Starvation Mode - Adaptive Thermogenesis and Weight Loss

What is usually meant by Starvation Mode?

The whole debate around “Starvation Mode” is that this term has taken a life of its own and through either exaggeration or inconsistent definitions resulted in a strange debate.

If you go and look at what is often called “Starvation Mode” you find:

I) “End of the World, You’ll Get Fat” - It means “the shutting down or crashing of one’s metabolism. This is often said to happen from having too little protein, too little calories or even from missing a meal. It is also said that your weight loss will plateau and you will not be able to lose fat because your body is starving and needs to hold on to anything it can. Some even say that you begin to gain weight as a result of your “ruined” metabolism.” (ref (1) – Oh, yeah, I’ll use references)

II) “72 hrs, only affects very lean, Minnesota!” - Or it “is a state in which the body is responding to prolonged periods of low energy intake levels. During short periods of energy abstinence, the human body will burn primarily free fatty acids from body fat stores. After prolonged periods of starvation the body has depleted its body fat and begins to burn lean tissue and muscle as a fuel source (2)” (This is from Wikipedia – an aside, the article the Wikipedia page references is a blog post and doesn’t even say that.)

III) “Long term adaptation” - Or when the term is the equivalent of “metabolic adaptation” or “adaptive thermogenesis” it can be defined as the metabolic and physiological changes that occur during weight loss that effect energy needs in a way that cannot be explained by the traditional BMR equations based on Lean Body Mass (LBM) and body composition. (3)

The “End of the World” definition is bananas. That is a myth and a good example of how good information becomes an exaggeration.

The 2nd definition of Starvation Mode is true but not really relevant to the average person dieting – however, for people on very low calorie diets (VLCD) for extended period this can be a concern. Gallstones, constipation are just some of the smaller adverse effects (7) Even for people just having a large calorie deficit. I’ll touch on this a bit later.

Then there is “Adaptive Thermogenesis”. This is not a sexy term and "Starvation Mode" sounds better. Alas, this Adaptive Thermogenesis concerns us all. As we lose weight, we need less energy to move around and if we lose some muscle along the way, well, our muscles use less energy in daily activity. But that isn’t all of it.
Adaptive Thermogenesis is a reduction in energy needs beyond the standard calculated needs from the amount of lean body mass. The person that lost weight needs less energy than the lean person at that same weight. It is complex; it is related to true changes in metabolic and physiological function and what I’ll focus most on.

By the way, when someone says that starvation mode doesn’t affect the obese or those with larger fat stores they are only correct in regards to definition (II). Adaptive Thermogenesis (III) especially affects people that have been overweight (3).

TL;DR Starvation Mode has different meaning to different folks. The idea of “I’ve crashed my metabolism because I didn’t eat lunch” is a myth. But other definitions, lean mass loss from extended low energy intake and adaptive thermogenesis from regular weight loss are scientifically supported phenomena. This last is likely important to you.

Why is it important? And the article review ... finally

Boy, I’m long-winded. Why all the interest in adaptive thermogenesis then? So far it’s a myth or not of much concern to most people. Let’s get to the meat – the part that affects most people, explains some of the difficulties in weight loss and suggests that we need diet breaks. This is getting back to definition (III) up above.

A review of a science review

As of June of 2013 an review article of some of the theories and research on adaptive thermogenesis in humans was placed in public access in PMC. “Adaptive thermogenesis in humans” by Michael Rosenbaum and Rudolph L. Leibel not only covers this research but also presents some points of interest in weight loss strategies.

There are interlocking control mechanisms, feedback loops, which work to maintain energy stores relatively constant without our conscious effort – the way these work is complex and the links between exercise, diet and the mechanism of appetite are not clear (ok, that’s basic, we are warming up. (4) )

Most diets fail (80-90%) and for those that are successful, maintenance will probably require “a lifetime of meticulous attention to energy intake and expenditure.” Well, that’s why we are all here, right?

