In part 1 of this series, we discussed diabetes remission, introduced the 'personal fat threshold' hypothesis, and the energy systems that regulate fat and carbohydrates.
To summarise, the key points are listed below:
- Type-2 diabetes could be defined as a disease of 'energy toxicity' within the cells, where the body struggles to regulate the energy it gets from food
- Obesity is a risk factor for type-2 diabetes, but because individuals at a healthy weight can also develop type-2 diabetes, obesity alone cannot explain the underlying cause of type-2 diabetes
- Type-2 diabetes remission can be achieved with a significant reduction in fat stored in the liver and pancreas
- Your body primarily stores fat in the form of subcutaneous fat in adipose tissue (the fat found beneath the skin)
- The 'personal fat threshold' hypothesis states that every individual has a limit at which they can no longer store fat in their adipose tissue as subcutaneous fat. It's at this point the body will have to store fat in the organs including the liver and pancreas.
Why is this important?
If we can understand what influences our body's ability to store and utilise incoming food energy (i.e. fat, carbohydrate, and protein), we can look at strategies to improve these systems (i.e. nutrition, sleep, and exercise), and thereby improve our overall health.
If your weight is currently within the 'healthy' range for BMI, and you're living with type-2 diabetes, then focusing on weight loss alone is unlikely to be beneficial to your health. However, focusing on interventions that will improve the function of your body's energy systems will result in improved health, while still allowing you to maintain your current weight.
On the other hand, if you're currently overweight or obese and living with type-2 diabetes, it’s important to focus on improving the function of your body’s energy systems alongside weight loss, as weight loss alone may not guarantee better management of your type-2 diabetes in the long-term.
To improve your understanding of the body's energy systems, we'll start by reviewing the body's largest 'pool' of stored energy: adipose tissue.
- Understanding what influences our body's ability to regulate food energy can allow us to develop a more individualised approach to the management of type-2 diabetes
- Your approach to better managing type-2 diabetes may differ depending on whether you're a healthy weight, overweight, or obese.
Fat utilisation and storage
As you'll recall from the part 1 article, fat is transported around the body in transportation molecules known as chylomicrons. They act like boats that move fat around the body to either be stored in the adipose tissue or used by the cells for energy. As a reminder, you can see this process represented in the image below. In this article, we'll be focusing on the circled area of the system.
What happens to fat when it enters the adipose tissue?
The adipose tissue has two ways of storing fat in its cells:
1) It can either expand existing fat cells (known as adipose tissue hypertrophy)
2) It can create new fat cells (known as adipose tissue hyperplasia)
Whether your body has the ability to increase the fat cells in size or in number will influence your personal fat threshold. Research has shown that increasing the number of fat cells (hyperplasia) is more beneficial than the cells increasing in size (hypertrophy). We'll be discussing this in more detail in part 3.
What's the body's main energy source?
The body's two primary sources of fuel are fat and glucose (carbohydrate). The brain will prioritise using glucose as its energy source because fat can't be transported directly into the brain (it needs to be converted to ketone bodies first).
However, when it comes to the rest of the body (i.e. the muscles and our internal organs such as the heart and liver), there are two key factors that determine what fuel the cells will use for energy, these are:
- Carbohydrate consumption
- Exercise intensity
1. Carbohydrate consumption
As we mentioned in part 1, when we consume a carbohydrate rich meal, insulin levels will rise and move the glucose out of the bloodstream and into the cells to be used for energy, while fat will be 'spared' and stored.
So in the context of the hours after a carbohydrate rich meal, glucose becomes the primary fuel source for muscles and organs. If we were to consume a lower-carbohydrate meal, your body would switch to burning fat sooner.
2. Exercise intensity
In healthy individuals, fat is the preferred source of fuel at rest and lower-intensity exercise. This is represented in the graph below. In lower to medium-intensity exercise, fat is the 'dominant' fuel with less glucose being used. It's only when you perform higher-intensity exercise that body starts to use more glucose and it then becomes the 'dominant' fuel.
This is because the body can burn glucose at a faster rate compared to fat. When you're at rest, or doing low-intensity exercise such as walking, your body doesn't require the 'fast fuel' of glucose, so it mainly burns fat.
How is fat utilised for energy?
When a cell needs to use fat for energy (such as cells in the muscle or heart), chylomicrons will transport fat into the cell's mitochondria to be 'burnt' for energy. Mitochondria are known as the 'powerhouses' of the cell. It's here that a cell will take nutrients, such as fat and carbohydrate, and 'burn' them to create energy. This is represented in the image below.
What determines whether fat is used for energy or stored in the adipose tissue?
There are two key factors:
1) Energy balance
When we consume too much energy, our cells (in our muscles for example) will be required to 'take in' a lot of energy. Too much energy within the cells can be damaging as this can impact on their ability to function properly. To prevent this, cells will block or slow down the transportation of fat from chylomicrons into the cell. The cells essentially tell the boat that the port is full and to continue on its journey!
The chylomicrons will then transport fat to the adipose tissue to be stored as subcutaneous fat.
