewer calories than someone who is naturally of a lower weight, because of decreased energy expenditure. And it’s equally hard to maintain a weight higher than is natural, for both lean and fat people, because of the increased energy expenditure that occurs. Physiology acts to maintain whatever weight range is natural for each of us.

The Rockefeller research found that an average non-obese adult eats about 900,000 kcal of food each year and that the typical 20 or so pounds of weight gained between the ages of 25 and 55 represents a remarkably small net ‘imbalance’ between calories in and calories expended — about 0.3%. This weight control, despite wide fluctuations in daily diet and activity, is beyond anyone’s conscious control, they found, but the result of intricate biological mechanisms.

It is not diet or behavior that is the key determinant of obesity, the Rockefeller University researchers explained. Despite intense research in dietary composition, they added, there is no evidence that the proportions of protein, carbohydrates and fat determine obesity or affect weight or body composition, either.

The adult years are characterized by a gradual and persistent physiologic increase in body weight, numerous researchers have observed. As the Israeli doctors noted, the typical American adult gains 3–5 kg [6.6-11 pounds] per decade beyond the age of 20 years, which translates into about 10–15 kg [22-33 pounds] between the third to fifth decades. They believe that this “age-related weight gain evolved as a protective means to ensure sufficient energy stores for basic metabolic needs during late adulthood, when energy consuming activities are curtailed due to age-dependent muscle mass loss. Thus, age-related weight gain is a major driving force of human longevity and perpetuity.” The added that adipose tissue into older age also guards against times of famine.

It makes evolutionary sense. They argued that in ancient times, young hunters needed muscles for hunting but as they aged and were less able to hunt, their bodies converted muscles into fat stores to enable non-hunting older men to be more able to survive into old age. In fact, they said, aging is associated with profound changes in body composition, particularly about a 15% decrease in muscle mass after age 30, and insulin resistance which allows fuel to be deposited as adipose tissue. Rather than being pathological or blamed on modern life, they said these are natural changes with evolutionary roots to help ensure longer life until children are fully mature and able to hunt and become independent.

Given the reproductive cycle, they added, the last born would complete training around a parental age of 45-50, just as the weight gain curve begins to naturally flatten out. “Long term epidemiological studies show that there is no substantial gain after this age.”

The Israeli hypothesis supports that weight gain with age may be a biological way to guarantee survival and longevity, even today, they wrote. As we’ve seen, that’s exactly what the science continues to show.

© 2007 Sandy Szwarc