Before You Jump on the Snacks – Insulin – Intermittent Fasting Bandwagon…

The reason I am writing this post is that I wanted to give a counterpoint to something I saw a couple of weeks ago on LinkedIn (Figure 1).

At the time, I commented on this post the best I could but wanted to clarify some of my ideas here to give you a different perspective on snacking and its relationship to insulin, weight loss, and intermittent fasting based on my own understanding of the interaction between the three.

Figure 1. Snacks Cause Insulin to Spike, Insulin Promotes Fat Storage, Fat Storage is the Antithesis of Weight Loss.


Snacking, as most Americans know it, consists of eating junk food.  Junk food is high in calories and too many calories causes obesity.  Junk food, i.e., highly processed, highly palatable food, not snacks per se, causes obesitySo, if the first part of the statement read, “JUNK FOOD” I would be more on board with it.

I recently wrote a post trying to explain that not everything you eat between meals is a snack.  The junk food people eat between meals should be known as indulgences, not snacks.  And therefore, different rules should apply to them.  For many people snacks are viewed as somewhat benign, smaller portion, “cheats”, that they can get away with between meals.  We need to change this type of thinking and draw clear lines between snacks and indulgences and when and how many we consume each day.

Insulin Secretion

Secondly, I understand that you can’t get all of the nuances of nutrition into a couple lines of text, but you do not trigger insulin every time you eat.  Insulin is triggered/released from the beta cell of the pancreas when blood glucose rises to a sufficient level/threshold after a meal containing either carbohydrate or gluconeogenic amino acids.  In people with obesity, this system is broken and insulin and blood glucose may remain elevated even in the absence of snacks or carbohydrate intake.

In response to a meal or snack, the amount of insulin secreted from the pancreas and the time it takes to be removed from the blood stream is dependent on a number of characteristics of the meal and the individual’s body.  The slower the increase in blood glucose after a meal, the lower the rate of insulin that will be secreted.  If the meal contains fat, fiber or protein, digestion takes longer and blood glucose rises more slowly.  If the carbohydrate is of a lower glycemic index, blood glucose should also rise more slowly and the rate of insulin release is lower.

How insulin sensitive or resistant is the individual?  Insulin sensitivity is a measure of how efficiently insulin is able to lower blood glucose after a meal challenge.  The more insulin sensitive you are, the less insulin is required to dispose of the glucose.  The more insulin resistant you are, the more insulin is required to dispose of the blood glucose.  In general, endurance athletes are very insulin sensitive and diabetics and people with obesity are very insulin resistant.

I also find it very interesting that prolonged fasting has been shown to actually increase insulin resistance in the short-term to preserve (the finite supply of) blood glucose for the tissues that prefer it (brain and spinal cord).  Intermittent fasting over the course of weeks leads to a reduction in insulin resistance, likely due to weight loss and a negative energy balance.

The bottom line is, the amount of insulin released is very dependent on

  1. what and how much you are eating
  2. how insulin sensitive or resistant you are
  3. whether you are in energy balance (more insulin sensitive in short-term negative energy balance)
  4. what your glycogen stores look like (example: people are more insulin sensitive after lowering glycogen stores through a single bout of vigorous exercise).
Snacks and Weight Gain

Epidemiological evidence has shown that in some cases more frequent meal consumption is actually associated with a LOWER body weight.  Said differently, people who ate more snacks, actually weighed less.  This is why we had personal trainers and gyms recommending people eat 5-6 small meals/day to boost their metabolism and the number of daily calories they burned.  They phrased it as “meals”, but I guess you could say they were “pro” snack.  Here is the link to a review on eating frequency as well as all the observational papers I have found on the association between eating frequency and body weight.

Snacks and Weight Loss (RCTs)

Researchers have also looked at whether the incorporation of a snack into a diet program actually enhances weight loss due to an increase in dietary adherence and a decrease in the feelings of deprivation while dieting. 

There is evidence to suggest that snacks on a weight loss program are either a) unharmful or b) helpful but no clear evidence to suggest they are harmful (prevent) to weight loss.  Now of course it depends upon the type, frequency, and quantity of the snacks being eaten.  When done thoughtfully and intentionally, it doesn’t seem like snacking will ruin your diet.

Insulin, Weight Loss, and Weight Gain

The idea of “healing” insulin is almost spiritual.  Insulin is a hormone that doesn’t need to be healed.  There is and can be, however, beta cell burnout and death from having to be overworked for years and years.  The same type of phenomenon happens to the heart (heart failure) in response to chronically elevated blood pressure.  So, I get it that it is healthy to reduce the stress on the beta cell by increasing the insulin sensitivity of the individual or reducing the carbohydrate load of the diet.

There is fairly consistent evidence for insulin dysregulation/dysfunction being predictive of future weight gain but our ability to work in the opposite direction – to try to make someone more insulin sensitive/reduce resistance doesn’t seem to be very effective for weight loss.  The same dynamic that I mentioned earlier with eating 5-6 meals/day seems to apply.  Observationally, people who ate 5-6 meals/day weighed less than those who eat 3 or less.  However, when randomized controlled trials were conducted to see if eating 5-6 meals was better for weight loss than eating 3 meals, there was no difference between the groups.

Although not stated directly, it was mostly implied here that hyperinsulinemia (high secretion of insulin/high blood insulin) will reduce weight loss but by controlling insulin, in this case through intermittent fasting, weight loss will be greater.  The relationship between insulin secretion and weight loss is blurry with conflicting evidence for and against its relationship.  I wouldn’t base my weight loss programming on it.

