This blog may be the most important blog I have written about weight loss yet. If you have a difficult time losing weight or keeping it off, this blog is for you. I talk about what happens to our metabolism when we cut back on calories, how this prevents further weight loss, makes it difficult to maintain weight loss and what we can do to change this. This blog will show you that weight loss is much more complicated than burning more calories than you are taking in.
One of the biggest effects that weight loss has on our body is that it decreases the amount of energy we use each day in order to prevent further weight loss. This is the biggest hurdle with maintaining weight loss. Since this is such an important issue, I will start the blog with this.
Weight loss and decreased energy expenditure
- Energy expenditure – refers to the amount of energy (calories), that a person uses to breathe, circulate blood, digest food, and be physically active. To prevent weight gain, energy intake (caloric intake) must be balanced with energy expenditure.
Our daily energy expenditure results from the following: 60-70% of the energy used is for resting metabolic rate (RMR), thermic effect of feeding (10%), and thermic effect of activity (about 30%). Resting metabolic rate measures the amount of calories you burn at rest. Thermic effect of feeding refers to calories (energy) used to break down and digest food, and thermic effect of activity is calories burned through daily movements and exercise.
One six-month studied looked at calorie restriction in overweight men and women and measured various biomarkers (1). There were four groups: calorie restriction, controls (people that made no changes to their lifestyle), calorie restriction + exercise and very low calorie group. After 3 months, every group but the control group, lost weight and fat mass. Everyone in the reduced calorie groups experienced a decline in their 24 hour energy expenditure as well as their sleeping energy expenditure.
When lean or obese subjects maintain a 10% loss of body weight, their 24 hour energy expenditure decreases by 20-25% (2). This decreased response in energy expenditure, is 10-15% below what can be accounted for by the loss of fat and lean mass; meaning there are other signals besides changes in fat and lean mass that tell our body to reduce our 24 hour energy expenditure. This also means that the person would not lose weight since the decreased energy expenditure would counter the decrease in daily energy intake. When our 24 hour energy expenditure drops, our body slows down in an attempt to make us regain the lost weight.
Exercise may offset the decrease in energy expenditure
Hopefully, all of this talk about energy expenditure is not confusing. If it is, you can think of energy expenditure as our metabolism. Although they are not the same thing, they are related. When you hear slow metabolism you likely think of someone that has a hard time losing weight. When our 24 hour energy expenditure decreases, it is similar to slowing down one’s metabolism, making it more difficult to lose weight and to keep it off.
A 6 month study found that calorie restriction led to an abrupt drop in energy balance, which soon adjusted and reached a new equilibrium (steady state) at the new lower weight; the subjects no longer lost any more weight (3). All components of daily energy expenditure decreased in response to the calorie restriction. Basically, their metabolism slowed down in response to the weight loss and they stopped losing weight.
Surprisingly, the low calorie + exercise group did not experience metabolic adaptations and decrease in energy expenditure (their metabolism did not slow down). It was also found that calorie restriction led to subjects naturally decreasing their level of physical activity. This may be an adaptation by our brain to reduce calories burned from activity to prevent further weight loss. This means that calorie restriction with exercise may preserve our metabolism better than only calorie restriction. This can have implications for maintaining long term weight loss while preserving daily energy expenditure.
Twin Studies- the importance of genetics in weight loss
A well controlled study with 14 pairs of obese, female, identical twins put them on a calorie restricted diet and measured changes in metabolic efficiency and weight loss after 40 days (4). The researchers found great within pair (as opposed to between different pairs) similarity in weight loss and the metabolic changes. The average weight lost was highly correlated within twin pairs but not between different pairs of twins. There were 12.8 times more variability for changes in weight between the twin pairs than within twin pairs. Changes in metabolic efficiency were highly correlated within twin pairs but not between pairs.
This study found that weight loss and changes in metabolism from a calorie restricted diet were very similar between sets of twins; much more similar than between the different pairs of twins. Although all of the subjects lost weight and had metabolic changes from their calorie restricted diets, these changes proved to be heavily influenced by the genetics of the subjects.
Hormonal changes from under eating
There are a few hormonal changes associated with under eating, but the effect on thryroid hormones may be the most important. Thyroid hormones change in accordance with weight loss. Production and release of thyroid hormone increases our daily energy expenditure by increasing blood pressure, heart rate, and the use of energy by our muscles. Someone with a thyroid hormone deficiency is labeled as hypo (low) thyroid, and usually experiences low blood pressure, bradycardia (slow heart beat), lethargy and tends to gain weight easily. Someone with an excess of thryroid hormone (hyper-thryoidism) experiences high blood pressure, tachycardia (fast heart beat) and tends to lose weight. So, having low levels of thyroid hormone is associated with a slower metabolism (and easier weight gain) and having high levels of thyroid hormones is associated with a fast metabolism and a harder time gaining weight.
