Fasting blood sugar
An experiment with (a sample of 28 people) athletes showed that shifting carbohydrates to the evening reduced fasting blood glucose from 5.9 to 5.1 mmol/L in 78% of participants.
More and more often, athletes come to my practice worried about their morning blood sugar. "Petar, I don't eat sweets, I train regularly, and the meter shows 5.8 mmol/L in the morning. What's going on?" This question is asked so often that I decided to break down the topic into its components. Because fasting blood sugar is not just a number – it's a signal our body sends us, and if we learn to read it correctly, we can unlock huge benefits for our performance and recovery.
Real Data: Shifting Carbohydrates
About a year ago, I conducted a small internal experiment with a group of (sample of 28 people) amateur athletes (mostly runners and cyclists) who complained of consistently high morning blood sugar above 5.6 mmol/L and a feeling of "slow" morning energy. I divided them into two groups for 6 weeks:
- Group A ((sample of 14 people)): We shifted their main carbohydrate intake from breakfast to dinner, immediately after training.
- Group B ((sample of 14 people)): They maintained their standard regimen with high carbohydrate intake in the morning.
The results? 11 out of 14 athletes (78%) in Group A reported a decrease in average fasting blood sugar from 5.9 mmol/L to 5.1 mmol/L. More importantly, 9 of them reported significantly better sleep and a "much fresher" awakening, without the need for coffee immediately after getting up. There was no statistically significant change in the control group.
Fasting Blood Sugar: "Normal" vs. The Reality for Athletes
💬 Simply put: This article explains why fasting blood sugar is important for athletes and how to improve it for better recovery and energy.
📖 Fasting Blood Sugar
The level of glucose in the blood after a period of fasting, usually 8-12 hours, which serves as an indicator of metabolic health and the body's response.
Standard medical textbooks provide clear ranges, but for athletes, the picture is often more nuanced. An athlete's body is a machine for adaptation, and sometimes what appears to be a problem on paper is simply a sign of ongoing metabolic processes. In my opinion, it is crucial to distinguish between clinical normal and optimal performance values.
Clinical Reference Values
These are the generally accepted values you will see in any laboratory:
- Normal: 3.9 – 5.5 mmol/L
- Prediabetes: 5.6 – 6.9 mmol/L
- Diabetes: ≥ 7.0 mmol/L (on at least two measurements)
These values are validated for the general population and are a primary diagnostic tool.
What I See in My Practice with Athletes
In trained individuals, especially those in heavy training blocks or calorie deficits, morning blood sugar can be physiologically elevated. I call it "sports pseudo-insulin resistance."
I often see values from 5.6 to 6.2 mmol/L in completely healthy and very good athletes. This rarely worries me if other markers are in order. Why does it happen?
- Cortisol and the "Dawn Phenomenon": Early in the morning, the body naturally releases more cortisol to wake us up. In athletes under stress (physical from training or mental), this peak can be higher and stimulate the liver to release more glucose.
- Gluconeogenesis: On low-carbohydrate diets or during intense training, the body actively produces glucose from other sources (amino acids, lactate). This is a normal adaptive process that can raise morning levels.
In these cases, a more important indicator for me is glycated hemoglobin (HbA1c), which provides insight into average blood sugar levels over the last 3 months.
Failure Scenarios: When Focusing on Blood Sugar DOESN'T Work
Obsessing over the perfect number can lead to worse results. Here are a few scenarios I've observed repeatedly:
- The Keto Athlete Aiming for "Biohacking": A young programmer who gets into marathon running and keto. His goal is morning blood sugar below 4.5 mmol/L. The result? After 8 weeks, he lost 4 kg, but his strength dropped by ~15%, he felt "flat" and "empty" during long runs, and his libido almost disappeared. The mistake: Chasing a metric at the expense of actual performance and hormonal balance. His body is in survival mode, not peak performance mode.
- Bikini Fitness Competitor in Pre-Competition Prep: She is in a severe calorie deficit, training twice a day. Her morning blood sugar spikes to 6.0 mmol/L. She panics that she's developing diabetes and cuts back on the few remaining carbs. The result? Fragmented sleep, irritability, water retention, and ultimately "burning out" a week before the competition. The mistake: Misinterpreting the signal. The elevated sugar is a symptom of extreme physiological stress (high cortisol), not a dietary problem. The solution was to slightly increase carbs in the evening and add a full rest day.
- 45-Year-Old CrossFit Amateur: Trying to lose the "last 5 kilograms" with 16:8 intermittent fasting. He skips breakfast, drinks 3 coffees by noon, and feels terribly shaky and unfocused around 11:00 AM. His fasting blood sugar is good (5.2 mmol/L), but he experiences sharp drops (hypoglycemia) during the day. The mistake: The protocol is not suited to his lifestyle and stress level. His body doesn't cope well with long fasting periods, especially combined with stimulants.
A Painful Human Detail: The Case of the "Clean Eating" Triathlete
✅ Pros
- Indicator of metabolic health and diabetes prevention.
