Most mammals including humans inhabit niches where food is available with reasonable certainty. A lot of mammals, including humans, choose to store food in order to further secure fairly continuous access to energy, instead of storing it internally as fat. As you point out, obesity would negatively impact on fitness in the highly competitive environment mammals, including humans have evolved. There's also a loss of calories associated with first ingesting food, depositing it as lipids (which costs energy), and then catabolizing it. It's much more energy efficient if you have a stack of nuts stored away to eat when you're hungry. Endothermic animals are simply not very well adapted to long periods of starvation, losing up to several % of body weight per day.

There are, however, interesting exceptions - mammals that get seasonally obese. These are the hibernating mammals such as bears, which need to get really fat before going into hibernation, in order to survive the winter.

I think it would be very interesting to study leptin endocrinology in these animals (some limited studies have been carried out), to see what endocrine mechanisms allow this seasonal "overeating".

In our studies on fish, which can experience long, natural periods of starvation (e.g. during winter) we see leptin levels increase during starvation. To me this is logical as this would send an anorexigenic signal to the brain, telling the fish not to look for food when there's no food to be found anyway. A mammal, on the other hand, really needs to find food fairly quickly before it starves to death.

Thank you for your responses. Bjorn you answered a lot of my questions, it was a very good summary thank you.

What I am still wondering is the way in which we maximize the food we do eat.

So for example, during pregnancy hunger increases, but even if the same amount of food was consumed the body is able to access the calories and store them more efficiently so the person can gain weight. I am not an expert but there seems to be a number of body changes that allow this: hormonal, even changes to the composition of gut bacteria- http://www.sciencedaily.com/releases/2012/08/120802122506.htm

If our body is at times capable of gaining more energy from our food, why are we not permanently set up to do so? Why would we ever consume inefficiently?

There's a cost attached to every benefit. That's what the ecologists teach us through the principle "there's no such thing as a free lunch" or TNSTAAFL which conveys the idea that's it's impossible to get something for nothing. This is one of the classic "four laws of ecology", and it certainly applies to physiological mechanisms as well.

The intestine has two very important functions; one is nutritional uptake and the other is being a barrier against pathogenic bacteria.

In order to do both, there has to be a compromise in how "leaky" the intestinal wall can be. If the uptake mechanisms are maximized to obtain nutrients, this also increases the risk of pathogens entering through the gut wall, resulting in more inflammatory reactions in the gut, the inflammation being a sign of the defense mechanisms being activated beyond normal.

Perhaps, during special situations such as pregnancy, when it's extremely important for the mammal to maximize nutritional uptake due to the extreme nutritional demands of fetal growth and post-natal lactation, the interstitial function is shifted towards increased nutrient uptake, which compromizes the barrier function, but only for a limited time. However, to set the intestine on maximal nutrient uptake mode constantly, would seriously risk the health, well-being and life of the individual.

Hi all,

As far as I remember, during pregnancy the immune system is also boosted. Therefore, this might allow this shift of the intestines to a more efficient nutrient-uptake mode mentioned by Bjorn. However, since TNSTAAFL applies, I would not be so sure if during pregnancy woman enhance their assimilation efficiencies, and it might be the case that extra nutrients are either coming from an increased ingestion rate, or divergence from other organs and functions such as liver, muscle (as in many fish) and so on. Water retention also increase considerably, increasing the weight gained but not the energy content.

All the best!

Apologies if this has already been covered, but you also have to consider the logistics of vasculature and blood supply. The morbidily obese already struggle on that front. This along with sub-clinical inflammation as you have pointed out would not support longevity in humans. Interesting topic to discuss!

The change in the gut microbiome may explain the energy inefficient absorption of our digested foods. It does seem that it is the caloric excess, not the relative proportion of macronutrients, which determine weight gain. A proportion of those who become overweight and obese do have a favorable metabolic profile.

Part of the equation in this might be related to the "capacity-load" model, basically how you body (actually your internal organs) develop and under which load they operate. In some settings, low birth weight and malnutrition may "imprint" an unfavorable fenotype. So basically your body trades-off the development of certain organs. And you come out to the world with that "car". In today's world, we may be asking our cars to run different races (exposed to highly calories, for example). And not all of us have a Ferrari to run this race.

I have got to know of this "metabolic capacity vs metabolic load" concept and approach, and have used it, from Jonathan C K Wells. He defines this, in technical terms, as follows: "Holding current size constant, poor growth in fetal life and early infancy is strongly associated with increased disease risk. Holding birth size constant, increased childhood growth rates are associated with risk of the same diseases. A physiological model based on metabolic capacity (directly associated with early life growth) and metabolic load (directly associated with weight gain, rich diet and physical inactivity from childhood onwards) can explain these observations. A high ratio of load to capacity increases disease risk." Source: http://books.google.com.pe/books?hl=en&lr=&id=9FQXunlUvz0C&oi=fnd&pg=PA13&dq#v=onepage&q&f=false

Human evolution is a slow process. The best example is salt, we've been using salt for millions of years and our body still thinks salt is something strange when it enters to the body.

