Just a spoonful of…

Sugar.  When we consume highly refined, energy dense food or drink, particularly if it contains sugar or HFCS we become “awash” in excess calories.  Our metabolism no longer has to be efficient and instead has to try to “waste” calories.  If we are also inactive and sitting at a desk all day, it has no way to get rid of the calories but to store them as body fat. Indeed, storage in the adipose tissue is promoted by insulin, the activity of which is stimulated by high blood sugar.

But fat cells are not inert – they are almost like the “Adipose”, a race of vaguely humanoid blobs of fat from the Dr. Who TV program. It is a complex and highly active metabolic and endocrine organ producing hormones and cytokines, with connective tissue, nerves, immune cells, and its own blood supply. 

Normally fat cells have an important role in maintaining triglyceride and free fatty acid levels, which are the preferred source of energy for muscle contractions at low activity level.  The heart, for example, normally gets 60-70% of its energy from fatty acids and triglycerides and 35% from carbohydrates, and the heart can also use ketone bodies.

In skeletal muscle at low exercise intensities, slow oxidative (type I) muscle fibers with a high capacity for fat oxidation are recruited first.  As intensity increases, faster (type II) fibers with higher glycolytic capacity are activated.  This parallels the changes in fuel selection from mainly fatty acids to mainly carbohydrate.

Fat is stored as triglycerides and has to be converted to fatty acid and glycerol, and transported and the release and availability of fatty acids is a relatively slow process. To keep the process going, at rest we continually recycle triglycerides to fatty acids and back.

When we do moderate exercise the recycling drops from about 70% to about 25% – the process to provide FFAs doesn’t have to do a “cold start”.   When we stop exercising, the recycling quickly increases back to 80% to recover the fatty acids into triglyceride stores and to protect against an overshoot of plasma FFA levels.

Even so…

Fatty acids have more energy per gram than carbohydrates, but produce a little less ATP per “cycle” and consume a little more oxygen per ATP produced, but overall this is still a very efficient process.   The energy yield from a gram of fatty acids is approximately 9 Kcal, compared to 4 Kcal for carbohydrates.

Cardiac muscle has a large number of mitochondria supporting continuous aerobic respiration via oxidative phosphorylation and is highly aerobic and resistant to fatigue. The heart can also recycle lactate, which is very energy efficient when lactate is oxidized to pyruvate, which itself can then be used aerobically in the TCA cycle.  So the “lactate” by-product of exercise is an efficient source of energy for the heart, which has to increase its output to support the exercise that’s producing the lactate.  Isn’t that quite amazing?  Both lactate and ketone bodies are also usable by the brain, but that’s another story.

The metabolism of glucose requires less oxygen than metabolism of fatty acids to produce the same amount of energy stored in the form of ATP.  However each gram of palmitate produces significantly more energy (3.6 times greater) than that derived from a gram of glucose.  So it makes sense that we get about 70% of our energy from fatty acids at low levels of activity.

When we go to high intensity activity, we switch to more glucose metabolism, and this occurs very rapidly if, for example, we are in the ”fight or flight”  mode brought about by adrenaline (epinephrine).

Weight loss

With the proper training we can stimulate biogenesis of more mitochondria and more efficient use of fatty acids at higher levels of activity.  We can use more fat as we exercise, and that is very efficient.  Think of it like a hybrid car engine that more efficiently uses high-energy stores (battery) and adds the gasoline motor when needed.  And the gasoline engine tops up the batteries.  In our case we are quite capable of producing glucose from stored fat (gluconeogenesis), but if we always have excess glucose, we never need to.

When oxygen is abundant (e.g. we’re only walking!) and food is scarce, there is an advantage in utilizing fatty acids for fuel as opposed to using glucose.

So our bodies make the most out of such energy sources as are available, and mitochondria are the place where this energy is made available.  Fatty acids must be activated in the cytoplasm by conversion to the acyl-CoA derivative (which uses some energy) and then enter mitochondria for beta-oxidation, which takes place in the mitochondrion.

