What exactly is Fat, and why is it so hard to get rid of?

Fat is an enigmatic opponent.  From both the nutrition side and the weight loss side, fat presents a number of challenges.  It's high caloric value, ubiquity, and the fact that it usually tastes delicious make it difficult to  effectively control dietary intake of fats.  At the same time, once your body has stored excess energy as body fat, it is notoriously difficult to get rid of.  Many people find that even when they are "doing everything right" it seems impossible to significantly reduce the amount of fat they are carrying on their bodies.  In this post, we'll cover precisely what fat is, how it's digested and absorbed and what metabolic conditions encourage (or inhibit) the human body to "burn off" excess fat stores.

First off, what exactly is fat?  Chemically speaking, fat is a combination of fatty acids connected by a glycerol molecule.  Because that is probably not a meaningful description for the average person, I have included the

handy image to the left to illustrate this concept.  Once the free fatty acids are connected to the glycerol backbone to form one large molecule, it is called a triglyceride.  Their are many different types of fatty acids that can be attached to the glycerol backbone, and these differences are what define your different types of fat, such as saturated fat, mono-unsaturated fat, poly unsaturated fat and the ominous trans-fat.

To briefly summarize these different types of fat:

Saturated fat - The primary fat derived from animal sources.  It is composed of straight fatty acids that have no double bonds.  Because of this, the molecules can pack closely together and form more dense fats.  This type of fat is often solid at room temperature.  There is also some concern that high levels of dietary saturated fat are correlated with an increased rate of cardiovascular disease.  However, most research studies have failed to show a strong causative relationship between saturated fat intake and cardiovascular disease.  It is more likely that cardiovascular disease is impacted by total nutrition and lifestyle choices, rather than simply eating saturated fat. 

Unsaturated Fat (Mono-unsaturated and Poly-unsaturated) - The primary type of fat found in plant sources.  For example, olive oil contains roughly 70% unsaturated fat, in the form of oleic acid (a type of fatty acid).  Unlike saturated fat, unsaturated fats are composed of fatty acids that are bent or kinked.  As a result, they can't pack together as closely, are often less dense and are often liquid at room temperature.  Several studies have suggested that there are a number of potential health benefits from certain types of unsaturated fats, such as those found in olive oil (oleic acid).

Trans Fat - An artificial type of unsaturated fat generated from a chemical process known as partial hydrogenation. Partial hydrogenation is used to make liquid fats more solid, such as in the production of margarine from vegetable oil.  Not all partially hydrogenated fats contain trans fat.  In fact, with recent research showing that trans fat could potentially increase the risk of coronary heart disease and raise "bad" (LDL) cholesterol levels, many products are now being produced without trans fats.

Probably more important than understanding dietary fat, is understanding how your body processes that fat and how it impacts your own stores of body fat (aka adipose tissue).  First off, it is important to keep in mind that a complete fat molecule, a triglyceride, is not readily absorbed by the digestive system.  It must first be enzymatically digested into fatty acids and glycerol, which are illustrated in the first image.  Unlike the larger triglyceride molecules, free fatty acids are readily absorbed through the intestines. The digestion of triglycerides into free fatty acids is performed by enzymes in the intestines called lipases. These enzymes are the target of the drug orlistat, also known as alli (OTC) and Xenical (Rx).  Orlistat prevents the digestion of triglycerides into free fatty acids and therefore prevents the body from absorbing the fat.  During treatment with these medications, a substantial amount of ingested dietary fat remains as undigested triglycerides, passes through the digestive system and is excreted in the stool.

Fat contains more calories per gram than carbohydrates or proteins.  Fat has approximately 9 calories per gram compared to an average of 5 for carbohydrates and 4 for proteins.  Because of that, it is a lot easier to consume excessive calories if you are eating a diet that is high in fat.  However, simple caloric intake is not the whole story when it comes to weight gain or loss.  In many cases, consumption of fat can be a beneficial part of a healthy diet.  Fat is often associated with high protein, low carbohydrate foods, like meats or eggs.  Research has suggested that many sources of fat could have beneficial health effects, like olive oil or cod liver oil.  A moderate amount of fat in a meal promotes a sensation of satiety that you often don't get with high sugar/high carbohydrate meals.  The biggest danger is when you combine high fat food choices with choices that are high in sugars and simple carbohydrates.  As we will see in the next section, the combination of high sugar/high fat creates metabolic conditions that favor the storage of large amounts of energy as body fat.

