Differential effects of intermittent feeding and voluntary exercise on body weight and lifespan in adult rats. Goodrick CL, Ingram DK, Reynolds MA, Freeman JR, Cider NL. J Gerontol. 1983 Jan;38(1):36-45.
Male wistar rats were housed in laboratory cages or activity-wheel cages at eight 10.5 or 18 months of age. Part of each cage group continued to be fed ad libitum, whereas the remaining animals were fed every other day. Compared with the ad libitum condition, intermittent feeding decreased body weight and increased lifespan at both ages in both caging conditions. Compared with the caged condition, voluntary exercise in activity wheels reduced body weight only in the 10.5-month-old group fed ad libitum but produced no effect on survival of either age group. The results suggest that intermittent feeding can enhance survival in mature rats even beyond ages at which body weight growth usually ceases, whereas voluntary exercise appears to have an early threshold beyond which increases in longevity are not observed.
The “intermittent feeding” in this 1983 paper turns out to be what is now becoming somewhat popular as a version of intermittent fasting, and makes me wonder why nobody was talking about intermittent fasting back then. Instead everyone was talking about running, while intermittent fasting might have saved Jim Fixx.
In this paper rats of two different ages were tested, starting with 10.5 month old rats (not quite mature, or not at peak body weight), and 18 month old rats (fully mature based on body weight). A number of things were interesting from this study, first being that the intermittent fasting diet improved lifespan a full 36% in the 10.5 younger rats, and 14% in the 18 mature rats. However the researchers said the difference and improvement was misleading because both fasting groups had a mean lifespan of 27 months, while the everyday eating rats in the younger experiment lives were shorter at 20 months compared to 23 months in the older ad lib eating rats, which they thought was likely due to survivorship bias in the older ad lib rats. Exercise surprisingly to me had no statistic or consistent effect on lifespan in either the ad lib or intermittent fasting animals, though roughly looking at their graphs the exercising groups generally did appear slightly ahead of the sedentary animals.
In the discussion they mentioned limits with regards to the degree of dietary restriction generally being beneficial with survival. Restricting food intake >50% shortening lifespan and 10% restriction was beneficial, but increasing lifespan only 7%, less than usually seen with ~30% restriction or the every other day diet.
Interestingly in this study the fasting rats given access to a running wheel did tend to spontaneously exercise more, 2-3 times as much compared to the ad lib rats with a running wheel. So it’s not like fasting animals get old and decrepit, doddering on in old age, but rather were apparently healthier and aging substantially slower.
My favorite variety of intermittent fasting is still “time restricted feeding” variety which I think has a lot going for it, but in the research the every other day diet does have the preponderance of evidence behind it. I think we all hope (and some evidence suggests it’s true) that the every other day fasting findings are applicable to time restricted feeding. My apprehension regarding the every other day time plan has been that it’s supposed to be particularly arduous, while time restricted feeding soon becomes a pleasure. However I’m typing here on Tuesday afternoon, having not eaten anything since Sunday night. I’m calling mine “No Meals Mondays” and I have to say it’s not that bad. Last week around 42 hours of fasting I felt like I was entering an altered state of consciousness/feeling of calm or “meditative” state that made me wonder if this is what it feels like to not have ADHD. It was kind of like Adderall without the irritability. So we’ll see if I get that later today, but being almost 11 months into my intermittent fasting lifestyle, it’s kind of fun and definitely interesting to see what you can take and what it feels like.
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Chad Reilly is a Physical Therapist, obtaining his Master’s in Physical Therapy from Northern Arizona University. He graduated Summa Cum Laude with a B.S. Exercise Science also from NAU. He is a Certified Strength and Conditioning Specialist, and holds a USA Weightlifting Club Coach Certification as well as a NASM Personal Training Certificate. Chad completed Yoga Teacher Training at Sampoorna Yoga in Goa, India.