The total ATP per mol of tripalmitoyl glycerol is 3 x 129 = 387 (for the fatty acid) plus 20 for glycerol = 407.  Subtract the 2 mol ATP for each FA to form acetyl – CoA derivative = -6, and the total yield is 401 ATP per mol of triglyceride.  Compare this with the 36 mol of ATP per mol of glucose.

So how much energy do we have stored?  If I weigh 68 kg and have 14% body fat that’s about 9.5 kg of fat.  I could go down to about 11% body fat and the difference is about 2.2 kg of fat.  If my basal (resting) metabolism is 1500 kcal a day and I have 2,200 g of triglycerides available, I have sufficient stored energy for about 11 days.   If I was at 22% body fat, I’d have more stored energy.  So being at the top end of the “normal” body fat range might be an advantage if something such as an illness, accident, earthquake, or surgery prevents me from being able to eat much for 10 days.

Normally we use more of whatever nutritional energy is available (in our body), so if we have low blood glucose and insulin levels and high free fatty acids, we’ll use mainly fatty acids. If we consume carbohydrates, then blood glucose levels will rise stimulating insulin secretion and depressing fatty acid levels.  So we’ll use mostly glucose.  And if there’s too much glucose to use, we’ll store it in adipose tissue.

In fact storing energy “wastes” about 5-10% of the energy, but that’s no problem when energy is available in excess.  There’s also a “loss” in recovering the stored energy from fat stores, but again that’s an unavoidable “cost” of “banking” and transporting the energy.

Whole body lipid oxidation increases progressively during prolonged aerobic exercise, even when work rate is maintained constant.  It has to, because we can’t store enough energy in the form of glycogen to sustain long duration exercise.  Our capability to do long duration exercise is thus dependent on our ability to use FA as a fuel.  An increased capacity of mitochondria is therefore an advantage for endurance exercise and we can train so as to upregulate the mitochondrial capacity to oxidize FA.

During exercise at moderate intensity (60% VO2 max) we get about a 40% increase in free fatty acid (FFA) availability which remains significantly elevated for at least 3 to 6 hours after exercise, before decreasing to about 10% after 24 hours.   The increase is proportional to the net energy used in the exercise and is higher in people who have a low resting plasma FFA.   So why is this of interest when we exercise?

The best intensity for weight loss?

You will no doubt read about the differences in opinion regarding weight loss – whether exercise at low, moderate, or high intensity, or interval training has the best effect.

It is all to do with calories!

But it’s not a simple equation.  Carbohydrate is the major fuel source during moderate- to high-intensity exercise, although the contribution of fatty acids increases with training. What is interesting is that substantial fatty acid use continues after exercise. It’s called post-exercise lipid oxidation (PELO) and this uses about the same energy during recovery regardless of whether exercise was at 55% or 75% of HRR when the amount of energy used during exercise is the same.  So it is efficient to use high-intensity interval training because you will use the energy more quickly, and get a better cardiovascular benefit, and still get the same PELO effect and contribution to weight loss.

For a simple calculation, during moderately intense exercise at 65% HRR we may use about 10 kcal a minute for 30 minutes. That’s 300 kcal, with perhaps 100 kcal of those coming from lipids. After exercise, at rest over the next three hours, we will use about the same total of 300 kcal, of which about 80-90% or about 250 kcal come from lipids providing that the energy used is not replenished with carbs or fat straight after exercising! [Note below.] This increased use of lipids continues for many hours.

So if someone is trying to lose weight it may be appropriate to have a small (typically half-portion) protein shake after exercise but to wait a few hours more before eating. If you are also on a calorie restricted diet, your body will increase its lipid oxidation over a period of time (it takes at least a few days or weeks) as its limited carbohydrate energy stores are reduced and glucose homeostasis becomes increasingly dependent on gluconeogenesis.

[Note:] Most of us are eating almost all the time and so this effect does not happen. If we want the beneficial effect of exercise on total energy expenditure and from post-exercise lipid oxidation, we need to be in a temporary fasting state.  We can quickly get used to the feeling – it will soon become a friend!  It’s NOT hunger.  You can take advantage of this process in the 2MD weight loss program.

Agree? Disagree? Do share your opinion.