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?