More On The Starvation Response
Written by Jeff Novick, M.S., R.D.
Friday, 09 January 2009 00:00
Continuing on the topic of the starvation response, as I discussed the true starvation response is a metabolic adaption that occurs when body fat drops below essential levels and not just from a reduction in calories, sustained over time, or not.
While this starvation response is often reported in free-living subjects on diets, we have to ask why we do not see this response in controlled studies.
Remember, what happened in the Keys study was unique, because these subjects were truly "starving". First, they started out thin, with little reserves. They were "fit" men. They had used up all their fat reserves and had gone below 5% body fat. Very careful records were kept. As the pictures I posted showed, they were "skin and bones." They were now starting to burn up essential body tissues
(vital organs) which drives metabolism. Remember, most of metabolism isnt driven by just muscle tissue, but the muscle tissue of vital organs. Not peripheral muscle tissue. They also lost over 25% of their intital body weight. And they started out as thin and fit.
Ouside of the Keys study, and another one of two done on actually starvation (i.e., people during hunger strikes) who again, went below 5% body fat, or the same in animals you cant find this effect in the literature where weight loss stops WHILE there is still significant fat reserves left. Just the opposite, We have studies were subjects were "fasted" (on no food but water) for over 6 months and they have never entered this starvation mode. Why? They still had over 5% body fat .
In the biosphere, we saw something similar. They lived for 2 years under a form of food restriction. It took almost 18 months to see their weight begin to stabilize. However, the average BMI upon entry was around 19-20 and during the 1st year they lost over 15% of their intial weight putting them at a BMI or around 16-17. Again,they were close to starving, and the normalization of weight was not due to the starvation response but an increase in food supply (they increased their rations as they were closer to the experiments end) and a decrease in activity levels.
"The gradual stabilization of body weight in the biospherians thus appeared to be the result of a steady increase in food supply and energy intake"
So even after starting at a BMI of around 20, and going for over 18 months on a VLCD, they hadnt reached the starvation mode. They did experience a relative
decrease in RMR due to their loss of body weight but never to a point where they stopped losing weight for most of the experiment, when energy intake changed.
As someone does get closer to the point of true starvation, there are metabolic changes that occur that causes the body to become metabolicially more efficient so the few remaining reserves will last.
In 1998 the American Journal Of Clinical Nutrition re-analyzed the results of the Key's studies and was able to show that it was due to the percentage of fat reduction below an essential point.
Am J Clin Nutr. 1998 Sep;68(3):599-606.
Adaptive reduction in basal metabolic rate in response to food deprivation in humans: a role for feedback signals from fat stores.Dulloo AG, Jacquet J. Department of Physiology, Faculty of Medicine, University of Geneva, Switzerland.
"... Taken together, these results in normal-weight men responding to severe food deprivation reveal anthropometric predictors for human interindividual
variability in the capacity for energy conservation and suggest that the adaptive reduction in BMR is partly determined by an autoregulatory feedback control system linking the state of depletion of fat stores to compensatory mechanisms that suppress thermogenesis."
As have other studies...
Int J Obes Relat Metab Disord. 2001 Dec;25 Suppl 5:S22-9.
An adipose-specific control of thermogenesis in body weight regulation.Dulloo AG, Jacquet J.
' "adaptive thermogenesis"... is dictated solely by signals arising from the state of depletion of the adipose tissue fat stores; it is hence referred to as the adipose-specific control of thermogenesis... supression of this adipose-specific thermogenesis during .. starvation ..leads to energy conservation,"
Annu Rev Physiol. 2006;68:223-51.
The comparative physiology of food deprivation: from feast to famine.
"However, below a critical fat store depletion, marked behavioral, metabolic, and endocrine changes occur."
As we can see, there is a phenomenon called the starvation response that is set off when the body goes below a very low level of fat reserves The reason is, with no more fat reserves left to burn for energy, the body will start going after muscle from vital organs and death would soon occur.
It is a unique situation. And, this unique situation is not the same thing that happens when someone who is overweight and/or over fat goes on a reduced calorie diet. At least not till they used most of their fat stores up.