Monday, November 21, 2011

Impaired Mitochondrial Function and Obesity, Part Two

Last time we learned that extremely obese people, weight-reduced people and possibly people who will eventually become obese all appear to have a harder time oxidizing fatty acids for fuel than do lean people. Because fatty acids are oxidized in the mitochondria, researchers have begun to look for mitochondrial defects as an explanation for this problem.

Possible defects
Several types of mitochondrial defects have been observed. These are summarized by M.M. Rogge and by J.A. Houmard in their review articles. (The picture above comes from the Rogge article.) Both reviews cite a 2002 study by Kelley et al. that used electron microscopy to show a 35% decrease in skeletal muscle mitochondrial area in obese vs. lean subjects. In some obese subjects, but not lean subjects, there were also large vacuoles which appeared to be degenerated mitochondria. Obese subjects also tended to have mitochondria with less clearly defined inner structure and narrower cristae.

The reviews also cite a 2005 study by Ritov et al. showing lower mitochondrial DNA content (fewer mitochondria) in obese subjects and reduced electron transport chain activity of the mitochondria, even after adjustment for the reduced mitochondrial content. This is consistent with a number of studies that show an increase in activity of certain glycolytic enzymes and a decrease in activity of other enzymes related to oxidative function in obese vs. normal-weight subjects. (See the two review articles for lots of references.) One of the important enzymes that has a lower activity in obese subjects is carnitine palmitoyltransferase 1 (CPT1, the smallest green rectangle in the drawing above), the enzyme that regulates and facilitates the entry of long-chain fatty acids into the matrix of the mitochondria.

To summarize, there are several possible reasons that obese people may have a harder time oxidizing fatty acids than they should. (1) They have fewer mitochondria. (2) They have smaller mitochondria. (3) Their mitochondria have structural problems that are visible by electron microscopy, and some of their mitochondria may even have degenerated completely. (4) Their mitochondria have reduced oxidative activity.

Now what?
So far, I have painted a fairly bleak picture. It’s even bleaker when you read the articles I’ve referenced and realize that while metabolic flexibility is poor in obese people, it’s even worse in people with type 2 diabetes. (I’ve cited only the lean vs. obese in this discussion, but many of my citations also include a type II diabetes group as well, and these typically perform worse than the obese subjects do.)

It’s possible that some people have a genetic predisposition against metabolic flexibility. However, because obesity and type 2 diabetes become more prevalent with increasing age, it’s also possible that we are gradually poisoning our mitochondria, so that our surviving mitochondria are the ones that prefer to metabolize carbohydrates. These survivors not only tend to shunt fatty acids into storage, but they also resist metabolizing the fatty acids that are mobilized out of storage between meals. Our low energy production (and the easy availability of food) encourages us to eat more carbohydrate to provide the ATP we need for daily life. We could propose various possible mechanisms for gradual mitochondrial poisoning but at this point it is only speculation. In any case, we can’t change our genetics, and we can only hope that what we currently call a healthy lifestyle is genuinely healthy for our mitochondria.

Possible interventions
On the positive side, there do seem to be a few things we can do to improve our metabolic flexibility. The first of these is mild-to-moderate exercise. In 2007 Solomon et al. described a 12-week program of moderate aerobic exercise in older obese people that improved (decreased) their respiratory quotient by 0.04. In 2010 Meex et al. asked older male type 2 diabetics to exercise twice a week for 30 minutes on a cycling ergometer and to perform resistance exercise once a week. Before the training program, their metabolic flexibility was about 60% of that of a group of matched controls. After twelve weeks, their metabolic flexibility was the same as that of the control group, and the protein content of their electron transport chain proteins had increased by 275%.

It is possible that more vigorous exercise may not be as helpful as mild-to-moderate exercise for restoration of metabolic flexibility. When the body’s AMP to ATP ratio increases, it activates adenosine monophosphate (AMP) kinase. In order to restore high ATP levels, the AMP kinase does a number of things including downregulation of physical activity and upregulation of feeding behavior. Because of this, it may be necessary for a mitochondrially impaired individual to titrate their exercise so that there is just enough to promote mitochondrial flexibility but not so much that it would cause an AMP kinase-mediated drive to eat more and exercise less.

Once metabolic flexibility is somewhat restored, it is important to take advantage of it. Because carbohydrate will always be metabolized first, it makes sense to decrease the availability of this substrate to the mitochondria. Meals should be low in carbohydrate, moderate in protein and relatively high in fat, to keep the mitochondria in fat oxidation mode as much as possible. Snacks should be avoided because each time carbohydrate is consumed, it moves to the front of the line in the mitochondrial queue. (For an interesting discussion of the effect of exercise, high-fat meals and improvement of the respiratory quotient in healthy young men, see Smith et al.)