If I had to choose one thing that people need to understand about fat metabolism, it would be this: Blood sugar is the master regulator of metabolism.  When your blood sugar is high, your metabolism shifts its focus towards storing energy (making fat).  When your blood sugar is depressed, your metabolism focuses on getting more sugar in your blood (burning fat).  Blood sugar plays this critical role because it regulates the production of the master metabolism hormones, insulin and glucagon.  High blood sugar leads to increased production of insulin, which causes fat cells to absorb free fatty acids and glucose from the blood and turn it into fat.  Furthermore, during periods of high blood sugar, the metabolic pathways required for burning fat (e.g. lipolysis) are shut down.  Therefore, it is virtually impossible to reduce body fat levels when you have elevated blood sugar levels.  This is illustrated in the figure above showing an increase in glucose (blood sugar) causes the pnacreas to release insulin, which inhibits lipolysis in the fat cell. In contrast, low blood sugar levels encourage the metabolic pathways that burn fat because low blood sugar decreases insulin levels and increases levels of glucagon, which encourages the release of stored energy.

So what is the key to creating the metabolic conditions that encourage your body to tap into its stored fat reserves?  The answer is two fold.  First, you need to avoid the conditions that inhibit fat catabolism (another word for "burning fat").  Those "anti-fat burning" conditions are created by high blood sugar levels.  Avoid eating foods that cause large increases in blood sugar, particularly sugars and simple carbohydrates.  Of course, it is also important to avoid excessive calorie intake in general (max=2000-2500/day) if you want to maximize your fat burning potential.  There are also several prescription drugs on the market that can help you maintain low blood sugar levels.  The second key is to encourage your muscles to utilize more glucose, thus lowering blood sugar levels by energy demand.  Exercise, obviously, is the most effect way to do this.   Both during and after exercise, your muscles have an increased demand for energy.  Even resting muscle has an elevated metabolic demand.  Increasing your lean muscle mass is also an effective way to encourage a decrease in body fat.  In addition to exercise, increasing the amount of protein in your diet encourages the growth and maintenance of lean muscle tissue.  In the end, the keys to decreasing your body fat levels sound like much the same advice you've always heard.

1) Avoid sugar and other foods high in simple and processed carbohydrates
2) Avoid excessive calorie intake (more than 2000-2500 calories per day)
3) Exercise
4) Increase your protein intake to encourage development of lean muscle mass

Additional options
1) Explore options about prescription medications that can help you regulate your blood sugar.
2) Explore protein supplements and other natural supplements that can help you increase your lean muscle mass.

References, Background Reading and Additional Information


Books:


How Fat Works
Philip Wood


Eat, Drink and Be Healthy: The Harvard Medical School Guide to Healthy Eating
Walter Willett, MD


Cereal Killer
Alan Watson


Good Calories, Bad Calories: Fats, Carbs, and the Controversial Science of Diet and Health
Gary Taubes


Online Resources:


Fat - Wikipedia


Insulin and Metabolism - Wikipedia


Fatty Acid Metabolism - Wikipedia


The Chemistry of Fat Metabolism

The Science of Acne

Scientific Research Papers:


PPARS and the Complex Journey to Obesity
Evans, et al. Nature Medicine 2004

Fructose Consumption: Considerations for Future Research on Its Effects on Adipose Distribution, Lipid Metabolism, and Insulin Resistance
Stanhope, et al.  Journal of Nutrition 2009 

Consumption of Resistant Starch (Dietary Fiber) Decreases Postprandial Lipogenesis in White Adipose Tissue in Rats
Higgins, et al. Nutrition Journal 2006

Understanding The Glycemic Index

The Glycemic Index (GI) is one of the most helpful and straight-forward tools available when it comes to choosing which foods to consume and which foods to avoid.  Utilized primarily by people to help treat their Type 2 diabetes, the Glycemic Index lists many common foods and their impact on blood sugar levels.  The Glycemic Index rates foods on a scale of 1 to 100 (sometimes the scale can go up to ~140).  A high score indicates that eating that food will cause a large increase in your blood sugar, while a low score indicates that a food will have a minimal effect.  Generally speaking, foods that are high in sugar, simple carbohydrates and are highly processed have high GI scores and foods that are low in carbohydrates and are relatively unprocessed have low GI scores.  To the left is an example Glycemic Index chart showing the GI values of some common foods.

So why is blood sugar important in the first place and what role does it play in your weight loss efforts?  The answer to this question, like so many others, can be better understood through the lens of human history. For the vast majority of human history the problem has been too little available food, rather than too much.  Furthermore, the widespread availability of refined sugar is a relatively recent phenomena, beginning in the mid 1800's.  For thousands of years of human development, there was no scoop of sugar to sweeten that hot cup of java (for most cultures, there was no coffee, either).  There were no sweetened cereals, no high-fructose corn syrup, no cookies, no candy and no soda.
In short, there was very little sugar in the human

