Spend any time reading about, or talking to others about nutrition, muscle gain, and fat loss, and one word that you’re sure to come across frequently is ‘insulin’.

I thought I’d put together a bit of an article, giving you the basics of what insulin is, what it does, and why it’s so important, not only in maintaining our physiology, but also when it comes to making positive changes in body composition.

We’ll also have a brief chat about terms such as ‘insulin sensitivity’, ‘carb sensitivity’, and ‘nutrient partitioning’.

Ok, so firstly, what is insulin?

Insulin is a hormone, produced by an organ called the pancreas. Remember a hormone is a chemical produced by your body that sends messages to other parts of the body, resulting in an action at the ‘target site’.

What does it do?

Insulin has a central role in regulating the metabolism and carbohydrates, proteins/amino acids, and fat within our body. It causes (amongst other actions) the following
  • the uptake of sugar (glucose) from our bloodstream, into our muscles, liver, and fat tissue
  • increased amino acid uptake into our cells; remember that protein is made up of lots of individual amino acids joined together
  • increased protein synthesis; the making of new proteins within our body
  • increased fat synthesis (the making of new fat in our body)
  • decreased breakdown and utilisation of fats (decreased lipolysis)
Generally, our insulin levels rise in response to us eating carbohydrates. Most of you have probably heard of diabetes, which is primarily a disease of carbohydrate/glucose regulation in the body, in which insulin plays a vital part. For people with diabetes, their body either doesn’t produce insulin, or the insulin they are producing doesn’t work very well.

Insulin sensitivity

This is a term that gets mentioned quite a lot these days, and rightly so; it’s important! What the term means, is basically how well the insulin in your body works, it’s that simple. Generally, the leaner you are, the more ‘powerful’ your insulin is, and conversely, the less lean you are, the less powerful your insulin will be.

Insulin resistance

This is related to insulin sensitivity. Insulin resistance refers to when your body is in a state of poor insulin sensitivity, that is, you cells are resistant to the effects of insulin. This is also very closely related to the term ‘pre-diabetes’ – in which patients are in a state of glucose and insulin metabolism that, if not corrected will most likely lead to diabetes. In these situations, often losing some bodyfat and improving food choices and timing is enough to prevent the development of type 2 diabetes. You’re probably starting to get the message that your glucose and insulin function is very heavily influenced by how lean you are!

Nutrient partitioning

This term sounds quite complex, however think of it simply as the process of your body sending nutrients to certain places within your body. For example, the leaner you are, the better your body is at sending carbohydrates into your muscles, rather than into fat storage. Similarly to insulin sensitivity, the opposite is true for those who are less lean.

This is part of the reason that as you progress through your MP programs (and get leaner whilst carrying more muscle) you begin to require more carbohydrates to keep progressing. Ever noticed how after say, a leg workout, for the rest of the day you legs look full and muscular? That’s due to them soaking up all the available nutrients, and (with the aid of insulin) ordering them to be shuttled in there to start the recovery process.

Carbohydrate sensitivity/carbohydrate tolerance

This term is essentially describing how much insulin your body will produce in response to you eating carbohydrates. This is partly the reason why two people with relatively similar levels of bodyfat may respond differently to (and require), differing amounts of carbohydrates in their diet. We all know the people who can eat high energy carbs at almost all meals, and not suffer…. then there’s the rest of us who start to get a little less lean if we consume too many high energy carbs outside the metabolic window.

So what does all this mean to us, and why does it matter in body transformation?

Well, quite simply, insulin is a key player when it comes to regulating bodyfat levels. Sure, there is a highly complex interplay between thyroid hormones, testosterone, estrogen, cortisol, growth hormone, the list goes on, but insulin is the big daddy!

This is where MP comes in. As we know, nutrient timing is a vital part of metabolic nutrition, and utilising MP nutrient timing protocols takes full advantage of insulin’s muscle building potential, whilst minimising its less desirable effects. Intense exercise (FIRE, and ICE anyone?) causes acute changes in our insulin function, and these changes allow us to use MP nutrition to not only enhance recovery and increase protein synthesis (muscle building!), but also decrease lipogenesis (the making of fat).

This makes the concept of the metabolic window vital, as avoiding high energy carbs outside the metabolic window helps keep our insulin metabolism quiet and stable, thereby allowing us to burn more fat, and gain more muscle! Importantly to note, there are advanced people doing MP who will consume small amounts of high energy carbs outside the window – yes, your metabolism really will become that good!

However it is also important to note that for those who haven’t completed many MP programs, or may be less advanced, regularly consuming high energy carbs outside the metabolic window may hinder your progress. This is a situation in which the less desirable effects of insulin can be ore dominant – such as turning off the breakdown of fats, and the making of new fats.

Therefore, if you want to improve your insulin metabolism and transform your body, concentrate on using a combination of intense exercise, and metabolically precise nutrient timing. Remember too, that the amount, and frequency of your high energy carb choices should not only be based on your goal, but also your current metabolic classification. If you’re unsure of ‘where you fit’, ask your MP certified trainer to metabolically classify you.

I know I haven’t given you any ‘new’ info as such, but hopefully this helps give you some more understanding about insulin, and importantly, why the metabolic window/nutrient timing is so vital.

Members of mp-body.com can download the audio of this article in the MP audio library.


Cornier M, A., et. al. (2005). Insulin sensitivity determines the effectiveness of dietary macronutrient composition on weight loss in obese women. Obesity Research, 13(4):703-9.
Diaz, E. O., Galangi, J. E., & Aguirre, C. A. (2006). Glycaemic index effects on fuel partitioning in humans. Obesity Review, 7(2), 219-26.

Gonzalez-Periz et al., (2009). Obesity-induced insulin resistance and hepatic steatosis are alleviated by omega-3 fatty acids: a role for resolvins and protectins. FASEB, 23(6), 1946-57.

Ivy, J. L., & Ferguson, L. M. (2010). Optimizing Resistance Exercise Adaptations Through the Timing of Post-Exercise Carbohydrate-Protein Supplementation. Strength & Conditioning Journal, 32(1), 30-36.

Lopes da Silva, M. V., & de Càssia Gonçalves Alfenas, R. (2011). Effect of the glycemic index on lipid oxidation and body composition. Nutr Hosp, 26(1), 48-55.

Pittas A, G., & Roberts, S. B. (2006). Dietary composition and weight loss: can we individualize dietary prescriptions according to insulin sensitivity or secretion status? Nutrition Review, 64(10), 435-48.

Poole, C., Wilborn, C., Taylor, L., & Kerksick, C. (2010). The role of post-exercise nutrient administration on muscle protein synthesis and glycogen synthesis. Journal of Sports Science and Medicine, 9, 354-63.

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