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Insulin Sensitivity and Fat Loss


Kilo Klub Member / Verified Customer
Kilo Klub Member
Verified Customer
Nov 15, 2006
Insulin Sensitivity and Fat Loss
by Lyle McDonald

Over the years, bodybuilding nutrition has divided itself into three fairly distinct categories (I’m going to leave out the ones I consider voodoo nonsense) which are high-carb/low-fat, moderate carb/moderate fat, and low-carbohydrate. Low carb-diets can be further subdivided into high or low fat as well as cyclical or non-cyclical. I discuss each in more detail in Comparing the Diets.

In theory, you can make arguments for or against any of these approaches in terms of superiority. In the real world, it’s not quite that simple. You can always find folks (and this is true whether they are bodybuilders or just general dieters) who either succeeded staggeringly well or failed miserably on one or another approaches.

Before going on, I want to mention that protein recommendations tend not to vary that significantly between diets and most of the arguments tend to revolve around the varying proportions of carbohydrate and fats in the diet and that’s what I’ll be focusing on here. Simply, I don’t consider low-protein fat loss diets in the equation at all for the simple fact that they don’t work for anybody but the extremely obese. Any dieting bodybuilder or athlete needs 1-1.5 g/lb lean body mass of protein on a diet. Possibly more under certain circumstances.

My general experience has been that individuals who respond very well to high-carbohydrate/lower fat diets tend to do very poorly on low-carb/higher-fat diets. They feel terrible (low energy and a mental fog that never goes away), don’t seem to lean out very effectively and it just doesn’t work.

This cuts both ways: folks who don’t respond well to higher carbs do better by lowering carbs and increasing dietary fat. Sometimes that means a moderate carb/moderate fat diet, sometimes it means a full blown ketogenic diet. I should also note that some people seem to do just as well on one diet as another.

Some of this may simply be related to adherence although this tends to be less of an issue in bodybuilders (who take obsessiveness to a new level). Carb-based diets make some people hungry even if they follow all the ‘rules’; so they eat more and don’t lose fat effectively. For many of those people, reducing carb intake allows better calorie control in the long-term. People who hate moderation tend to like cyclical ketogenic diets, they can handle no-carbs during the week and massive carb-ups on the weekend; moderate carbs drive them crazy.

But how does all of the above help the neophyte dieter looking to diet down. Put differently, how can someone know ahead of the fact what diet might be optimal for them? Current research is starting to explore a link between diet and genetics and suggesting biological differences in how people respond to diet; that might explain some of the real-world results I described above.

With regards to fat intake, studies have identified what researches call low and high-fat phenotypes (phenotype is just a technical word for the interaction between your genetics and your environment) (1). Some people appear to be better able to increase fat burning in response to higher fat intakes; they stay lean in the face of such an intake. Others, however, do no such thing. Other aspects of metabolism and appetite were associated with being either a high- or low-fat phenotype.

Unfortunately, no practical way of determining which one you might be ever came around. It was also never exclusively determined if the effect was due to inherent biology or simply adaptation to a habitual diet. But the point still stands, biologically, some people seem better able to increase fat oxidation in response to higher fat intakes than others. I think this goes part of the way to explaining the response (good or bad) to high-fat ketogenic diets. People who upregulate fat oxidation well tend to thrive on them; people who don’t just get bloated and don’t lose fat well.

More recently, an interaction between diet effectiveness and both insulin sensitivity and insulin secretion after a meal has been proposed (2). Noting that all of the research to date has been on obese individuals (not dieting bodybuilders), I still think it explains some of what is going on. As well as allowing us to predict ahead of time which diet someone might do best on.

A Very Brief Primer on Insulin Secretion and Sensitivity

To understand the research I want to talk about next, I need to briefly discuss two different but somewhat related aspects of insulin metabolism: insulin sensitivity/resistance and insulin secretion.

As I imagine all of the readers of this know, insulin is a storage hormone released in response to eating with carbohydrates having the largest impact on insulin secretion, protein having the second greatest and fat having little to no impact on insulin secretion. Insulin sensitivity refers to how well or poorly the body responds to the hormone insulin. Individuals who are insulin resistant tend to have higher baseline insulin levels because the body is releasing more in response to try and overcome the resistance.

And while a great majority of insulin resistance is determined by lifestyle (training and diet play a huge role, as does body fatness), so do genetics. At the same bodyfat level, insulin sensitivity can vary nearly 10 fold for genetic reasons. So it’s possible that even lean athletes and bodybuilders could have some degree of genetic insulin resistant (I’ll talk about how to determine this at the end of the article). As it turns out, individuals also differ in how much or how little insulin they release following a standardized meal; some people release more insulin than others in response to a meal. While this can be related to baseline insulin sensitivity, it doesn’t have to be.

It turns out that both issue relate to fat/weight gain or loss (2). In contrast to what is generally believed, good overall insulin sensitivity tends to correlate with weight/fat gain and insulin resistance is thought to be an adaptation to prevent further fat/weight gain. However, some research suggests that a tendency to release too much insulin in response to feeding may predispose people towards weight/fat gain. One huge confound in all of this, mind you, is that high insulin secretion tends to make people eat more. Studies of diabetics find that decreasing insulin secretion with drugs tends to cause a spontaneously lower food intake (2).

The Impact of Insulin Sensitivity or Insulin Secretion on Response to Different Diets

While the research is in its infancy, there have been studies examining the weight loss response relative to either insulin sensitivity or insulin secretion. For the most part, no major difference in terms of weight loss has been found in subjects with different insulin sensitivities (2). However, at least one study found that the specific diet given interacted with baseline insulin sensitivity to determine the magnitude of weight loss (3). In that study, obese women with either high or low insulin sensitivity were placed on either a high carb (60% carb, 20% fat) or low carb (40% carb, 40% fat) diets.

