“Igniting” Your Metabolism


Blog #53   Andrew Siegel, M.D.

Our metabolism is defined as the sum of biochemical reactions that occur within our cells in order to sustain our lives.  Our metabolic processes include anabolism (the constructive process in which simple compounds are converted into complex substances, for example, the building of muscle and bone) and catabolism (the destructive process by which complex substances are converted into simpler compounds, with release of energy).  Our metabolism balances anabolism and catabolism to keep our weight as stable as possible, despite varying caloric intake and energy expenditure.

Why would we want to “fire up” our metabolism?  By revving it up, it will help us achieve “fighting” weight and reveal our sinewy body, so we will ultimately look and feel better. Another even more compelling reason is to achieve optimal health—when we attain fitness by way of healthy eating, weight reduction, regular exercise and avoidance of tobacco, we reduce our risk for diabetes by 90%, heart disease by 80%, and cancers by 50-60%. Another motivation is to add quality and quantity to our life.  Finally, stoking our metabolism will help us gain energy.

Our muscles are key players in our metabolism, as they are quite metabolically active—each pound of lean muscle burning 50 or so calories/day.  It is easy to understanding why muscles are such large consumers of energy (calories) if you think of the service that muscle cells provide as compared to skin cells, fat cells, liver or kidney cells, etc.  Some of the many roles that muscles play include skeletal muscle contraction for activities required in our daily existence such as movement, chewing, talking, etc.; cardiac and diaphragm muscle contraction that proceeds without interruption, often for eighty or more years; and smooth muscle contraction in our blood vessels, intestines and urinary tract.

With a sedentary existence and aging, there is a gradual loss of muscle mass and a resultant slowing in our resting metabolism.  By building and maintaining our muscle mass with strength training, our resting metabolic rate will be higher and we will burn more calories. Exercise is also our endogenous stress reducer, lowering cortisol levels, suppressing our appetites and helping us burn carbohydrates before they have a chance to be stored as fats. Additionally, exercise serves to increase the “insulin sensitivity” of muscle, which means that our muscles become more efficient at utilizing carbohydrates as fuel.

On a practical basis, the term “metabolic” is often used to refer specifically to the breakdown of food and its transformation into energy.  Metabolic reactions and processes take place in our cells to convert biochemical energy from dietary nutrients into ATP (Adenosine TriPhosphate), the metabolic currency of our cells. The energy stored in ATP can be used to drive any process requiring energy.  Every cell in our body requires energy to function and energy demands are greater when cellular work is increased.  Just as our cars will expend more fuel when we travel at 60 mph as opposed to 25 mph, so our cells will need to use more energy when increased demand is placed upon them, for example, muscle cells during physical exertion as opposed to resting.

ATP provides the energy for any and every cell, whether that cell is a brain cell, an intestinal cell or a muscle cell. ATP is a molecule of adenosine bound to three phosphates.  It requires extra energy to bind the second to the third phosphate and this provides the power source of the cell.  When energy is needed, the ATP is broken down into ADP (Adenosine DiPhosphate), releasing the third phosphate and a burst of energy.  So ATP can be thought of as a fully charged battery and ADP as the spent battery.  When the battery is recharged, the phosphate is reattached to ADP to make ATP.  This can only occur in the presence of oxygen (oxidative metabolism). Our dietary calories provide the energy needed in order to reattach the phosphate to ADP, that is, to recharge the battery.

We do not keep a ready store of ATP available, but are capable of manufacturing it on demand. Our cells in amounts appropriate to their specific needs can then use the energy in ATP.  This oxidation of nutrients occurs via a complex series of chemical reactions within the mitochondria of our cells. These chemical reactions are dependent upon oxygen use and they speed up as demand increases.  The ultimate result of oxidative metabolism is the production of ATP.  This is absolutely clean combustion, with the waste products being only water and carbon dioxide.  Similarly to how our cars use a mixture of fuel and oxygen that is ignited for combustion to drive the pistons, so our bodies need oxygen and fuel (nutrients).

Unlike our vehicles, we are capable of providing energy to our cells even when oxygen demand exceeds supply.  When oxygen becomes scarce—that is, when oxygen demand exceeds supply because of the intensity of exercise, non-oxidative metabolism kicks in.  This encompasses the breakdown of glucose or glycogen.   Glucose, circulating in our blood, or glycogen, stored in our liver and muscles, is converted to lactic acid under such anaerobic conditions.  This takes place in the cytosol (fluid within the cell), as opposed to oxidative metabolism, which takes place within the mitochondria.

Our energy source is flexible, as we are capable of using any nutrients— carbohydrates, fats, or proteins—whether they come from a recently ingested meal, or are stored in our liver and muscles (glucose storage in the form of glycogen), or are derived from fat reserves.  Obviously, it is desirable to use our fat and carbohydrate stores for energy and not our protein (muscle and internal organs).  Protein catabolism for energy usually happens only with starvation, when carb and fat stores are depleted; under these circumstances we lose lean body mass to provide the energy for survival.

At the lowest exercise intensities, the energy from the consumption of fats is our main energy source.  At intermediate energy intensities, the energy derived from carbohydrates and fats is roughly balanced. When oxygen delivery is compromised by a further increase in exercise intensity, the anaerobic system kicks in, which uses primarily carbohydrates as its energy source. There is a good reason that glycogen is stored in our muscle cells— the source of energy for our muscles is conveniently located in the very cells where it is needed and can be rapidly accessed when the demands on muscle are intensified. So, for example, sleeping will use fats as a primary energy source.  On the other hand, sprinting will use carbohydrates.  Activities at intensities between these two extremes will use a graded mixture of these two fuel sources.