So what happens when we lose weight? Well, the key eye opener phrase for me from this article is this. Read it and weep.

“Maintenance of a 10% or greater reduction in body weight in lean or obese individuals is accompanied by an approximate 20%-25% decline in 24-hour energy expenditure. This decrease in weight maintenance calories is 10–15% below what is predicted solely on the basis of alterations in fat and lean mass. Thus, a formerly obese individual will require ~300–400 fewer calories per day to maintain the same body weight and physical activity level as a never-obese individual of the same body weight and composition. Studies of individuals successful at sustaining weight loss indicate that reduced weight maintenance requires long-term lifestyle alterations. The necessity for these long-term changes is consistent with the observation that the reduction in twenty four hour energy expenditure (TEE) persists in subjects who have sustained weight loss for extended periods of time (6 months – 7 years) in circumstances of enforced caloric restriction in the biosphere 2 project, bariatric surgery and lifestyle modification.”

Wait, what? Repeat that?

Whether you go and live in a cave, get surgery or just change eating habits that 10% weight loss results in lower energy needs for 6 months to 7 years.

How does this work? The study goes to point out that:

- It’s not the work of digestion that changes (TEF)
- There is possibly a small change (10-15%) in resting energy needs (commonly called BMR or REE)
- The majority of the reduction in energy needs is around the non-resting energy expenditure – that is the energy used during exercise and in normal non-exercise activity.
It basically means you twitch less. Or you perform less physical activity (Other studies suggest that since physical activity doesn’t decrease – it increases in rats that lose weight. And logically people that lose weight by lifestyle changes are more active…)

So what is going on?
- Possibly muscles are more efficient? (lots of data that support that)
- Neuroendocrine function changes? (even more data that support that)
- Autonomous nervous system is affected and there is less “tone” of the sympathetic nervous system? – Let’s be reductive here – we twitch less? (lots of data for that)
- Brown adipose tissue (brown fat) is doing something? (early speculation and no clear evidence)
- Leptin, an important mediator in energy intake and use, is modified? (most certainly, but to what extend and what about the receptors…)

Let touch two of these in detail:

1) How are muscles more efficient?
It could be speculated that with people who were always lean, they didn't necessarily get there by exercising all the time, so their muscles didn't learn how to become efficient. Meanwhile, for a lot of people who get lean from fat, they got there by exercising and their muscles became more efficient in the process.

Several studies have shown that this is a very real observed reslut of leaning out and the possible mechanism is, aside from the speculation above, that there are actual changes in muscle physiology:

Here are the mechanisms the authors discuss (13):
- thyroid hormone changes affect proportion of muscle fiber types which in turn use proportionally different energy paths (fatty acid oxidation vs glycogen stores)
- possible changes in muscle fiber recruitment

2) What are these neuroendocrine changes? At the simplest level they correspond to increased adrenal activity and decreased thyroid and gonadal activity. This gets complex quickly and involves how hormones help regulate and balance things like activity, mood, hunger and body temperature and interact with the autonomous nervous system (the part of nervous system that works without our concious will). So what does increased adrenal activity really mean? Well, it results in an increase in appetite, reduced metabolic activity, and an increase in cortisol levels, which in turn result in loss of LBM and "increase the partitioning of stored calories to fat." (3) A variety of pathways such as those that regulate things from sexual drive to cognition to bone reconstruction are also modulated down.

Now, it’s really a mixture of all these listed things and the weight of each is interesting but how does this impact the average Joe or Jane losing weight?

In short one could imagine that metabolic, endocrine and autonomic systems are “fighting” the weight loss. This can be seen when Leptin is administered to subjects that have lost just 10% weight (neat table here (8)) the effects of weight loss on decreased energy expenditure, the autonomic function and the neuroendocrine function are reversed. (Great! Where do I get me some Leptin, right?)

TL;DR Losing weight creates a bigger impact on TDEE than you might think...