Generally, similar to Bjorn, I think that obesity was not adaptive in environment that human have evolved and it is still eliminated by natural selection. We can not observe evolution process during our life (only microevolution) but it is scientifically proved that obese people are more likely to suffer from various diseases, can have shorter life, and handicaped fitness.

See Int J Obes (Lond). 2008 Nov;32(11):1611-7. Epub 2008 Oct 14.

Thrifty genes for obesity, an attractive but flawed idea, and an alternative perspective: the 'drifty gene' hypothesis. Speakman JR.

http://www.ncbi.nlm.nih.gov/pubmed/18852699

Dear Juliet

Your question while interesting doesn't address one of the more fundamental issues regarding obesity and that is energy expenditure. In our increasingly technology based society there is a shift towards chronic inactivity, which in itself is a significant risk factor to health. A fact established many years ago using data from London bus drivers and conductors (Morris, 1953, The Lancet). We have evolved to be active not inactive (even when we moved away from hunter gathering to farming this still involved hard physical work). So previously our energy expenditure was high compared to modern day living.

We are a very efficient biological machine, compare energy utilisation in muscle against the internal combustion engine and you'll see what I mean. Pregnancy is probably not an ideal model as many of the previous commentators have pointed out in trying to explain the evolution towards obesity. Without a substantial shift in the general attitude towards increasing physical activity I think we are probably evolving into couch potatoes and that's a significant contributor to the obesity epidemic.

Derek just made a very good point in explaining one of the causes why we are getting fat. Apart from our reduced activity, the food we ingest has also changed. In general natural food have always less energy content per gram than our new packed food. You will be surprised to see that can see that any normal cookie usually have ~ 2000 KJ per 100 g. So lower activity and an increased energy content in the food are among the causes that explain our overweight. However, this still does not answer why we have not evolved be fatter. Well, we might wanna ask instead why we should have evolved in this direction?. Well besides some of the reasons already pointed out, reproduction and matting have not been mentioned yet. Reproduction, in all animal kingdom, acts as a big selection force. Therefore, if overweight compromise our ability to find a good healthy mate and reproduce, then this trait wont be particularly selected.

A good question and an interesting topic! I'll simply add that as we're discussing it we should make sure we don't think about fat simply as excess stored energy- but rather as a dynamic part of the endocrine system, linked to many aspects of metabolism

Juliet- specifically to address your question, "why would we consume inefficiently" I'd recommend looking into the metabolic and dietary factors that regulate leptin production.

Thank you all for your responses, there is a good range of perspectives coming in. I understand human evolution has not yet had a chance to react to obesity, and that is at the heart of my question.

Obesity has never been a factor in our survival until now, in the past weight gain was limited and protection against starving to death. With famines a regular occurence, i would have assumed the body would maximize energy intake at all times. However across a lifetime there are times when the body can more readily put on weight, which means the rest of the time there is energy being wasted. Why?

Juliet, here is a good article which touches on some of the topics you're referencing: http://www.t-nation.com/free_online_article/most_recent/control_leptin_and_control_your_leanness&cr=

Don't let the fact that it's published on a bodybuilding site fool you. The authors are clever guys with good info and a useful perspective.

If you know the cause of overweight it is obvious that the fat is only there to protect vital organs from damage from accumulated toxins that all cause inflamation...

reduce toxic load and overweight disappears ..neutralise or evacuate by proper hydration ..

Jessica Jaimes, We have been using salt for centuries .... was that the chemical form ??? NaCl or was it natural salt that contains so much more then 2 elements ... use Himalayan salt and your body will not see this as a strange substance !!!!! The chemical produced NaCl is strange to the body and will remain so for as long as you use it... Himalayan salt has 83 minerals in ionic form dissolved in it... the way nature makes it for us ....not industrial forms

Another aspect not yet mentioned is that the availability of excess food to larger populations plus no major disturbances by war or disease has only been available to our present generation.

From what I know, its the other way around - we are very well adapted to getting obese. As mentioned by Derek Ball, aerobic breathing is a very efficient energy producing machine. If we were not well adapted towards fat accumulation, losing weight would be so much easier than it is. We are programmed to eat and most of the signal produced by our digestive system, except for Ghrelin, is intended to signal satiety and stop us from eating, and still does a poor work (as anyone who struggles to lose weight or maintain low weight post diet would tell you). We become energy efficient, not only following pregnancy, but following weight-loss diet as well.

On the other hand, one of the fewer life-expectancy incraesing treatments is calorie restriction.