When talking metabolism, it is helpful to think of our glycogen as our “small fuel tank.”  Once the fuel in the liver and muscles is exhausted, our “large fuel tank”—our fat—needs to be tapped to provide energy.  In contrast to the limited carbohydrate storage in our liver and muscles, our bodies abundantly store fat.  Depending on how much fat we have, many days to weeks of energy can be provided.

Lou Schuler, a fitness journalist and author of many popular books about strength training and nutrition, wrote a nice article entitled “Reprogram Your Metabolism” in March 2012 Men’s Health. I have borrowed several of his concepts and highly recommend going to the source and reading the entire article.

As stated earlier, our metabolism balances anabolism and catabolism and is able to strike a metabolic balance to keep our weight stable. Interestingly, if we eat excessive calories, our bodies compensate by speeding metabolism and conversely, if we consume too few calories, our bodies compensates by slowing metabolism.

Essentially, we are hard-wired to adapt to famines and starvation by metabolic slowing and holding on to fuel (fat) and to adapt to times of feasting and plenty by metabolic speeding and fuel burning. The process of eating, in and of itself, notches up the metabolism as the body gets “exercise” in the digestive process, particularly if a high fiber load is eaten.

Metabolic flexibility allows us to easily shift gears between the two nutrient fuels—carbohydrates and fats—that we use for energy purposes. Metabolic inflexibility is a situation in which the body loses the ability to selectively burn carbohydrates or fats in accordance with the demands of the situation. Schuler believes that a big part of the rampant obesity problem is metabolic inflexibility.  Food scientists working for the industrial food complex have mastered combining sugar, salt and fat into highly available, highly palatable, highly stimulating, highly rewarding, highly caloric, highly addictive concoctions that have sabotaged our metabolic systems.   After such a massive glucose load, insulin is released so that the energy can be used and stored. When insulin levels become chronically elevated, we start accumulating fat; when levels are low, we burn fat for fuel.  Insulin is all about increasing fat storage and decreasing fat burning—this is why diabetics on insulin injections typically get fat.

Under circumstances of chronic consumption of processed junk foods it thus becomes very difficult to tap our fat stores because of the chronic insulin elevation. This fosters a metabolic disaster, thwarting our metabolic flexibility and resulting in burning carbohydrates as fuel and leaving fat stores intact.  The resultant low carbohydrate levels lead to hunger, and since the metabolically inflexible have chronic low carbohydrate stores, they are always hungry.  So here we have a very dangerous situation—a fat abdomen, high levels of insulin, low carbohydrate stores, and hunger.  This paradox—hunger in the face of obesity—can only be resolved by lowering insulin levels so that the fuel stored as fat can be tapped as an energy source.  The only way to achieve this is to lower carbohydrate intake and ramp up physical activities.  As a consequence of intake of fewer carb calories and better quality carbs (whole grain, fruits, vegetables, legumes, etc.), insulin levels will decrease, fat will melt away, hunger will dissipate and we will lose weight, gain energy, feel and look better and avoid many preventable diseases.

Pearls to “jump start” your metabolism:

  • Resistance training, a primarily anaerobic activity, is capable of mobilizing fat stores into the bloodstream from storage areas. Carbohydrates are the main energy source while lifting, but during the recovery period, it is mostly fat that is burned as fuel.
  • Interval training—alternating short periods of intense effort with longer periods of recovery—trains the body to become metabolically flexible and shift between the fuel sources, as the arduous effort burns mostly carbohydrates while the recovery burns mostly fats. 
  • Any intense physical activity will produce positive changes, whether it is heavier weights, faster efforts, and longer or more frequent workouts.
  • The foods that fill us up fastest and satisfy hunger longest are high quality, high-fiber carbs (whole grain pasta, whole grain brown rice, whole grain breads, legumes, whole fruits and vegetables); lean protein sources (easy on meat and dairy); and healthy fats (vegetable and seafood-origin)
  • Cut carbohydrates to decrease insulin levels and increase fat mobilization—especially sugar, simple white carbs, liquid calories and high fructose corn syrup.
  • Avoid giant meals in which the caloric load will be stored as fat; substitute with multiple smaller meals in which the calories will be used for immediate energy.
  • Eat slowly and savor food to minimize overeating.
  • Home cooked, non-processed meals are virtually our only means to ensure that we are getting wholesome, high-quality nutrients without hidden calories.
  • Limit after dinner snacking—that way we will burn fat during sleeping and not the fuel contents of our nocturnal nosh.  If we restrict our evening snacking to one piece of fruit, we will wake up in the morning with less to pinch on our waistlines.  This will also help prevent the sleep disturbance from having to digest foods when going to sleep.
  • Get adequate amounts of quality sleep, since sleep deprivation causes decreased levels of leptin (our appetite suppressant), increased levels of ghrelin (our appetite stimulant), increased levels of cortisol (causesincreased glucose levels and promotes fat deposition).
  • Minimize stress to minimize cortisol release; if you can’t eliminate it, manage it.
  • Caffeine can potentially ramp up our metabolism, in addition to boosting athletic performance and helping focus.

Andrew Siegel, M.D.

Author of Promiscuous Eating: Understanding and Ending Our Self-Destructive Relationship with Food


Now available on Amazon Kindle


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