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?

11 comments:

Exceptionally Brash said...

Thank you so much for posting this. You are really helping explain the details of these very interesting studies.
Many low carb diet plans start with a very strict plan for a few weeks, and then many back off and add a few carbs back in.
Do these studies explain why this works for some and not for others? I know that for me, I get out of fat-burning mode very quickly, and seem to lose the want to burn fat after a few carb meals.

Stargazey said...

You may be right, Exceptionally Brash. In Part Two I'll try to explain some of the practical side to all of this.

From the unfortunately bad experiences I've seen with zero-carb, I would say that cutting out all carbs is not a good idea. You risk missing out on phytonutrients, and you risk elevating your blood sugar from all that protein. But there are some tricks that can help you cope with poor oxidation of fatty acids. Low-carb is one of them.

David Isaak said...

One thing that nags at me about the Houmard research is that, if I understand correctly, the formerly obese subjects achieved their weight reduction through gastric bypass or something similar.

From a metabolic point of view, I might expect these people to be different from those who achieved their results through targeted diet changes and increased exercise. (Just as I would expect the metabolic effects of losing 25 pounds through exercise to differ from losing 25 pounds by cutting off a leg.) In general, the results of these experiments seem to suggest a half-dozen others that need to be done.

In any case, though, I'm eager to see your second installment!

Joe Cannon said...

Maybe I missed it, but has the RQ been measured in large numbers of overweight and "normal" weight people to see if there is a difference?

Joe
Joe-Cannon.com

Stargazey said...

Joe Cannon, I don't think RQ has been measured in lots of lean and obese people, but it has been measured in a few. Google "obesity" plus "RQ" or "RER" and you should find some representative articles. An example is Lower Excess Postexercise Oxygen Consumption and Altered Growth Hormone and Cortisol Responses to Exercise in Obese Men. The presence of metabolic syndrome may contribute to a higher RQ/RER in overweight people, as indicated here, Fat oxidation at rest predicts peak fat oxidation during exercise and metabolic phenotype in overweight men.

berto said...

Great stuff, Stargazey.

I was way too swamped with other reading to allow me to read up on mitochondrial dysfunction, but I was interested in doing so at some point - so thanks for the introduction.

I'm eager to find out "what can be done about it", and I'm thinking that it's related to removing the supply of exogenous glucose and waiting until the little engines adapt to a new environment...

Thanks again.

-Al

Blaisjp said...

Well, that explains the lady down the hall who only eats carbs because she says fats make her fat and is skinny as a rail. Then there is me that only eats fats because carbs make me fat. So, are you insulin resistent or carnitine palmitoyltransferase resistent. I'll take the insulin resistance because it's easier to pronounce.

majkinetor said...

Low carb community that want to loose weight should aim to optimize all pathways related to fat burning. Bringing insulin down may not be enough. That sole action will make FFA available but that still doesn't mean they will be burned. If something is wrong with beta oxidation pathway, bringing insulin down may even promote fat accumulation outside of the adipocytes which is never good.

So, IMO, we should supplement specific molecules.

Carnitine might be obvious and it turns out that grape seed extract promotes expression of CPT:

"GSE supplementation increased mRNA levels of lipolytic genes such as carnitine palmitoyltransferase-1 (CPT-1) and decreased mRNA levels of lipogenic genes such as acetyl CoA carboxylase (ACC)."

Some level of carnitine palmitoyltransferase II deficiency is not to be excluded, it looks like its not that uncommon.

Externally carnitine come from meats so vegetarians are deficient.

Internally, carnitine biosynthesis require methionine and lysine and vitamin C, vitamin B6, niacin, and iron.

As you are as strong as your weakest link, deficiency of any of above will be problematic, especially coupled with exercise and inadequate diet. Vitamin C deficiency is 100% certain for all people due to inborn error in metabolism. Lysine is EFA, and you easily become deficient too. Premenopausal woman often have iron issues and those can be aggravated with infections.

Choline deficiency diet is known whey to induce NAFLD in lab animals.

To further support mitos, CoQ10 is essential as this is the one that people older then 30 are deficient for sure.

So, I suggest, for those that do not want to wait for part 2 to start with:

1. 60-100mg CoQ10, if possible in reduced form
2. Choline, 600 mg
3. Acetyl-Carnitine or Carnitine around 500mg
4. Vitamin C, 6-10g
6. Vitamin B complex, 50mg each

Diet should be low carb, high fat, low omega-6, high omega-3, every day nuts (rich in Lysin and other amino acids are important minerals like Mg)

David Isaak said...

@majkinetor: "...Lysine is EFA.."

Huh?

majkinetor said...

Not EFA but EAA, I misspelled.

Essential Amino Acid.

That means human body can not make it and you must take it from the diet, so with inadequate diet you can become deficient.

David Isaak said...

Oh, I know about Essential Amino Acids. But your typo had me thinking there was some classification of lysine I had missed before!