If you crash in energy one to three hours after a meal, a blood sugar dip often plays a role. In a study of more than 1,000 healthy people, the dip two to three hours after eating predicted hunger and energy better than the peak that came before it (Wyatt, 2021). For an athlete, that afternoon slump explains more than you think.
I notice athletes almost always blame an energy dip after eating on sleep or training volume. Sometimes that is right. But just as often the cause sits in how your blood sugar peaks and then drops too far.
This guide explains how that dip forms, which blood values tell you about it and what you can do.
What is a blood sugar dip?
A blood sugar dip is a sharp drop in your blood sugar after a peak, usually one to three hours after a carbohydrate-rich meal. Your body releases insulin to catch the peak, and sometimes it overshoots slightly. You feel the result as tiredness, hunger and a loss of focus.
The peak itself is not the problem. It is the speed and depth of the drop afterwards that hits your energy.
That explains why you sometimes feel worse at three in the afternoon after a big sandwich than you did before lunch.
Why am I tired after eating?
Tiredness after a meal often comes from the dip that follows your blood sugar peak. A fast peak triggers a strong insulin response, and that can push your glucose just below your starting point. At that moment you feel weak, hungry and less sharp.
In the Wyatt study the link was clear. People with larger dips reported more hunger and ate more later in the day (Wyatt, 2021).
That is exactly why a simple meal adjustment often already softens your afternoon dip.
The size of your dip is partly personal. Two people can respond very differently to the same meal.
Blood sugar dips and sport: what happens around your training?
Around your training a blood sugar dip can actually help you fall apart. If you eat something sugary just before exercise, your insulin peaks, after which your glucose can drop quickly at the start of the effort. This is sometimes called rebound hypoglycemia, and you notice it as early fatigue.
Research shows that some athletes who take carbohydrates shortly before training get this transient dip (Jentjens, 2002). Not everyone is affected, but those who are sensitive feel it right away.
Timing matters more here than the amount. A gel fifteen minutes before the start lands differently than the same gel an hour before.
If you want to test it, simply move your last carbohydrates earlier and watch how you feel in the first ten minutes.
Which blood values say something about your blood sugar?
Three blood values together tell you the most about your blood sugar: fasting glucose, HbA1c and fasting insulin. Glucose is a snapshot, HbA1c gives your average over weeks and insulin shows early whether your sensitivity is dropping. The table below links each value to what it measures.
| Blood value | What it measures | Read on |
|---|---|---|
| Fasting glucose | Your blood sugar at that moment, after a night of fasting | Blood test for fatigue |
| HbA1c | Your average blood sugar over the last eight to twelve weeks | HbA1c in non-diabetics |
| Fasting insulin | How much insulin you need to keep your glucose stable | Fasting insulin and HOMA-IR |
So a single glucose value within the range does not rule out a sensitive blood sugar balance.
Fasting glucose: a snapshot
Fasting glucose measures your blood sugar after a night without food, and it is the most commonly used value. If it sits neatly within the range, that says something, but not everything. A normal fasting glucose does not rule out big peaks and dips after meals.
For an athlete that is the trap. You can be lean and trained, with a tidy fasting glucose, and still have sharp dips during the day.
So you read glucose best next to HbA1c and insulin, not as a loose number.
HbA1c: your average over weeks
HbA1c shows how high your blood sugar sat on average over the past eight to twelve weeks. It averages your peaks and dips, so it is less sensitive to what you ate on one morning. That makes it a more stable marker than a single glucose measurement.
Even in people without diabetes, a slightly higher HbA1c says something about your metabolic health. You can read more about that in HbA1c in non-diabetics.
Combine your HbA1c with your complaints and you get a fuller picture than with glucose alone.
Fasting insulin: the early warning
Fasting insulin often shows years earlier than glucose or HbA1c that your sensitivity is dropping. Your body keeps your blood sugar normal for a long time simply by making more insulin. You see that extra effort back in a raised fasting insulin.
For athletes who want to track their metabolic health, this is a valuable early marker. Read how to interpret it in fasting insulin and HOMA-IR.
A single measurement is a start. The trend over time tells you more.
Can you have a blood sugar dip without diabetes?