If you would like t read more I reccommend R. Leibel articles on the subject:

http://centennial.rucares.org/index.php?page=Weight_Loss

Great question. Clearly, in the extreme, obesity causes health risks that are of a short-term and intermediate-term nature with regard to reproductive fitness. Lack of ability to forage for food (due to immobility), soft tissue infections, impaired glycemia with complications of diabetes, congestive heart failure, excess steroid hormones and other illnesses (eg, polycystic ovary syndrome) can impair fertility and other aspects of reproductive fitness. Keep in mind that typical body fat stores even in a relatively lean human can sustain energy requirements for a couple months; if the environment is so poor in nutrients that food is likely to be scarce for longer than a few months, then it is unlikely that humans can inhabit that area for many generations, so that is not an evolutionary need. There may also be conditioning that occurs at the level of the individual human. For instance, babies born to mothers with food deprivation during pregnancy tend to be heavier in adulthood, likely due to being "programmed" for survival in the face of scarcity. Keep in mind that humans are intermediate in terms of survivability without food. Some birds will die, for instance, in a few days without food. We cannot go all winter without eating, but we're pretty hardy!

Dear Juliet, in your second post you wonder why we as humans don't react to obesity, the short answer is we do. Excess storage of fat in adipose tissue initiates an adipokine response that is inflammatory in nature. The chronic release of these hormones leads to an increased propensity for cardiovascular disease, cancers, diabetes.

Yes it is an interesting question. In my opinion Mother Nature has determined when, where and how much fat a given species should store. Throughout human history the optimal amount of fat has been determined through our evolutionary experience. Humans likely store a little more fat in the fall before the winter. Organisms often do this by consuming ripe fruit loaded with fructose. My friend Richard Johnson summarizes this well in his book “The Fat Switch”.

Humans also shift to fat storage when the food starts to run out in a pre-famine environment. In this situation the human body will likely give up lean body mass to store a little extra fat. You actually have to measure body composition to see this shift. The obesity epidemic clearly does not reflect a normal physiological state. We now consume massive amounts of excessive fructose leading to continuous rather than intermittent insulin resistance.

Eventually the brain is adversely affected and it takes a healthy brain to properly auto-regulate fat stores. We now call this pathological state Carbohydrate Associated Reversible Brain syndrome or CARB syndrome. People with CARB syndrome will inappropriately store fat at virtually any caloric intake. They are more or less stuck in a permanent fat storage mode.

Having significant brain dysfunction (CARB syndrome) and excessive body fat clearly does not improve human survival in a traditional hunter-gatherer setting. Imaging a 300-pound hunter-gatherer trundling across the savanna chasing game!

Regina Wikinski . In old times, adipose tissue was the way to tranport energy useful in hungry times. Now, In industrialized countries , epidemic obesity affect , not only apparently well nutriion population, but also the people with unsatisfactory way of life, anemic malutrition related to low intake of proteins, enriched in carbohydrates,, sugar drinks instead of milk, lack of access to drinking water, Potassium yodine. Calcium, Vitamins and vaccines. In this case you can see obese children affected with Type 2 diabetes mellitus. On the other hand, remember Somalia, Ruanda and other african populations. Education for health and a good managed budget to help poor populations is a social urgent need. Scientific world must understand and also become responsible for what happens.

Interesting topic. First of all, I think that evolution is a time consuming process in first line, and it would certainly take lots of generations (hundreds of years at least) for a population to adapt to a higher level of food intake and to deselect the disadavantages by a different metabolism. Since starving periods are not that infrequent even during the last centuries in highly industrilized regions, there have been interruptions of high food intake periods all the time deselecting humans with the need of higher food intake avoiding a simple solution to be evolved.

But generally, there are two major possibilites of adaption, either to avoid side effects of obesity by generating a higher need of food intake or to generate new mechanisms avoiding the secondary disadvantages of being obese.

Both mechanisms have been described, for the second just think about the habitants of the Italian village Limone, which are genetically protected from heart attacks and become older than anyone else.

The first mechanism can be seen by comparative body height studies in highly industrialized countries which show a steadily increasing body height resulting in humans with a higher need for more food (which is a way avoiding to become fat).

So I think, there is already adaption, but the amount of food intake we can assure in some countries/locations in the world this time, is simply unique and could not be forseen by evolution.

Hi Juliet -

In human evolutionary biology, the most credible conceptual framework currently is a refinement of Barker's "thrifty phenotype" hypothesis, and integrates mainstream evolutionary theory about phenotypic plasticity during development. I suggest you check out Ken Kuzawa's work - and recent critiques of his ideas - and this will bring you up to speed.

Best -

Donna Holmes

Washington State University

Obesity decreases chances of survival in pre-historic times; however, no such selection factors currently exist to prevent human obesity.

This link may be will help you

http://learn.genetics.utah.edu/content/metabolism/obesity/

Regards

Not sure if this has been said. Some essential fatty acids can not be made by humans. The conversion of ALA to EPA/DHA is not high. Of course in the past this could have been higher. ALA is an omega 3, and these are ingested less than omega 6, which leads to more inflammation. So from a human evolution one could claim that fat has an energy function, but also an essential function. In case of DHA that is also found in the human brain more. So either we had a good ALA-EPA-DHA conversion rate and we lost it, or we never had that. The latter would also have some implications for the view on fat and evolution

It could be related to change in diet and life style.

The diet contains more carbohydrates and fats, while the life style change to sedentary life.

A vintage Question that is even more pertinent today as humans are becoming more obese:

Worldwide obesity has nearly tripled since 1975.

https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight

We do not need obesity, but energy