2) Insulin levels
We discussed in the part 1 article how one of the roles of insulin is to act like a key and move glucose out of the blood and into the cells, but insulin also plays a key role in fat regulation.
When insulin levels are high (i.e. after a carbohydrate rich meal), your body switches into a state of 'fat storage'. Insulin tells the adipose tissue to take in more fat to be stored, and the cells use carbohydrate (glucose) for energy instead.
This prevents the body's cells from being 'overrun' with energy from both carbohydrate and fat. As mentioned above, this excess energy in the cell can be damaging and prevent it from functioning properly.
These two factors are often interlinked. With excess energy storage, you'll typically see high insulin levels too.
- Fat cells in the adipose tissue can take in excess energy in two ways, by expanding in size or in number
- Fat is utilised in the mitochondria of the cells to be 'burnt' for energy
- Apart from the brain (which will primarily use glucose for energy), the body's preferred fuel source will depend on how many carbohydrates you eat and your exercise intensity
- Excess energy intake and/or high insulin levels determine whether the body will use fat for energy or store it in the adipose tissue
How do carbohydrates influence this process?
As mentioned above, the two main factors that determine whether fat is stored or used for energy are: excess energy intake and insulin levels. Eating a large amount of carbohydrates (particularly refined carbohydrates found in processed foods) can not only lead to excess energy intake, but it can also result in high insulin levels.
In part 1 of this series, we discussed how carbohydrates enter the cells from the bloodstream with the support of insulin. Following this, carbohydrates (in the form of glucose) are then either used for energy or stored as glycogen for later use. As a reminder of this process, take a look at the image below.
While we have a large capacity for storing fat in our adipose tissue (a lean adult male will have around 50,000kcal stored), we have a very limited capacity for storing glycogen (around 1500kcal in our muscles and 400kcal in our liver).
Imagine you have two suitcases, one is a lot smaller than the other. The smaller suitcase represents your glycogen stores, so this is the place that the carbohydrates from the food you've eaten get stored if they're not used by the body for energy. The larger suitcase represents your fat stores.
When you fill up your small suitcase by putting too many clothes in (i.e. maxing out your glycogen stores by eating more carbohydrates than your body can store), you need to find another place to put the rest of your clothes (or in this case, the rest of the glucose). It would make sense that you’d probably put the rest of your clothes in the bigger suitcase.
In the case of the body, it will convert the excess glucose into fat so it can be stored in the adipose tissue. It's transferring energy from one form (carbohydrate) to another (fat).
The storage of fat in the adipose tissue (whether it's from fat, or converted from glucose) is a protective mechanism. It's preventing excess energy build-up in the cells, excess energy circulating in the blood stream, and the accumulation of fat in our internal organs.
- Consuming excess carbohydrates influences the two main factors leading to fat storage by contributing to excess energy intake and increasing levels of insulin in the body
- We have a large capacity for storing fat (50,000+kcal), but we only have a limited capacity for storing carbohydrate as glycogen (~2,000kcal)
- When you've reached your glycogen capacity, excess carbohydrate will be converted to fat in the liver or the adipose tissue
- Storing excess energy in the form of subcutaneous fat in the adipose tissue is a protective mechanism
Why is this important?
If the ability to store fat in the adipose tissue is what's protecting our cells and organs, then what happens when it can no longer do this?
It's at this point that fat will start to be stored within the internal organs in the form of visceral fat, and chronic diseases such as type-2 diabetes and non-alcoholic fatty liver disease will start to develop or accelerate.
An example of this is displayed in the image below. Participants A and B have a similar BMI and also have the same volume of total fat stored around their 'trunk' (ASAT + IAAT = total trunk fat). However, participant B has a much higher volume of intra-abdominal adipose tissue (IAAT, i.e. fat stored within the internal organs) and a lower volume of abdominal-subcutaneous adipose tissue (ASAT i.e. fat stored around internal organs beneath the skin), compared to participant A.
This is indicating that participant A has a greater capacity for storing fat in the form of subcutaneous fat in the adipose tissue compared to participant B. And therefore, will have a much lower risk of developing type-2 diabetes.
- When an individual reaches their personal fat threshold, increased levels of fat will be stored in the internal organs
- This can't be predicted by BMI or total fat storage (body fat percentage)
- As the volume of fat stored in the organs increases, so will our risk of developing type-2 diabetes and other chronic diseases
Take home message
The body's ability to store fat in the adipose tissue as subcutaneous fat is an important process for ensuring:
- Excess energy doesn't build up in the cells
- Excess energy doesn't build up in the bloodstream
- Fat isn't stored within the internal organs
This is the key to understanding the 'personal fat threshold' hypothesis. Our risk of developing chronic diseases such as type-2 diabetes and non-alcoholic fatty liver disease can't be solely predicted by our weight, or body fat percentage. The most important factor is where the fat is stored, rather than how much.
In part 3 of this series, we'll review the adipose tissue in more detail, and look into how insulin resistance has an influence on our personal fat threshold.
Article written by Robbie Puddick, Registered Nutritionist and Second Nature Health Coach