Physiology of Fat Loss

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By Lauren Killey

Many clients begin their health and fitness journey with the goal of reducing ‘stubborn’ fat or ‘spot-reducing’ to drop fat from particular areas of the body. Though you cannot choose where your client will lose fat, you can use protocols to help with getting rid of stubborn fat. To do this effectively, you should have an in-depth understanding into the physiology of fat loss.

Fat cells contain beta and alpha (stubborn) fat receptors, both of which influence fat loss. In order to lose body fat, we need to first release fatty acids from the fat cells into the blood stream. Those fatty acid then need to be transported through the bloodstream to the target tissue where they will be used, and finally they need to be oxidized to create ATP. This process of ‘fat loss’ involves three key phases:

1. Mobilization/Lipolysis
2. Transportation
3. Oxidization

Fats, or triglycerides, are made up of three fatty acids and a glycerol backbone. The first step in the fat loss process, is to mobilize the fat or get the fat out of the cell, so it can be transported. If you cannot get the fat out of the cell, you cannot burn it off! In order to do this we need to stimulate the beta receptors and inhibit the alpha receptors using nutrition, training, and supplementation.

In order to mobilize the fat cells, we need to release stress hormones (catecholamines) – epinephrine (adrenaline) and norepinephrine (noradrenaline). These will bind to the beta fat receptors and activate the hormone sensitive lipase enzyme which will catalyse the release of the fatty acids into the bloodstream.

These stress hormones are released through physical stress caused by exercise, stimulant based supplementation or a more aggressive calorie deficit. So, your first step in the fat loss process will be ensuring that your client is eating in a calorie deficit and using metabolic, or conditioning focused training and cardio protocols which will stimulate enough of a stress response within the body to release the stress hormones necessary to mobilize fat from the cells.

The second phase of fat loss is transportation. Once the beta receptor is stimulated and the fatty acids are released into the blood stream, they will bind and be transported via the protein albumin to the area in the body which requires energy, for example, the muscle cells.

The final stage of the fat loss process is oxidation. Once the fatty acids have been transported to the muscle cells – or any cell which requires energy – they will be burnt off as fuel (oxidized).

This process will be dictated by an individual’s body fat levels. A client who is leaner, may find it is harder to ‘get the fat out of the cells’ as they have a higher ratio of alpha to beta receptors. When alpha receptors are stimulated by low levels of noradrenaline, they inhibit lipolysis. A high amount of adrenaline and noradrenaline is required to overcome the inhibitory effects of the alpha receptors. That is why simple caloric restriction with low intensity exercise my not work effectively for targeting stubborn fat.

We also need to get blood to the fat cells to allow blood flow to transport the fat acids out of this area. Areas with a high density of alpha receptors have poor blood flow and it is challenging to get catecholamines to these areas. Fortunately, when we inhibit the alpha receptors, we also take care of the blood flow issue.

When trying to target the ‘stubborn fat’, we need to BLOCK not stimulate the ALPHA receptors. This will require a higher catecholamine response. So, the best protocol to use with clients will be HIIT style conditioning work. This HIIT training should be performed above 80-90% of max heart rate as this will increase noradrenaline and adrenaline.  This will help to release a lot of the fatty acids into the blood stream, which will likely come from some of those ALPHA sites.  BUT since this type of effort utilizes glucose as the primary fuel source, although a lot of fatty acids will be mobilised and released into the blood stream…not all of them will be oxidised.

Post-training, we have a mild increase energy expenditure, but a lot of the fatty acids which are being transported around the blood, will ‘reesterified’ into the fat cells or restored as fat. The afterburn from HIIT is only 30-60 calories. Not that much! To ensure that all the fatty acids are oxidized, some steady state cardio should be performed after HIIT training, since steady state cardio oxidizes primarily fat as fuel. The fatty acids are already in the blood stream, and do not have to be released from the cell, meaning this is a more ideal time to oxidise fat.

The protocol we suggest is:

  • HIIT (10 to 20 mins) to mobilise the fatty acids into the blood stream.
  • Follow with 5 mins of complete rest to allow the fatty acids to be released in the blood stream
  • Finish with 20 to 40 mins of steady state to oxidise beta and alpha fat receptors.

This style of training is intense and should not be used all the time. We recommend no more than 2-3 times per week. Using a periodised nutrition and training approach will allow you to coach your clients through their fat-loss journey using different protocols at each stage, to maximize their results. 

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