diet, with the exception of the fructose that naturally occurs in fruit and the lactose that naturally occurs in milk.  This increase in sugar consumption is illustrated by the chart at right.  Instead, the human diet was composed of whole grains, meats, fish, eggs, vegetables, fruits, milk, tubers and other basic foods.  Against this dietary backdrop, thousands of years of natural selection optimized the human body to utilize this diverse array of basic foods.  However, in our recent history, advancements in farming, shipping, food processing and lifestyle have led to dramatic changes in the average diet, changes that are inconsistent with our historical diet.  Many of the metabolic functions that humans have evolved to survive cycles of abundance and famine wreak havoc on our bodies when confronted by our modern diet.   While occasional peaks in blood sugar levels are completely normal and well tolerated by the body, consistently elevated blood sugar and frequent spiking of blood sugar levels has a wide range of deleterious effects.  The most well known effects of excessive blood sugar levels are weight gain and insulin insensitivity, also known as Type 2 Diabetes.

So how can utilizing the Glycemic Index help you control your weight?  Because using the glycemic index can help you moderate your blood sugar levels, and high blood sugar levels are directly responsible for weight gain.  High blood sugar creates the metabolic conditions which stimulate your body to store energy as fat and glycogen. Specifically, high levels of glucose in your blood stimulate your pancreas to release the natural

hormone, insulin. Insulin causes fat cells (adipocytes) to absorb the glucose in the blood and convert it into fat for storage.  At the same time, high levels of blood sugar and insulin repress another natural hormone, glucagon.  Glucagon stimulates your body to produce glucose by burning stored energy, such as fat.  So basically, when you have high sugar levels in your blood, your body is focusing on storing energy instead of burning it.  This makes it extremely difficult to lose weight.  The solution is to use the Glycemic Index to help you make better choices about what to eat and what to avoid.  Avoiding high GI foods will help make your other efforts, like exercising or watching your total caloric intake, that much more effective.  It will also help you avoid getting Type 2 diabetes, which can have a devastating impact on your quality of life.

Like anything else, the Glycemic Index is not perfect and there are a couple of important caveats to be aware of.  First, the glycemic index is based on blood glucose levels, so it can underestimate the negative impact of some foods, particularly those with high levels of similar sugars, like fructose.  Take for example your average non-diet soda.  It is basically carbonated sugar water, and you would logically assume that non-diet soda would have a GI value near 100. However, soda is sweetened with high fructose corn syrup, which must be converted into glucose by the enzymes in your body.  As a result soda has a GI value near 65 because it doesn't raise your blood sugar as high as if it were sweetened by glucose.  But it's a hollow victory, because while the peak may not be as high, it will elevate your blood sugar for a longer period of time because it takes time to convert all of the fructose into glucose. In general, if you are trying to regulate your blood sugar and lose weight, you should do your best to limit sugar intake across the board.  The other caveat is that certain very high fat foods are low in sugars and simple carbohydrates and appear to have very attractive GI values.  Eating vast amounts of bacon, for example, is going to have negative consequences, regardless of what the Atkins Diet plan says.  The best approach is to use the Glycemic Index with a healthy dose of common sense to help you make intelligent dietary decisions.

In summary, the Glycemic Index can help you avoid foods that can unhealthily raise your blood sugar.  Low GI value foods are most often those foods which are less processed and contain lower levels of sugars and refined carbohydrates, like flour.  Additionally, foods with low GI values are more likely to be more nutritious, more filling and slower digesting, helping you better regulate your hunger levels.  All of these factors can help you make progress towards gaining control over your weight.

References, Background Reading and Additional Information

Books:

The Glycemic Index Diet for Dummies
Merl Raffetto

The New Glucose Revolution What Makes My Blood Glucose Go Up ... and Down? 101 Frequently Asked Questions About Your Blood Glucose Levels
Jennie Miller, Kaye Foster-Powell and David Mendosa

500 Low-Carb Recipes: 500 Recipes from Snacks to Dessert, That the Whole Family Will Love
Dana Carpender


Online Resources:


Glycemic Index - Wikipedia

Glycemic Index Database

The Glycemic Index and Disease

The Science of Acne

Scientific Research Papers:


Dietary Determinants of Changes in Waist Circumference Adjusted for Body Mass Index – a Proxy Measure of Visceral Adiposity.  Romaguera, et al.  PLoS One. 2010


Association of glycemic index and glycemic load with risk of incident coronary heart disease among Whites and African Americans with and without type 2 diabetes: the Atherosclerosis Risk in Communities study. Hardy, et al. Annals of Epidemiology. 2010

A Family History of Diabetes Mellitus is Associated with Poor Glycemic Control and Increased Metabolic Risks among People with Diabetes: Data from the National Health and Nutrition Examination Survey 1999-2004. Kuo, et al. Internal Medicine. 2010.

A calorie is a calorie, or is it?

