Blood Glucose
It seems like everyone is hung up on sugar- should you eat it, should you not? Does it make food better or worse? Is it hiding like the boogeyman in our packaged foods, waiting to send us into a metabolic spiral? Personally, we think sugar gets a bad rap. While it is true that you should enjoy all of life’s wonders in moderation, sugar may be one of the single greatest tools to support your performance. Carbohydrates are one of the 3 major macronutrients, and they happen to be your body and brain’s primary source of fuel. Many endurance athletes likely intuitively understand this relationship with carbohydrates, as drinks with sugar and carbohydrate-rich snacks are always in their cage, bib, or running belt. So why do we need carbohydrates and what is all this hubbub about blood glucose?
Glucose Physiology
In pop culture, conversations about sugar usually refer to table sugar or sucrose. This is a mid-complexity carbohydrate made of two basic carbohydrate units- glucose and fructose, which can also be referred to as sugar. Glucose, a 6 carbon molecule, is the basic unit that every starch or sugar, from bread to soda, is broken down and converted to for your body’s use for energy production and mechanical (or mental) work.
When we eat, our body breaks down carbohydrates into their simplest form, shuttles them to the liver to be converted to glucose, and then sends them into the bloodstream. When blood glucose levels rise after a meal, insulin is released to help facilitate glucose movement into cells to provide instant energy or store for later use in the form of glycogen or fat. When we have not eaten for some time, glucagon is released and targets fat cells and the liver to produce glucose from stored molecules, sending that glucose into the blood to maintain energy production in your body.
Other hormones are involved, but glucagon and insulin are the major players in blood glucose regulation. These hormones are secreted antagonistically from the pancreas to maintain baseline levels of sugar in your blood. They can be disrupted in disease states, as seen in Type 1 and 2 diabetes or metabolic disease. Outside of pathology, however, your body is constantly fighting with glucagon, insulin, and other hormonal regulators to keep blood glucose values within a normal range (70-100 mg/dL).
Glucose Physiology in Exercise
As previously mentioned, exercise is a stressor in that it disrupts homeostasis; messing with the baseline glucose levels your body is trying so hard to maintain. When we lift weights, run, or bike, our muscles have an enhanced energy demand. Carbohydrates must be used to maintain mechanical work and muscle contraction. Luckily, our muscles can store carbohydrates in the form of glycogen after we eat to provide energy to these working tissues. However, this resource is finite. Once we deplete glycogen in muscles during prolonged exercise, we must use carbohydrates from the blood. As mentioned, these can be released from the liver, or manufactured glucose from fat metabolites.
So if I workout long enough, my fat will get converted to glucose to be used for muscle fuel, I will be shredded and happy, and all will be well, right? Not really. Higher intensities or long-term exercise require fast fuel that can be easily broken down without oxygen, or exogenous substrates after internal stores have been depleted. Unfortunately, fat does not fit this bill. To make matters more confusing, fat requires energy from carbohydrates for its personal oxidation. Takeaway? You cannot escape carbs. For this reason, carbohydrates must be consumed in exercise lasting longer than 60- 90 minutes 1,5.
Carbohydrate Fueling In Exercise
Beyond sheer energetics (calories in and calories out), carbohydrate feeding during exercise has been shown to enhance endurance performance. Even if you theoretically had enough stored fat (which you do) and were exercising at the optimal intensity to utilize it, eating sugar would make you faster and decrease your perception of exertion 10, 11. And recall, fat is burned in the fire of carbohydrates- meaning you need glucose oxidation for fat oxidation.
Although there have been reported placebo effects of carbohydrate supplementation (uh oh, did we just drop the veil?), the general rule of thumb for exercise is to maintain blood glucose levels, despite mounds of blogs, data, and anecdotes telling you to eat as much carbohydrate as humanly possible, and this will linearly increase your performance in a 1:1 fashion. In fact, carbohydrate oxidation can only occur at about 1 gram per minute, and eating more than this may be obsolete and upsetting to the gut. For this reason, the best rule of thumb is to try and maintain your blood glucose within your normal range, and eat about 60 grams of carbs per hour for exercise bouts lasting longer than 60-90 minutes. If you feel like eating more carbohydrates helps you and it doesn’t hurt your stomach, by all means, munch away 1,5,11,12.
Continuous Glucose Monitoring Technology
As with many aspects of health and performance, personalized real-time data is becoming more and more accessible. Continuous Glucose Monitors (CGMs) first emerged in Diabetes management, but have leaked into the fitness community. While they are awaiting FDA approval for non disease states, there are companies who will sponsor your use of this technology.