And I am going to use a “straw man” argument here but, if insulin is so important to weight loss, I cannot understand why diets that drastically reduce insulin secretion by lowering or eliminating carbohydrate (i.e., Keto, low-carb, Atkins, high-fat) do not result in more weight loss over time than a more normal mixed diet?  If insulin is so important wouldn’t these diets continually win out?

Lastly, if insulin was the primary driver of weight gain, a high-fat diet shouldn’t lead to weight gain (or as much weight gain).  But it does.  Our food environment is loaded with carbohydrate so oftentimes people who eat a high fat diet are also eating a high carbohydrate diet (in terms of total calories, not a percent of calories).  If you overfeed people with fat, there will not be a prolonged insulin spike, but people will still gain weight thus uncoupling the insulin, weight gain hypothesis.

Intermittent Fasting

Finally, intermittent fasting (there are many forms) doesn’t seem to be any more effective than continuous energy restriction on weight loss.  In a review by Rynders et al., 9 out of 11 studies comparing intermittent fasting regimens with continuous caloric restriction showed no difference in weight loss between diet types.  With that being said, there are so many studies on intermittent fasting with various levels of restriction and diet therapy/education that it is very difficult to draw any solid conclusions between the studies.

If you look at the literature (individual graphs of weight loss across time), there is a fairly consistent effect of intermittent energy restriction providing better (although nonsignificant) weight loss results.  But the differences are quite small and, in my opinion, do not warrant the fervor and absolutism of INTERMITTENT FASTING IS THE BEST THING EVER!!!!!  There is far too much certainty portrayed in the snacks – insulin – intermittent fasting hypothesis on weight loss as fact when the evidence is actually pretty conflicting.

Intermittent fasting works just as well as continuous caloric restriction (traditional dieting) but really not any better.  You can intermittent fast if you want to, but you don’t have to, as some have suggested.

The Bottom Line

The explanation/story/narrative behind intermittent fasting is very compelling.  From the time we wake up till the time we go to bed, we never stop eating.  We snack way too much, and this is not normal or healthy.  Evolutionarily speaking, we are designed to switch between eating and fasting; eating when food is available and fasting between meals (oftentimes not by choice I might add).

In the daily meal cycle there are two states, the “fed” state and the “fasted” state.  The fed state lasts from the time you consume a meal (or snack) until the time that meal is completely digested and “put away” (0.5 – 3 hours depending on meal size).  When eating a normal, mixed meal (fat, protein, and carbohydrate) the body secretes insulin and carbohydrate is stored as glycogen, fat is repackaged and stored as fat, and the amino acids from protein are used to build/repair tissues.  Carbohydrate is the primary fuel burned for energy in the fed state.

In the fasted state, the hormone glucagon (insulin’s opposite to some degree) is secreted causing glycogen to be broken down into glucose (in the liver) and released into the blood stream to help regulate blood sugar.  Fat is the primary fuel burned in the fasted state.  Protein cannot be stored for fuel so by the time you reach the fasted state, amino acids (the building blocks of protein) have already been burned as fuel during the fed state or incorporated into bodily tissues.  There is a very small “amino acid pool” available but amino acids do not greatly contribute to energy metabolism.

It has been argued, and rightfully so, that if you are constantly “snacking” you are also going to constantly be in a “fed” state.  Insulin will remain high, storage will be promoted (especially of body fat), and carbohydrate will be your main fuel source.  If you are burning carbohydrate, you are not burning fat.  If you’re not burning fat, you will never lose weight.

The ability to switch between the fed and fasted states and between carbohydrate and fat as the primary fuel is known as metabolic flexibility.  Most people with obesity are NOT metabolically flexible.  Ironically, many are either really good fat burners or really good carbohydrate burners.  They are essentially “stuck” in one state or the other and I don’t think this is the result of differences in diet, snacking or insulin.  Nor do I believe that we can “fix” insulin resistance through simple meal timing changes.  The insulin dysfunction in obesity seems like too big of a problem for such a simple fix.

When people with obesity lose weight, they generally also become more metabolically flexible.  But if there is essentially no difference in weight loss between intermittent fasting and continuous caloric restriction, aren’t you choosing a diet more based on lifestyle than any metabolic advantage? 

Intermittent fasting has a great story and with the failure of so many diets I can see why people would want to try it, but I don’t think the link between snacks, insulin, weight loss, and intermittent fasting is a strong as people like to think it is.  It’s a nice clean, easy to understand story but I’m not buying it.  If you are in a negative energy balance, you are, at some point of the day in a fasted state (and have a net lowering of insulin).  I completely agree that we should alternate between the fed and the fasted states.  But I also do not believe that we need to go 24-36 hours (or other similar prolonged periods) between meals to get this effect. 

Finding recipes, meal planning, grocery shopping, cooking, food logging, exercising 300 minutes/week, weighing yourself at least once weekly, controlling your food environment, preparing for tempting situations, and modifying your behavior aren’t sexy and you don’t get “something for nothing” (a metabolic advantage) but these are the things that are required to lose weight. 

It doesn’t matter whether you intermittent fast or continuously calorically restrict, you need to incorporate these behaviors into your life.  Changing one thing (intermittent fasting) won’t fix anything in the long-run.  You need to take a comprehensive, systematic approach to weight loss.  It’s boring, it’s not sexy, but if you do it right it works in the short and the long-term.

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