We've all heard the old axiom "a calorie is a calorie."  This simple fact leads to a simple weight loss solution: Consume fewer calories than you burn.  Seems straightforward, right?  Unfortunately, in real life, and particularly in biology, things are rarely that simple.  The problem with the "a calorie is a calorie" concept is that while a calorie itself has a clear definition, (one calorie is the amount of energy necessary to heat 1 mL of water up 1 degree Celsius), the question of how one measures calories with regards to human metabolism is a tricky business. Understanding the limitations of the calorie system and the difference between "good" and "bad" calories can help you make informed decisions that will better help you reach your goals.

People often talk about "burning" calories as if there is some sort of internal calorie-powered steam turbine in each of us.  I think that most people realize, or at least I hope, that nothing is actually burning inside your body.  If it was, you would know.  It would probably hurt.  However, when scientists are assessing the caloric value of a food, they do just that, they burn it.  A defined sample of the material is ground up and burned in a device called a bomb calorimeter (shown at right). The calorimeter measures the amount of heat that is generated and calculates the number of calories in that food.

In contrast, in the human body large molecules are broken down into smaller components, which are then absorbed.  Absorption of nutrients happens primarily in the intestines.  Generally speaking, your body has a very difficult time absorbing large molecules, so if something isn't fully digested, it isn't available for use as energy by your body.  The Diabetes center at UCSF made a nice diagram to illustrate this process (below).

Let's say you have a delicious baked potato, with a dash of chives and a dollop of butter, of course.   Once you've finished enjoying masticating and ingesting that potato, it travels down to the stomach, where it experiences some preliminary digestion in the acidic environment of the stomach.  That breaks the potato down into medium sized molecules, which are represented in our illustration as "complex starch".  If you were to take a really close look at these complex starch molecules, they would look somewhat like a snowflake, branching and interconnected chains of starch forming all sorts of complex shapes.  These large structures are too big to be absorbed by the nutrient absorbing cells that line the intestines. In the intestines, enzymes called "amylases" break down these large complex starch molecules into their individual components, sugars like glucose. These individual sugar molecules are rapidly and efficiently absorbed by the intestines and pumped into your blood stream, where they travel throughout your body.

But what does all this mean when it comes to counting calories and understanding the difference between "good" and "bad" calories?  It is important to understand that in nature, like in life itself, nothing is truly free.  It takes energy to maintain all of the components of the digestive system: the digestive organs, the enzymes, the digestive cells, ion balances, sugar transportation, etc.  The pieces in this complex machine are constantly breaking down and in need of repair or replacement.  As a result, your body places most of its emphasis on processing foods and nutrients that are the easiest to digest and have the most caloric value.  It's the age old story of the lowest hanging fruit getting picked first.  Often times, things that are difficult to digest just don't get digested at all.  Think about the last time you had some corn on the cob or whole grain brown rice, notice anything afterward? Some undigested material perhaps?

What it all boils down to is this: Certain foods have much lower available calories than others, regardless of what the label says.  On the other hand, with some foods virtually all of the calories will be digested and absorbed by your body.   The difference is based on almost entirely on two things.  First, the source of the calories: Protein, fats, complex carbohydrates or simple carbohydrates (sugars).  Second, is the structural complexity of the food itself.  There is a huge difference between an ounce of whole grain wheat, and an ounce of whole grain wheat flour.

Because the wheat flour has already been processed into a fine powder, it is more efficiently digested by the enzymes in the gut and a greater percentage of the available calories are absorbed by your body.  This simple fact is why virtually every single diabetes treatment plan includes substituting whole grain foods for processed grain foods.  It is also the basis for their different values in the glycemic index, which measures the impact of different foods on the levels of glucose (sugar) in your blood.

The take home message here is simple: If you want to control your weight, the first step is to replace foods that are highly processed and rich in simple carbohydrates with foods that are less processed like whole grains, meats, eggs, fruit and vegetables.  It's a good dietary decision for people in general, but absolutely critical if you want to control your weight and body fat percentage.

References, Background Reading and Additional Information:


Books:


Good Calories, Bad Calories: Fats, Carbs, and the Controversial Science of Diet and Health
Gary Taubes


Nutrition: Science and Applications
Lori Smolin


The Biggest Loser Cookbook: More Than 125 Healthy, Delicious Recipes Adapted from NBC's Hit Show
Devin Alexander, Karen Kaplan and Bob Harper


Online Resources:


A calorie is Not a Calorie - An in-Depth Review


Carbohydrates - Wikipedia


The Glycemic Index - Wikipedia


The Science of Acne, click The Science of Acne.




Scientific Research Articles:


Association between Dietary Carbohydrates and Body Weight.  Ma, et al.  American Journal of Epidemiology. 2005


Whole Grain, Bran, and Germ Intake and Risk of Type 2 Diabetes: A Prospective Cohort Study and Systematic Review. Munter, et al. PLoS Medicine.  2007