Original episode & show notes | Raw transcript
This document provides an in-depth explanation of the key sports nutrition concepts discussed in the Empirical Cycling Podcast episode featuring nutritionist Namrita Brooke, Ph.D. The goal is to move beyond simple recommendations and explore the physiological mechanisms behind fueling for performance.
A core theme of the discussion is that there is no one-size-fits-all nutrition plan. What works for one athlete may cause severe gastrointestinal (GI) distress in another.
Key factors contributing to this individuality include:
Genetics: Variations in genes can influence the number and efficiency of nutrient transporters in the gut.
Dietary History: An athlete accustomed to a high-carbohydrate diet will have a digestive system that is better adapted to processing carbohydrates during exercise compared to someone on a low-carb diet.
Training Status & History: A highly trained, veteran cyclist’s body is more efficient at utilizing fuel and diverting blood flow than a novice’s.
Environmental Conditions: Heat, humidity, and altitude all change the body’s physiological demands and can affect gut tolerance.
This individuality is why the podcast cautions against the “armchair experts” who give advice based solely on personal experience. True optimization requires systematic, evidence-based self-experimentation.
One of the most common issues for endurance athletes is GI distress. The podcast explains that this is often related to osmolality.
Definition: Osmolality is a measure of the concentration of dissolved particles (solutes) in a solution. In sports nutrition, this refers to the concentration of sugars, electrolytes, and other molecules in a drink or gel.
The Body’s Baseline: Human blood plasma has an osmolality of approximately 280-290 mOsm/kg. This is considered isotonic.
Hypertonic Solutions (>290 mOsm/kg): These are more concentrated than blood. Most gels and sugary drinks (like undiluted Gatorade or soda) are hypertonic. When a hypertonic solution enters the small intestine, the body must pull water out of the bloodstream and into the gut to dilute it before it can be absorbed. This process can lead to:
Bloating and cramping
Dehydration (as water is pulled from circulation)
Delayed gastric emptying
Hypotonic Solutions (<280 mOsm/kg): These are less concentrated than blood. Water and well-formulated sports drinks are hypotonic. They are absorbed rapidly and facilitate hydration.
The key takeaway is that consuming highly concentrated, simple sugars without sufficient water can turn your gut into a “knot” because your body is forced to divert resources to dilute the fuel before it can be used.
The concept of “training the gut” is not a myth. It refers to the process of adapting the digestive system to tolerate and process larger amounts of food and fluid during exercise. This happens via two primary mechanisms:
Upregulating Nutrient Transporters: The small intestine has specific “doorways” or transporters to move nutrients from the gut into the bloodstream. For carbohydrates, the main ones are SGLT1 (for glucose) and GLUT5 (for fructose). Regularly consuming carbohydrates during training signals the body to build more of these transporters, increasing the gut’s absorption capacity. An athlete who rarely eats on the bike will have down-regulated transporters, leading to a bottleneck where fuel sits in the gut unabsorbed.
Improving Gastric Emptying: This refers to the rate at which food and fluid move from the stomach into the small intestine. This is also trainable. By consistently consuming larger volumes during training, the stomach becomes better at handling and passing that volume onward, reducing feelings of bloating and sloshing.
Practical Application: If your goal is to consume 90g of carbs per hour, but you currently only manage 30g, you must increase your intake gradually over weeks of training. A sudden jump will overwhelm the system.
The podcast provides a tiered approach to carbohydrate intake, emphasizing that the context of the ride and subsequent training days is critical.
Low-Intensity (Endurance/Zone 2): 30-60 g/hour. The goal here is not just to fuel the current ride but to spare muscle glycogen for the next day’s harder workout. Even at low intensities, the body uses a surprising amount of carbohydrate.
Moderate-Intensity (Tempo/Threshold): 60-90 g/hour. As intensity increases, the body’s reliance on carbohydrates as a fuel source rises dramatically.
High-Intensity (VO2 Max/Anaerobic): Fuel before and after. It’s nearly impossible to digest food during maximal efforts because blood is shunted away from the gut to the working muscles. The strategy is to start with completely full glycogen stores and refuel immediately after the intervals if the ride continues.
The 90-120 g/hour “Pro” Level: This high rate is not determined by body size but by the maximum absorption and oxidation capacity of a highly trained gut. An 80kg male and a 50kg female pro can both absorb and utilize this amount because their transport mechanisms are highly upregulated through years of training.
The body’s ability to absorb carbohydrates is limited by its transporters.
The Glucose Bottleneck: The SGLT1 transporter for glucose becomes saturated at an intake rate of about 60 grams per hour. Consuming more than this in pure glucose form will not lead to greater absorption; it will just sit in the gut.
The Fructose “Side Door”: Fructose uses a different transporter, GLUT5. By consuming a blend of glucose (or its polymer, maltodextrin) and fructose, athletes can absorb carbohydrates through two channels simultaneously, pushing the total absorption rate above 60 g/hour and up towards 90-120 g/hour.
Optimal Ratios: Early research pointed to a 2:1 ratio of glucose-to-fructose. More recent findings, as mentioned in the podcast, suggest a 1:0.8 ratio may be even more effective and tolerable for some, especially at very high intake rates. (Note: Table sugar, or sucrose, is naturally a 1:1 blend of glucose and fructose).
A critical insight from the podcast is that on-bike nutrition cannot be separated from your overall 24-hour energy intake.
Getting a “Head Start on Recovery”: Fueling properly during a ride is the first step of recovery. It spares your internal glycogen stores, meaning you have less of a deficit to replenish afterward. This is paramount in stage races or heavy training blocks.
The Dangers of Under-fueling: An athlete who finishes a ride severely depleted may not be able to fully restore their muscle glycogen overnight. It can take 2-4 days to fully recover from a deep “bonk.” This creates a compounding deficit, leading to poor performance and fatigue in subsequent sessions.
Carb Loading & Water Weight: The fear of gaining weight from carb-loading is addressed as a misconception. For every 1 gram of glycogen stored, the body stores approximately 3 grams of water. This is not “bad” weight; it is functional, hydrated fuel that is essential for performance. The performance penalty from being under-fueled is far greater than the negligible penalty of carrying an extra kilogram of glycogen and water.
Early Morning Rides: To avoid rebound hypoglycemia (a blood sugar crash caused by an insulin spike followed by exercise), the advice is to eat immediately before getting on the bike or within the first 5-10 minutes of warming up. A pre-ride meal 30-60 minutes beforehand is more likely to cause issues for a short, intense session.
Solid vs. Liquid Fuel: Both have their place.
Liquids: Excellent for delivering high amounts of carbs while also hydrating, but osmolality is key. A hypotonic mix is ideal.
Gels: Convenient but are hypertonic and must be consumed with a good amount of plain water to ensure proper dilution and absorption.
Solids (Bars, Real Food): Fat and fiber slow gastric emptying, making them less ideal for high-intensity racing. They are better suited for lower-intensity endurance rides where the gut is under less stress.
MAURTEN & Hydrogels: The theory behind hydrogel technology is that it encapsulates the carbohydrates in a way that allows them to pass through the stomach quickly without triggering a high osmolality response. Once in the intestine, the gel dissolves, releasing the carbs for absorption. This may improve gut tolerance, especially at very high intake rates (100+ g/hour).
Caffeine: A known ergogenic aid that reduces perceived exertion and improves time to fatigue. Its effectiveness is highly individual. It does not appear to directly increase carbohydrate oxidation but improves performance through its effects on the central nervous system.