As mentioned earlier, carnitine palmitoyltransferase 1 (CPT1) is a major control point for the entry of long-chain fatty acids into the mitochondrion. A third strategy for improving fatty acid oxidation is to circumvent CPT1 by consuming medium-chain fats like coconut oil and butter, rather than fats that contain long-chain fatty acids. Medium-chain fatty acids are metabolized differently than long-chain fatty acids because they can diffuse across plasma membranes without the help of transporter proteins. Thus, they can find their way into the mitochondrial matrix and present themselves to the beta oxidation machinery, to the TCA cycle, and to the electron transport chain without the need to deal with gatekeeper CPT1 proteins that are either downregulated or present in insufficient amounts. According to Houmard, circumvention of the CPT1 chokepoint may be helpful in increasing fatty acid oxidation and decreasing insulin resistance. However, this line of reasoning involves a fair amount of handwaving and probably needs a clinical study or two to back it up.

There it is. Mitochondrial dysfunction may be a plausible explanation for some forms of obesity. If mitochondria fail to oxidize fatty acids, both ingested and de-novo synthesized fatty acids will be preferentially routed to and will tend to remain in storage. The fact that weight loss by itself does not improve fatty acid oxidation in mitochondria explains why it is so easy to regain weight on a diet that is fairly high in carbohydrate. The fact that mitochondrial defects can be accumulated over time explains why a person can eat all sorts of foods and remain a normal weight while he or she is young, but when middle-age approaches, as often as not, so will the middle-age spread.

There are lots of other explanations for obesity, and this may not be a definitive one. But if you suspect that it might apply in your own case, it may be worth it to try (1) a mild-to-moderate level of exercise, (2) a low-carb, moderate-protein, high-fat diet and (3) replacing some of the long chain fatty acids you've been eating with medium chain ones. Enjoy that exercise machine or walking program and bon app├ętit!

Tuesday, November 15, 2011

Impaired Mitochondrial Function and Obesity, Part One

As obesity increases around the world, it’s natural to wonder why so many people are packing on the pounds. The standard answer—calories in exceed calories out—sounds reasonable, but in practice the conscious limitation of calories does not seem to work very well for controlling obesity. A few weeks ago Peter at Hyperlipid described an idea about the obesity problem that’s completely different. Defective mitochondria. I’d like to expand on that here. If you already know about mitochondria, skip the next section. If you’re like me, you’ve forgotten what you knew and a review wouldn’t hurt. (If, on the other hand, you are a true-blue biochemist, you'll notice that I'm gliding over some details in order to make the explanation easier to understand.)

Mitochondria explained
Mitochondria are granular organelles found in the cytoplasm of most eukaryotic cells. They have an outer membrane, and a multiply-folded inner membrane. Inside the second membrane is a viscous matrix containing a large number of proteins used to produce energy for the cell. The picture of a mitochondrion above comes from a 2009 review article by M.M. Rogge, The role of impaired mitochondrial lipid oxidation in obesity. If you click on the picture to open it in a new window, it will be easier to follow this discussion.

The brown elliptical line represents the outer membrane of the mitochondrion. The gray area is a somewhat schematic representation of the inner membrane. That membrane actually follows the folds (cristae) surrounding the white matrix, but this level of detail would make the picture confusing. Just say that the gray area is the inner membrane. The whole mitochondrion resides inside the cytosol of the cell, which, as you will recall, has a cell membrane of its own.

At the top of the picture are three columns, representing the three macronutrients available to cells: Triglycerides (fats), Glucose (representative of carbohydrates) and Amino Acids (from proteins). These have already made their way inside the cell and are presenting themselves to the mitochondrion as potential sources of cellular energy.

(1) Triglycerides have to be broken down to free fatty acids and then converted to fatty acyl-CoA in order to cross the two membranes and enter the mitochondrial matrix. There they are converted to many two-carbon units of acetyl-CoA by beta oxidation and produce some energy. The two-carbon acetyl-CoA units are converted to more energy by feeding into the TCA/citric acid/Krebs cycle, illustrated at the center of the mitochondrion. High-energy molecules are produced (NADH and FADH2), and these go to the respiratory chain that resides in the inner mitochondrial membrane. This is represented by the yellow ovals labeled I, II, III and IV at the bottom of the drawing. The respiratory chain uses NADH and FADH2 to produce ATP, which in turn provides energy for the cell.

(2) Glucose is first broken down by glycolysis into two molecules of pyruvate in the cytosol. The pyruvate is transported across both of the mitochondrial membranes and is converted to two of the two-carbon acetyl-CoA units in the matrix. Just like the acetyl-CoAs from free fatty acids, the two acetyl-CoAs from a molecule of glucose feed into the TCA cycle and ultimately produce ATP through the respiratory chain.

(3) Amino acids are also converted into forms that can cross the mitochondrial membranes and feed into the TCA cycle. This is presented for completeness, but will not be discussed in detail.