Weight loss and maintenance of weight loss is associated with a small, but significant, decrease in circulating concentrations of T3 hormone, and an increase in circulating levels of the inactive thyroid hormone (rT3). This means we move towards hypo thyroidism, marked by easy weight gain and lethargy. A weight loss study found that those in the reduced calorie group experienced a decline in their plasma levels of thyroid hormones at 6 months. As we lose weight, our concentration of thyroid hormone decreases, slowing our metabolism and making further weight loss difficult. If we could somehow keep our thyroid levels up while losing weight, we would have an easier time losing more weight and keeping it off (a hormone called leptin may be the key, but I will talk about that in my next blog).
Cortisol is associated with appetite regulation. Studies are inconsistent and have found no changes, increases, or decreases in cortisol levels following weight loss and maintenance.
Autonomic nervous system changes following weight loss
- “Autonomic Nervous System- the part of the nervous system responsible for control of the bodily functions not consciously directed, such as breathing, the heartbeat, and digestive processes.”
When we restrict calories, our autonomic nervous system makes some changes. Our parasympathetic nervous system activity increases, which slows our heart rate and lowers our resting energy expenditure (slows our metabolism). The sympathetic nervous system is responsible for our feeding behavior, increasing our heart rate and directly stimulates our thyroid gland to release more thyroid hormone (increases our metabolism). Maintenance of reduced body weight is marked by a decrease in the sympathetic nervous system activity and an increase in the parasympathetic nervous system tone (our metabolism slows). These factors may play a big role in the changes in energy conservation and neuro-endocrine homeostasis which accompany weight loss. Our calorie burning sympathetic nervous system activity is decreased while our calorie conserving parasympathetic nervous system activity increase following weight loss. Our nervous system adapts to weight loss by slowing our metabolism down in an effort to regain lost weight.
Maintenance of lower body weight increases muscular efficiency
In a study, subjects lost 10% of their current body weight and maintained that loss throughout the study. The researchers measured the skeletal muscle work efficiency of subjects before and after weight loss. When the subjects lost weight, exercise and movements required less energy than before weight loss. Basically, the body adjusted the energy spent in activity (made it more “efficient”) so less calories were burned, thereby increasing the likely hood of regaining weight. As an example, when someone weighs 200 pounds their body burns a certain amount of energy to walk two miles. Now, if that person loses weight, that same walk will result in less calories burned, meaning the body has become able to burn less calories for a similar amount of work. It is amazing the adjustments our body makes to push us towards weight gain.
Why it is difficult to lose weight and keep it off
When you are obese, losing weight is fairly easy. Any reduction in daily caloric intake would result in some weight loss. Losing weight becomes a problem when you are already lean. Once your fat stores go down significantly, your body resists further weight loss. That is why it is so difficult to lose that last bit of weight.
So, you finally reached your weight loss goal. Congratulations! Unfortunately, the battle has just begun. Over 80% of people that have lost a significant amount of body fat return to pre weight loss levels soon after (2). If you took two people, of the same body weight and composition, but one of the people was formerly obese, the formerly obese person would have to consume 300-400 kcals/day less than the never obese person of the same body weight, because of the reduced energy expenditure (slower metabolism) that accompanied the obese persons weight loss, that is absent in the never obese subject. This is a big reason for why it is so difficult to maintain weight loss; our body is working against us (but you can’t blame it!). Essentially, someone that has lost weight has to work harder to keep it off than someone of the same build that never lost weight. Or to put it another way, our body works to return us to pre weight loss levels, while the body of someone similar that has never lost weight, works to maintain its current weight.
This is also the reason that for every person that is naturally thin, despite high caloric intake, you can find a handful of obese people. The norm favors energy storage, and the people that can eat whatever they want and remain thin are the outliers. These thin people would have starved to death during periods of famine, so they would have fewer ancestors.
Our body continuously monitors our energy intake and adapts
Have you heard the expression that if you cut 3500 kcal per week you will lose 1 pound? This would only be true if our body did not adapt to this decrease in calories. If our metabolism and endocrine system did not change in response to reduced caloric intake, we could lose one pound a week by cutting out 500 kcal per day. In the real world, this rarely happens.
Our body works like a finely tuned machine. It continuously monitors how much energy we are taking in and how much energy we are expanding. If our body was not sensitive to these changes, our weight would fluctuate rapidly, depending on our energy intake. Although you may not like this, if our body didn’t say, “Hey, Bob has been reducing his energy intake these past few days, I better reduce the amount of energy Bob is expanding per day,” we would lose a substantial amount of weight over time and eventually die of starvation. We can still lose weight slowly; it just takes a bit of fine tuning.