Yes, you can have blood sugar dips after a meal even without diabetes. This is called reactive or postprandial hypoglycemia, and it happens when your blood sugar drops too far one to five hours after eating. A single draw often misses this pattern.
Continuous glucose monitoring can make such dips visible in people without diabetes, because it measures across the whole day instead of at one moment (Nguyen, 2018).
That explains why your blood test can be normal while you still feel that dip.
If you recognise complaints such as shaking, sweating or sudden hunger after eating, that is a reason to take the pattern seriously.
What can you do about blood sugar dips yourself?
You soften most dips by adjusting the make-up and timing of your meals, not by eating less. Protein, fibre and fat alongside your carbohydrates flatten your peak, which makes the drop afterwards milder. That way you keep your energy steadier.
A few adjustments help most athletes:
- Always combine carbohydrates with protein or fat, so a sandwich with egg rather than a plain one.
- Move your last fast carbohydrates before a session to at least an hour ahead, or take them during the effort instead.
- Watch your sleep, because a short night makes your blood sugar more erratic the next day.
- Take a short walk after a big meal, which helps blunt the peak.
That way you build in stability without a strict diet.
Which blood test do you choose for energy dips?
For energy dips after eating you choose a test that measures your blood sugar together: fasting glucose, HbA1c and fasting insulin in one go. That way you see not only a snapshot, but also your average and your early sensitivity. That gives a fuller picture than glucose alone.
For most athletes a broad starting panel is logical. Our 360 Health blood test combines these blood sugar values with your other systems in one measurement.
If you want the full picture of your fatigue, read our complete guide to blood testing for fatigue.
If you prefer to test in a targeted way, pick the single value that matches your complaint from the table at the top.
What if your blood sugar is normal but you are still tired?
A normal blood sugar does not rule out fatigue, because your energy depends on more than glucose alone. Think of your iron store, your thyroid and your vitamins. A blood sugar dip is one of the puzzle pieces, not the whole picture.
According to Thuisarts and the NHG guidelines, a blood test is used in a targeted way for complaints, not as a loose check of everything at once. The RIVM also publishes population figures that help to put your own value in perspective.
My advice is simple. Follow the trend and watch your complaints.
If the tiredness persists after eight weeks while your meals and sleep are sound, discuss with your GP whether further testing makes sense.
Frequently asked questions
The questions I get back most from athletes who want to figure out their energy dips.
What exactly is a blood sugar dip? It is a sharp drop in your blood sugar after a peak, usually one to three hours after a carbohydrate-rich meal. You feel it as tiredness, hunger and less focus.
Can a blood sugar dip cause fatigue? Yes, the dip after a meal links to more hunger and less energy. In research, the dip predicted energy better than the peak before it.
Do I need to fast for a blood sugar test? For fasting glucose and insulin yes. HbA1c you can draw at any moment, because it averages your value over weeks.
Can I have a dip without diabetes? Yes, that is called reactive hypoglycemia. Your blood sugar drops too far after eating, even without diabetes.
Does eating fewer carbohydrates help against dips? Often the combination helps more than the amount. Protein, fibre and fat alongside your carbohydrates flatten your peak, which makes the dip milder.
Which value do I check first? A useful starting point is fasting glucose, HbA1c and fasting insulin together. That way you see a snapshot, your average and your early sensitivity in one go.
References
- Wyatt P, Berry SE, Finlayson G, et al. Postprandial glycaemic dips predict appetite and energy intake in healthy individuals. Nature Metabolism. 2021. PMID: 33846643.
- Jentjens RL, Jeukendrup AE. Prevalence of hypoglycemia following pre-exercise carbohydrate ingestion is not accompanied by higher insulin sensitivity. International Journal of Sport Nutrition and Exercise Metabolism. 2002. PMID: 12500984.
- Nguyen QN, Pepin SN, Chehade JM. Use of Continuous Glucose Monitoring in Detecting Reactive Hypoglycemia in Individuals Without Diabetes. Journal of Diabetes Science and Technology. 2018. PMID: 29845872.
- Thuisarts.nl / NHG. Blood testing. Accessed 2026.
- RIVM. Population figures and reference values. Accessed 2026.
Disclaimer
Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.
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