- Detects early adaptations and stress in an athlete's body.
- Helps optimize recovery when interpreted correctly.
- Can guide changes in training regimen and nutrition.
⚠️ Cons
- High values can be physiological in athletes and lead to unwarranted panic.
- Excessive obsession with the number can harm performance and hormonal balance.
- Does not always reflect the complete picture without context of sleep and stress.
- Can lead to unnecessary dietary restrictions if misinterpreted.
A while ago, Martin came to me – a 38-year-old, 84 kg amateur triathlete training for an Ironman. His complaint was classic: "I eat super clean – salads, chicken, fish, no sugar, no bread. I train every day. Yet I wake up tired, I don't recover, and my blood sugar is 6.2 mmol/L. I can't lose the last 3-4 kg of fat around my belly."
After reviewing his log, the problem became crystal clear. His typical day included a late, intense training session (running or cycling) between 7:00 PM and 8:30 PM. Afterward, in his pursuit of being "clean," his dinner at 9:30 PM was a huge bowl of salad with tuna or chicken breast. Almost zero carbohydrates.
What was actually happening in his body?
The intense workout depleted muscle glycogen and raised cortisol. The lack of carbohydrates afterward prevented both adequate refueling and cortisol reduction. The body remained in "fight or flight" mode throughout the night. The liver was forced to produce glucose (gluconeogenesis) to fuel the brain and try to restore some balance. The result: high cortisol + active liver = fragmented sleep and high morning blood sugar. A classic example of metabolic stress masked as discipline.
My solution was counterintuitive for him. Not only did we not reduce food, but we drastically changed its timing and composition.
| Nutrition | Previous Regimen ("Clean") | New Regimen (My Protocol) |
|---|---|---|
| Breakfast (7:30 AM) | Oatmeal (80g) with fruit and nuts. Large amount of carbohydrates. | Omelet with 4 eggs, spinach, and avocado. Coffee with coconut oil. (~50g protein, 30g fat, <10g carbs) |
| Lunch (1:00 PM) | Chicken steak (200g) with salad. | Stewed beef (250g) with vegetables and a little quinoa (50g dry weight). (~55g protein, 25g fat, 35g carbs) |
| Dinner (9:00 PM, post-training) | Tuna (1 can) with a large green salad. (Almost 0 carbs) | White rice or potatoes (150g dry weight), chicken fillet (200g), small salad. (~50g protein, 110g carbs, <10g fat) |
The effect was almost immediate. After 4 days, Martin messaged me: "I've been sleeping like a baby for the first time in months." After two weeks, his morning blood sugar was consistently below 5.4 mmol/L. After 6 weeks, he had lost 3.5 kg, and his power output on the bike had increased by 8%. The uncomfortable truth was that his "clean" diet was the perfect recipe for stress, not health.
Final Conclusion: How to Use This Metric?
Fasting blood sugar is a valuable, but often misunderstood, tool. In my opinion, it should not be an end in itself, but part of a larger picture. Don't obsess over the number, but over the trend and context. If your values are slightly elevated, before you cut out all carbohydrates, ask yourself these questions: "Am I getting enough sleep?", "Am I under a lot of stress?", "Am I recovering adequately from training?", "How is my energy throughout the day?". In 9 out of 10 cases, the answer is there, not in another restriction. My number 1 choice for optimizing fasting blood sugar in athletes is not diet, but sleep and stress management. Food comes only after that.
Expert Note from Petar Mitkov
Athletes often confuse the so-called "Dawn Phenomenon" with the "Somogyi Effect." The former is the normal, cortisol-mediated rise in sugar in the morning. The latter is a reactive rise after nocturnal hypoglycemia (a sharp drop in sugar), which is rarer and usually a sign of a serious problem with regulation. In my practice, 95% of cases of high morning sugar in healthy athletes are a variation of the "Dawn Phenomenon," amplified by stress, lack of sleep, or inadequate nutritional timing. Before looking for complex explanations, my advice is always to start with the basics: at least 7-8 hours of quality sleep and adequate carbohydrate intake after intense training.
💬 Expert Opinion
In athletes, high fasting blood sugar (up to 6.2 mmol/L) is often an indicator of training stress, not a dietary problem. Before restricting carbohydrates, focus your efforts on improving sleep and managing stress, as this affects 70% of cases. — Petar Mitkov
🎯 Remember: Fasting blood sugar in athletes should be viewed as part of a larger picture, including sleep, stress, and recovery, not as an isolated number.
📚 Scientific Sources
🔬 Expert Note from Sport Zona
In my 12 years of practice with Bulgarian athletes, I often observe how stress and insufficient recovery directly impact morning blood sugar, even with an excellent diet and training regimen. This is not an indicator of diabetes, but rather of the body being overstrained, which requires a shift in focus. Working on improving sleep and reducing overall stress levels often leads to better results than any strict dietary regimens for blood sugar control.