Draw a curve that shows desirable blood glucose levels during a day. A Test.

Last update and review: December 26, 2020.

A short summary.

This article contains tests that should stimulate your thinking and make you better at understanding and managing blood glucose levels. The example used in the article describes blood glucose fluctuations in a group of athletes who consume between 45% and 65% of their calories as carbohydrates. But blood glucose may spike excessively also in athletes and sedentary people who follow a low carb diet.

Fluctuations of blood glucose in a group of recreational athletes.

In a recent study, Thomas et al., investigated fluctuations of blood glucose in a group of recreational athletes. The study subjects, their training and other details are provided in the figures and tables below.

Continuous glucose monitoring data for the study subjects.

The 2-hour postprandial meal response is highlighted in red, and periods of exercise are highlighted in black.

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Individual graphs of blood glucose for each athlete.

The intakes of carbohydrates by the study subjects.

The following figure shows the average intake per day of carbohydrate, sugar, fiber, as well as the recommended daily intake of carbohydrate, with the upper recommended boundary of 65% of calorie intake and the lower recommended boundary of 45% of calorie intake.

A summary of the physical and metabolic variables of the group of recreational athletes from the study by Thomas et al.

The proportion of time spent at different blood glucose levels by the study participants.

The figure below shows the proportions of time spent at different blood glucose levels by the study participants. On the vertical axis, 24 hours is 1, or 100% of time.
For each athlete, the curves represent cumulative continuous glucose monitoring (CGM) data for 6 days of the study period.
The bottom figure is the CGM signal with 2 hours from the beginning of each meal and snack removed. The green band represents the normal range.

Analysis.

Almost half of the study participants spent a majority of their time with blood glucose of above 6 mmol/L. Almost all have blood glucose spikes reaching prediabetic levels. Only one participant spent substantial time below 4.0 mmol/L which was largely due to significantly lower energy intake compared to recommendations. It is the participant 3 (ATH 3). The athlete 3 (ATH 3) is a 50-year-old man who, most likely unintentionally, practices “caloric restriction” by consuming 1877 kCal/day while his requirement is 3661 kCal/day. ATH 3 is the closest to a “low carb diet” with about 250 grams of carbohydrates a day. ATH 3 occasionally experiences blood glucose spikes into a pre-diabetic area and has the highest fasting insulin secretion. ATH 3 has a phenotype that is far from optimal.

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Most of the athletes in the study by Thomas et al. eat at the lower level of the recommended carbohydrate intakes, and one, ATH 3, eats less than what is recommended. Yet, a majority of the athletes have suboptimal blood glucose fluctuations.

How reasonable and well-thought are the recommendations on carbohydrates intakes? But before discussing this question, let us explore the question of the test below.

A Test of Physiological Literacy.

1. Are there any athletes who are diabetic? If so, who of the athletes is diabetic?

Carbohydrates intakes recommended by the International Society of Sports Nutrition (ISSN).

Kerksick et al., 2018 (2), authored the 2018 update of the recommendations of the International Society of Sports Nutrition (ISSN).

Carbohydrate needs for recreational athletes according to the ISSN.

Kerksick et al., 2018 (2):

Individuals engaged in a general fitness program and are not necessarily training to meet any type of performance goal can typically meet daily carbohydrate needs by consuming a normal diet (i.e., 45–55% CHO [3–5 g/kg/day], 15–20% PRO [0.8–1.2 g/kg/day], and 25–35% fat [0.5–1.5 g/kg/day]).

Carbohydrate needs for athletes involved in 2–3 h per day of intense exercise performed 5–6 times per week according the ISSN.

Kerksick et al., 2018 (2):

Athletes involved in moderate amounts of intense training (e.g., 2–3 h per day of intense exercise performed 5–6 times per week) typically need to consume a diet consisting of 5–8 g/kg/day of carbohydrate to maintain liver and muscle glycogen stores.

Carbohydrate needs for athletes involved in high volume intense training (e.g., 3–6 h per day of intense training in 1–2 daily workouts for 5–6 days per week) according the ISSN.

Kerksick et al., 2018 (2):

Athletes involved in high volume intense training (e.g., 3–6 h per day of intense training in 1–2 daily workouts for 5–6 days per week) may need to consume 8–10 g/day of carbohydrate in order to maintain muscle glycogen levels.

The recommendations of the ISSN applied: a daily intake of 700(!) grams of carbohydrate in a group of young elite cyclists.

In a recent study that we reviewed in another article on this website, the researchers set the daily intake of carbohydrates 700 grams a day in a group of young elite cyclists:

The daily intake of carbohydrate was 700 grams a day(!) and the total caloric intake was 4800 kCal.

The researchers put the young athletes on a 4800 kCal a day diet. The daily intake of carbohydrate was 700 grams a day(!). How much the athletes were eating at baseline is not clear. However, since the athletes remained weight-stable, we may presume that the caloric intake was similar before the experiment.

Another application of the ISSN recommendations: daily intakes of carbs in a group of racewalkers.

Burke et al., 2017 (1), investigated the impact of different diets on the performance and physiology of elite racewalkers. There were three experimental diets. In the two high carbohydrate diets, the daily intake of carbohydrates was set at 8.6 grams per kilo of body weight.

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Health personalities: Shawn Backer’s blood glucose.

Shawn Baker is a health personality and a proponent of carnivore diet. Shawn Baker himself follows a zero-carb carnivore diet. Shawn Baker about his diet, fasting and postprandial blood glucose in 2018:

I’ve eaten anywhere from 4-6+ lbs of meat per day.

…my own lab values showed an elevated HBA1c of 6.3 and that was paired with an also high fasting glucose of 127.

When I eat my normal diet I have never seen a blood glucose above 135.

A Test of Physiological Literacy: Draw a curve that would show the proportions of time spent by Shawn Baker at different blood glucose levels.

 

2. Use the figure and the idea from the study by Thomas et al., Shawn Baker’s description of his diet and blood glucose, and draw a curve that would show the proportions of time spent by Shawn Baker at different blood glucose levels.

3. Draw a curve that would show the optimal distribution of time spent at different blood glucose levels, a curve that you would like to see for your own blood glucose.

A Test of Physiological Literacy: Can exercise cause hyperinsulinemia?

Thomas et al. wrote:

Athlete 09 (Ath09) and athlete 05 (Ath05) undertook the most exercise during the monitoring period, averaging 102 and 158 min/day respectively. This high training load could have contributed to the low time spent by the two athletes in the interval between 4.0 and 6.0 mmol/L of blood glucose, as exercise is known to increase blood glucose and induce hyperglycemia and hyperinsulinemia as a result of this catecholamine response. References: 17,19,20

3. Is the statement by Thomas et al. correct? Does exercise induce hyperglycemia and hyperinsulinemia?

This particular test requires knowledge of several important facts on the interaction of hormones. It may take time to figure out the entire picture. However, the importance of this particular piece of Physiological Literacy is highlighted by the fact that Thomas et al. struggle to provide a good explanation of their observations.

Selected references:

1. Burke et al. J Physiol. 2017 May 1;595(9):2785-2807.

2. Kerksick et al. J Int Soc Sports Nutr15, 38 (2018).