So there were four groups: high carb/insulin sensitive, high carb/insulin resistant, low carb/insulin sensitive, low carb/insulin resistant. The results were intriguing: insulin sensitive women on the high carb diet lost nearly double the weight as insulin sensitive women on the low-carb diet. Similarly, insulin resistant women lost twice the weight on the low-carb diet as on the high carb diet. Unfortunately, it’s not clear what caused the divergent results. The researchers mentioned a gene called FOXC2 which is involved in energy expenditure and found that it was upregulated in the individuals who responded best to diet; further research into this topic is needed (3).

Even less data relates to insulin secretion status and diet although a recent study suggests that it may (4). In that study, subjects were given either a high glycemic load (60% carbs, 20% protein, 20% fat) or a low GL diet (40% carbs, 30% protein, 30% fat diet) and weight loss was examined relative to baseline insulin secretion. In that study, subjects with high insulin secretion lost more weight on the low glycemic load diet while subjects in the low insulin secretion group lost slightly more on the high glycemic load diet.

Getting to the Point

Overall, I think the limited data available on both high and low fat phenotypes as well as how individuals with differing baseline insulin sensitivity/secretion respond to diets supports the observations occurring in the real world in terms of both subjective feelings on a given diet as well as the weight/fat loss response. So how can we put this to use?

Unfortunately, there’s no easy way to see if you’re a high or low fat phenotype so I’ll focus on insulin sensitivity. There are a lot of complicated and impractical ways to determine insulin sensitivity and insulin secretion. All involve blood work and looking at either baseline insulin or blood glucose or how insulin changes in response to a meal.

However, in practice, there are signs as to whether you have good insulin sensitivity or not and possibly whether you over-secrete insulin. Here’s two very simple questions to ask yourself regarding your response to diet.

1. On high-carbohydrate intakes, do you find yourself getting pumped and full or sloppy and bloated? If the former, you have good insulin sensitivity; if the latter, you don’t.

2. When you eat a large carbohydrate meal, do you find that you have steady and stable energy levels or do you get an energy crash/sleep and get hungry about an hour later? If the former, you probably have normal/low levels of insulin secretion; if the latter, you probably tend to over-secrete insulin which is causing blood glucose to crash which is making you sleepy and hungry.

I consider it most likely that superior bodybuilders couple excellent insulin sensitivity with low insulin secretion in response to a meal. This would tend to explain why bodybuilders have often gravitated towards high carb/low-fat diets and been successful on them.

At the same time, mediocre bodybuilders frequently get less than stellar results from that same diet. Lowering carbs and increasing dietary fat seems to be more effective in that case some of the low-carb bulking strategies out there probably work better for those individuals. The same goes for fat loss. Cyclical low-carb diets such as my Ultimate Diet 2.0 or the more generic cyclical ktogenic diet (CKD) described in my first book The Ketogenic Diet allow such individuals to briefly enjoy the benefits of heightened muscular insulin sensitivity.

Putting it Into Practice

If you have good insulin sensitivity and low insulin secretion, odds are you will do well with a traditional bodybuilding type of diet which means high protein, highish carbs and low fat. Let’s say you’re consuming 1 g/lb of protein at 12 cal/lb. That’s 33% protein. If you go to 1.5 g/lb, that’s 50% protein. That leaves you with 50-67% of your calories to allocate between fat and carbohydrate. 15-20% dietary fat is about the lower limit as it becomes impossible to get sufficient essential fatty acids below that intake level. So, at 1 g/lb, your diet will be roughly 33% protein, 47-52% carbs (call it 45-50%) and 15-20% fat. If protein goes to 50% of the total, carbs should come down to 35% of the total with 15% fat.

If you’re not insulin sensitive and/or have high insulin secretion, a diet lower in carbs and higher in fat (don’t forget that protein can raise insulin as well) is a better choice. Assuming, again, 40% protein, a good starting place might be 40% protein, 20-30% carbs and 20-30% fat. A further shift to a near ketogenic (or cyclical ketogenic) diet may be necessary, 40% protein, 10-20% carbs and the remainder fat may be the most effective. If protein is set higher, up to 50% protein, carbs would be set at 10-20% with the remainder (20-30%) coming from dietary fat.

Summing Up

Hopefully the above has given you some insight into choosing what might be an optimal fat loss diet without having to go through so much tedious trial and error. However, please don’t treat the above as more than a starting point. Adjustments to diet in terms of calories or nutrient intake should always be based on real world fat loss. You should be tracking your fat loss every 2 weeks (4 at the most); if you’re not losing at a reasonable rate (1-1.5 lbs fat loss/week), you need to adjust something.


Blundell JE, Cooling J. High-fat and low-fat (behavioural) phenotypes: biology or environment? Proc Nutr Soc. 1999 Nov;58(4):773-7.
Pittas AG, Roberts SB. Dietary composition and weight loss: can we individualize dietary prescriptions according to insulin sensitivity or secretion status? Nutr Rev. 2006 Oct;64(10 Pt 1):435-48. Review.
Cornier MA et. al. Insulin sensitivity determines the effectiveness of dietary macronutrient composition on weight loss in obese women. Obes Res. 2005 Apr;13(4):703-9.
Pittas AG et. al. A low-glycemic load diet facilitates greater weight loss in overweight adults with high insulin secretion but not in overweight adults with low insulin secretion in the CALERIE Trial. Diabetes Care. 2005 Dec;28(12):2939-41.

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