Metabolic flexibility
When a meal of fats and carbohydrates is eaten, both substances are taken up into cells. Although both macronutrients are available to be converted into energy, typically the mitochondrion will use the carbohydrate first. The insulin that is secreted in response to carbohydrate ingestion inhibits fatty acyl-CoA oxidation and routes fatty acyl-CoA toward fat synthesis in the cytosol. Insulin enhances glucose oxidation by upregulating the enzyme that converts pyruvate to acetyl-CoA and feeds it into the TCA cycle. By a multistep feedback mechanism this also inhibits carnitine palmitoyltransferase 1 (CPT1, the smallest green rectangle in the drawing), the enzyme that mediates the transport of fatty acids into the mitochondrial matrix.

In normal cells after an hour or two, insulin will decline and less glucose will be available to the mitochondrion. Free fatty acids will still be present in the cytosol and will finally be allowed to transit as fatty acyl-CoA into the mitochondrion via carnitine palmitoyltransferase 1. Once inside the matrix, they will produce energy through beta oxidation, the TCA cycle and the respiratory chain. This is called metabolic flexibility. When carbohydrate is present, the mitochondrion will preferentially use carbohydrate. When free fatty acids are present but carbohydrates are in short supply, the mitochondrion will normally switch over to using fatty acids for fuel.

Mitochondria use different amounts of oxygen when they metabolize carbohydrates and fats. This is expressed as the Respiratory Quotient (RQ) or the Respiratory Exchange Ratio (RER). When carbohydrate is used as fuel, more CO2 is produced for a particular amount of oxygen consumed and the RQ is higher. The RQ number for pure carbohydrate is approximately 1.0. When fat is used for energy, less CO2 will be produced for a particular amount of oxygen and the RQ will be lower. The RQ for pure fat is about 0.7. The RQ for protein varies with the specific amino acid content but is about 0.8. Now we get to the meat (pun intended) of the matter.

Impaired metabolic flexibility
Since the early 1990’s, evidence has been accumulating that obese individuals have a depressed ability to oxidize free fatty acids in skeletal muscle. It further appears that defects in the mitochondria of skeletal muscle are responsible for this impaired lipid oxidation. Two review articles that discuss these phenomena are Intramuscular lipid oxidation and obesity by J.A. Houmard and The role of impaired mitochondrial lipid oxidation in obesity by M.M. Rogge.

It is possible to measure the relative use of carbohydrate or fat for fuel by the mitochondria by measuring the Respiratory Quotient. However, it is also possible to measure the ability of mitochondria to oxidize fatty acids by infusing radiolabeled palmitate (a free fatty acid or FFA) into a patient and subsequently measuring the appearance of radiolabeled CO2 as an indication that the palmitate has been oxidized.

Houmard cites an article in which Thyfault et al. compared [13C] palmitate oxidation in three groups of women. They studied lean controls (average BMI was 23), extremely obese women (average BMI was 41) and weight-reduced women (had undergone gastric bypass surgery at least a year before, had lost at least 100 pounds and had an average BMI of 34). When they infused [13C] palmitate into these women, the results were surprising. The lean controls oxidized about 66% of the [13C] palmitate in the basal state and about 85% of it during exercise. However, not only the extremely obese women but also the weight-reduced women oxidized much less palmitate under basal and exercise conditions. In addition, the low percentage of [13C] palmitate oxidation was almost identical in the extremely obese and the weight-reduced women. One would hope that weight reduction would improve metabolic flexibility, but apparently it does not.

According to Houmard, the decrease in free fatty oxidation by extremely obese and weight-reduced subjects is supported by a series of studies done at East Carolina University in Greenville, North Carolina. As shown in the figure above, biopsies of skeletal muscle, muscle homogenate and primary muscle cell culture all showed a large decrease in fatty acid oxidation by extremely obese subjects (and in some cases by weight-reduced subjects) when compared with lean controls. Both in vivo (real life) and in vitro (test tube) studies seem to confirm that obese subjects and weight-reduced subjects have difficulty with the oxidation of fatty acids.

Even pre-obese subjects may be destined for fatness because their mitochondria prefer to oxidize carbohydrates rather than fats. Rogge cites two longitudinal studies (Zurlo et al., 1990 and Seidel et al., 1992) that indicate that normal weight subjects who demonstrated preferential oxidation of carbohydrates rather than fatty acids were more likely to gain weight over time. However, these findings were not supported in a subsequent longitudinal study published by Katzmarzyk et al. in 2000. It is possible that some of us are doomed to become fat because we start our lives with mitochondria that prefer to oxidize carbohydrates and oxidize fatty acids relatively poorly. Or not. The data from the literature is not overwhelming on this.

To be continued…
That’s probably enough for this time. I have a bunch more to say, but there is a limit to how much science can be absorbed at one sitting. I do promise that it won’t be two months before I publish Part Two: How can defective mitochondria explain the difficulty some people have with the oxidation of fatty acids and what can be done about it?