Calorie restriction is similar to hibernation
- “Hibernation is characterized by low body temperature, slow breathing and heart rate, and low metabolic rate.” Often associated with low temperatures, the function of hibernation is to conserve energy during a period when sufficient food is unavailable. To achieve this energy saving, an endotherm will first decrease its metabolic rate, which then results in a decreased body temperature.”
You can think of conserving our fat stores in the face of reduced caloric intake as a sort of human hibernation; meant to get us through times of reduced food intake, similar to a hibernating squirrel. There are many metabolic similarities between animals in hibernation and humans on a restricted calorie diet, such as decreased body temperature and decreased metabolic rate.
Is it possible to lose weight when your body is in “hibernation” mode? Highly unlikely. Your body is hanging on to your fat stores for dear life! When animals hibernate, their metabolic rate is often 5% of what it normally is (5). Studies from Rhesus monkeys have shown that a 30% restriction of calories resulted in a lower 24 hour body temperature compared to a control group.
Following weight loss, as we maintain our lower body weight, our body does everything in its power to make us eat more and gain the weight back. Studies have found that people maintaining weight loss have a diminished satiety response (food is not as filling) despite our decline in energy expenditure (metabolism). The body lowers our energy expenditure so we need fewer calories to gain weight, and it makes food less filling, so we are tempted to eat more.
Why it is easier to gain weight than lose
Ever notice how it takes months to lose ten or more pounds, yet you can easily put on a few pounds a week with no effort? Turns out our body favors weight gain much more than weight loss. Following weight loss, when subjects overeat, a few changes are made by our body. There are increases in circulating thyroid hormones T3 and T4, sympathetic nervous system tone, daily energy expenditure and circulating leptin concentrations, while there is a decrease in the parasympathetic nervous system tone (our metabolism greatly increases). These are opposing responses compared to what happens during weight loss. Following over feeding after weight loss, initially our body revs up our metabolism and switches from calorie saving mode to calorie burning mode, when presented with excessive caloric intake (which would limit our weight gain). Unfortunately, this response does not last.
Unlike weight loss, the adaptations our body makes to overfeeding are short lived. Studies in mice have found that the previously described metabolic changes following over feeding, only subsist for a few months at most. With increased caloric consumption, the body stops or is unable to adjust to this increased intake, and begins putting on weight. Any increase in metabolism is not able to prevent weight gain when faced with a high caloric intake for an extended period of time.
How administration of leptin can help keep weight off
High leptin levels reduces hunger and low leptin levels increase hunger
A very important molecule related to weight loss and gain is the hormone leptin. It regulates our appetite. Leptin is derived from fat molecules and circulates in weight stable individuals in proportion to the fat mass of the person. During weight loss, as our hunger increases, circulating leptin levels decrease, and as hunger decreases, circulating leptin levels increase. When rodents are administered leptin, they reduce their intake of food. Leptin suppresses our intake of food by producing a neuropeptide, and reducing the expression of other neuropeptides. When we reduce our fat mass through weight loss, leptin concentrations fall, which stimulates our intake of food.
It turns out that leptin is a very, very important hormone. Administering leptin to leptin deficient humans and mice decreases energy intake, increases energy expenditure, increases the sympathetic nervous system, and normalizes thyroid function. Interestingly, when we administer leptin to lean or obese humans who are not leptin deficient, the dose required to induce weight loss produces plasma concentrations of leptin that are over ten times the normal leptin concentration. If we restore leptin levels to pre weight loss levels in people that are maintaining weight loss, their energy expenditure increases, thyroid hormone concentrations and sympathetic nervous system activity increases and energy intake is increased.
Now you can understand why it is so difficult to lose weight. Our body alters its nervous system, metabolism, and hormone production in response to a reduced caloric intake. Because our metabolism usually slows down following a reduced calorie intake, it becomes very hard to keep losing weight. When faced with what it perceives as starvation, our body enters hibernation mode, where it does everything in its power to conserve the remaining fat stores. And when you lose the weight, the body continuously tries to go back to the pre weight loss weight, by decreasing feelings of fullness and keeping our metabolism suppressed.
If two people are the same weight, but one of them was previously obese, the previously obese person would gain weight when eating the same caloric intake as the never obese person (our body tries to return to the previous weight while the never obese person’s body does not want weight gain).
Hopefully this blog did not deter you from trying to lose weight. While weight loss is heavily influenced by genetics, even people genetically susceptible to obesity can lose weight. The problem is that the weight loss has to occur very slowly. Calories have to be continuously reduced by a small amount, so the body does not think we are starving and will not decrease our metabolism. In addition, new research on the hormone leptin looks promising. Administering leptin to obese people and those who have lost weight, may counter some of the metabolic effects of weight loss, and make it easier to lose and keep off the weight long term.
My next blog will focus solely on leptin and its role in weight loss. You do not want to miss it!
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