Overweight? When to Eat vs. Exercise? The Science of Insulin Sensitivity And Nutrient Timing
Does Nutrient Timing Even Matter?
One of the most common questions in the fitness industry, particularly around tactical approaches to weight and fat loss, is how training and nutrition should be timed, relative to each other.
I’ve asked myself in the past, a few variations of this question - and to be clear, there are some many different contexts in which this can be asked and answered.
Two key features of metabolic disease, type II diabetes and even cardiovascular disease are postprandial insulinemia and peripheral insulin resistance - which means the appropriateness of the level of the hormone insulin that your body secretes following a meal, and how well your body responds to that insulin to lower your blood sugar.
For overweight/obese individuals with insulin resistance (at risk of developing pre-diabetes), there is clinical evidence supporting a role for nutrient timing in improving insulin sensitivity in as little as 6 weeks!
If you’ve ever been curious about interaction between nutrient timing and exercise and how that may affect these features of insulin resistance, inappropriately-high insulin levels (common in “pre-diabetes”), read on!
What Is Insulin?
Insulin is one of the many hormones (signaling chemicals) which regulates blood sugar levels, among other processes involved in overall “energy balance”. It is secreted by specialized cells of the pancreas, and has an effect on a range of cells of the body including the liver, brain, muscle, fat and others. Simplistically, insulin signals cells to remove glucose (sugar) from the circulation and to use it to generate energy, or for storage for future use.
What Is Insulin Resistance?
When cells and tissues respond abnormally to insulin, and do not remove glucose from the bloodstream as efficiently as they should, the tissues (also called “the periphery”) are said to exhibit “insulin resistance”. This typically results in the pancreas secreting even more insulin as a compensatory mechanism to attempt to make up for the tissues being less sensitive to insulin overall. This typically leads to a situation where the body is insulin resistant and has higher-than-appropriate levels of insulin (aka insulinemia) particularly after a meal (postprandial).
Insulin resistance and insulinemia can therefore be thought of as two sides of the same coin, and the result is often high levels of sugar (glucose) in the blood. Depending on how elevated these levels are, one may be diagnosed with “Pre-Diabetes” or overt “Type II Diabetes” (T2D). [There is another form of diabetes, known as “Type I Diabetes”, which occurs via entirely different mechanisms and will not be addressed here].
Overweight, Obesity, Insulin Resistance and The Road To Diabetes
One of the most common risk factors for the development of Type II Diabetes is Obesity. Obesity is generally defined as a complex medical condition in which an excessive amount of body fat is present in a person. The causes are multifactorial including genetics (familial history), environment, nutritional and activity or exercise habits. Obesity commonly results in increased risk not only of diabetes, but hypertension (high blood pressure), heart disease and other cardiovascular disorders, stroke, sleep apnea, and certain cancers and overall commonly results in diminished quality of life.
Reducing body fat to more healthy levels therefore represents not only a cosmetic goal, but very meaningful health, longevity and quality of life objective.
The implementation of nutritional plans and approaches aimed at reducing caloric intake to support weight loss, and integration of exercise regimens that affect the output side of the “energy balance” equation (energy in (food) vs energy out (metabolism and movement) will in general, when applied consistently, result in weight and body fat loss.
Nutrient Timing vs Exercise For Insulin Sensitivity In Obesity
Identifying means of improving insulin sensitivity and reducing postprandial insulin levels, based on the discussion above, may be important targets for reducing risk of progressive metabolic disease, pre-diabetes and overt T2D.
Exercise has been shown to have acute effects on insulin sensitivity, particularly within skeletal muscle, as you might expect, which need fuel in the form of glucose to continue contracting to support movement and exercise and repair and recovery post-exercise. Longer-term effects in skeletal muscle of repeated exercise typically include increased oxidative metabolic capacity and even how much fat is contained within muscle.
With the body able to utilize glucose or fatty acids for energy generally and during exercise, nutrient availability at the time of exercising can be expected to influence what is used for fuel. A study examined whether providing carbohydrates before vs after moderate-intensity exercise (cycling) in the morning influenced the immediate and/or longer-term insulin sensitivity of obsese male participants.
Without extensively providing the data from the study, the investigators showed that in providing nutrition after exercise, there was a general increase in lipid (fat) use by skeletal muscle, while the opposite was true. In providing carbs before exercise there was an increase in whole body use of carbs for energy (including in muscle) and a reduction in lipid (fat) metabolism. This result is not surprising, given that glucose is typically the preferred fuel source, so if provided prior to exercise, the body will preferentially use glucose over fatty acids.
More interestingly, the study found that over the course of 6 weeks, with chronic exposure to morning exercise, whole body lipid metabolism was about 2-fold greater in participants who exercised fasted vs those who were allowed a meal before exercise, at the expense of carb utilization.
Exercise before rather than after carb intake resulted in measures of improved insulin sensitivity - postprandial insulin levels were lower, and measures of insulin sensitivity were higher.
It is important to note, that over the course of the 6-week study, the observed changes in overall body composition were similar regardless of carbohydrate vs exercise timing.
Practical Implications For Fasted Exercise
While this relatively small and brief study provides some clues for deciding whether to perform moderate intensity exercise before or after breakfast, this is only a single piece to a puzzle within a specific population.
What it does suggest, is that for individuals who are overweight or obese and at risk of developing pre-diabetes, exercising fasted in the morning may improve whole-body insulin sensitivity.
The potential to improve insulin sensitivity over a short course of 6 weeks is a highly significant finding and therefore exercising prior to breakfast would be an attractive first strategy for overweight or obsese individuals with underlying insulin resistance seeking to improve insulin sensitivity and halt progression to pre-diabetes or overt T2D.
While not providing (at least on a 6-week timeframe) a significantly different reduction in overall body fat as compared with having breakfast prior to exercise, other interesting nutrient timing strategies can be effectively integrated to accelerate body fat loss. These additional strategies will be discussed in upcoming blog posts, so stay tuned!