CGMs are inserted to the arm or abdomen. A thin filament senses glucose levels in the interstitial fluid- the space between your blood vessels and cells, and a smartphone can be used to display live values 2. It has been suggested that CGM technology in healthy individuals could be used for early diabetic screening, lifestyle optimization, stress monitoring, and athletic performance 6.
CGMs For Athletes
Data on CGMs in athletic populations is still emerging, and is quite honestly incomplete. While further studies should be completed, early investigation suggests promising use case scenarios for CGM technology. For example, one study evaluating blood glucose in ultra marathon runners via CGM found that elevations in glucose levels were associated with running speed, but only to a point. While the study did not evaluate how CGM could be used to inform in-exercise fueling in real time, these data suggest that there is a minimum threshold for blood glucose elevation that must be achieved to optimize performance that could be monitored by CGM 7.
Additionally, early data suggest that CGM could be a useful tool in evaluating and promoting recovery from exercise. For example, exhaustive exercise has been shown to impair glucose tolerance as seen through elevated overnight glucose values, in addition to stimulating glycemic variability (fluctuations in blood glucose) for many days 4,13. These are markers of metabolic dysfunction, and could be heralds speaking to a need for prolonged, attentive recovery. Studies wielding overtraining models similarly exhibit impaired glucose tolerance in humans following the excessive training paradigm. CGM could fill an important need in this space by acting as a messenger for rest, recovery, and nutritional intervention. For example, using CGM to maintain elevated blood glucose following exercise could allow athletes to maximize glycogen resynthesis in preparation for future training 10.
Blood Glucose- More Than Carbohydrates?
To best utilize CGM technology for both athletic and healthy populations, it behooves us to understand the factors that affect blood glucose concentrations- and it is more than just the carbohydrates you eat. Glycemic index describes the effect that any given carbohydrate- from honey to potato- will have on one's blood glucose concentration over time 14. However, a potato could affect one individual's blood glucose differentially based on the time of month (if the individual menstruates), their sleep habits, their individual stress levels, and many other external factors.
Menstrual Cycle and Blood Glucose
Fluctuations of hormones across the menstrual cycle not only trigger changes in the uterine lining and mood, but have potent effects within the body. The menstrual cycle can be split into two phases: follicular and luteal. The follicular phase happens after menstruation leading up to ovulation, and is characterized by relatively low estrogen and a very low progesterone to estrogen ratio. The luteal phase occurs after ovulation and is associated with high progesterone and estrogen levels, as well as a high progesterone to estrogen ratio. The hormone composition in the luteal phase can lead to impaired glucose tolerance, slow glucose appearance rates (attenuated production of glucose from the liver) and reduced glucose disappearance rates (uptake and use by tissues). Some studies have also reported impaired performance and glucose dynamics in exercise during the luteal phase. This suggests that carbohydrate feeding in exercise may need to be altered based on the time of cycle, and that menstruating people will benefit from awareness of their carbohydrate tolerance dependending on the phase of the cycle 3, 8, 15.
Stress and Blood Glucose
It may be quite intuitive that stress can affect our physiology and, more specifically, metabolism. When we are chronically stressed from psychosocial stressors such as school, work, relationships, etc., our body secretes Cortisol at rates that are not normal in the human body across the day. This hormone blocks the action of insulin, keeping glucose in the blood, and even promotes the production of more glucose. This leads to elevated blood glucose levels that can damage insulin sensitivity, glucose tolerance, and blood vessels. In fact, studies evaluating mice under models for human depression and anxiety exhibit hyperglycemia (elevated blood glucose) and hyperphagia (excessive caloric intake). Subsequently, it is important to evaluate stress levels in order to maximize recovery, glucose utilization in exercise, and overall well being. CGM technology may have the potential to help attune individuals to their own stress levels based on observed blood glucose data 6, 9, 17.
Sleep and Blood Glucose
Sleep physiology has become popular science, with spheres of health, medicine, and media becoming enamored with its importance and how to maximize sleep efficiency. Well, no surprise, sleep is intimately tied to blood glucose metabolism. In fact, restricting sleep to 4 or 5 hours can significantly impair next day glucose and insulin tolerance. Even allowing subjects to sleep as much as they could on weekends following 5 days of sleep restriction was insufficient to return their glucose dynamics to baseline. In turn, consistent sleep of sufficient length is extremely important in modulating blood glucose dynamics during wake 16.
