Fuelled by science: my T100 triathlon journey as a nutritionist-in-training

Last Sunday, I took on the London T100 triathlon, my biggest endurance event yet. Just shy of an Ironman 70.3, it comprised a 2 km swim, an 80 km bike ride, and an 18 km run, totalling 100 km. Not bad for a Sunday morning.

Having completed a few half-marathons and Olympic-distance triathlons, I was confident I could cross the finish line. But I knew my performance, and how I felt afterwards, would depend entirely on my fuelling strategy.

A bit of background

I am currently studying for an MSc in Human Nutrition at St Mary’s University, Twickenham. This is my third degree, following a BSc in Neuroscience and an MSc in Microbiome Research at King’s College London, and I have also worked for two years as a private health consultant.

On a personal level, my teenage years were defined by a battle with an autoimmune condition. In 2013 I spent time as an in-patient on the UCL teenage rheumatology ward. It was during this period that I discovered the transformative power of a food-as-medicine approach. The moment I consumed anything processed, or containing refined sugar, gluten, or dairy, my symptoms worsened: an excruciating rash, joint pain, temperatures, and chronic fatigue.

My body allowed no margin for error. I learned early that the wrong fuel is like putting diesel in a petrol car.

In contrast, the right diet, rich in micronutrients to modulate cell signalling and calm inflammation, amino acids to repair the gut lining, and whole-food fibre to nourish the microbiome, can influence the trajectory of chronic disease, supporting the body’s foundational healing rather than merely masking symptoms with medication.

From education to practice

Combining personal experience with academic grounding, I have spent the past few years immersed in longevity and metabolic health, science writing for continuous glucose monitoring companies and coaching on the NHS National Diabetes Prevention Programme, where I see first-hand how poor diets drive chronic disease.

The evidence is clear: poor diet is the leading modifiable driver of chronic disease [1]. In the UK, 63% of adults are overweight and 27% are obese. Obesity-related illness costs the NHS over £6 billion annually, and rates of diet-related disability and premature death are climbing [2]. Poor diet is responsible for 11 million deaths a year, one in five deaths globally, making it more impactful than smoking [1].

Yet nutrition remains under-used in primary care, particularly when it comes to tackling excess sugar, refined grains, and ultra-processed foods, all of which undermine gut health and metabolic resilience.

The dichotomy: health-focused nutrition vs endurance fuelling

So there I was, training for a demanding endurance event and facing a paradox. Everything I know tells me added sugars and refined carbohydrates drive disease. Yet to get around the course, I needed adequate energy.

In perfect timing, I took a module in sports nutrition as part of my MSc this year. I am no longer someone who follows a restrictive diet; I eat mostly whole foods, healthy fats, protein, and fibre, and I limit carbohydrates to feel mentally sharp, but I love a pizza every now and again. When I ramped up my training, my hunger, especially for carbohydrates, increased too, and I listened. What I learned in that module helped me understand exactly why.

Research on fuel use during exercise (Hargreaves and Spriet, 2018) shows how the body’s preferred energy source shifts with intensity [3]:

• At rest, most energy comes from fat oxidation, particularly plasma free fatty acids, with minimal contribution from carbohydrate stores.
• At moderate intensities (around 65% VO₂ max), fat still provides a substantial proportion of energy, but carbohydrate use, particularly muscle glycogen, rises sharply.
• At high intensities (around 85% VO₂ max), muscle glycogen becomes the dominant fuel, while fat use drops substantially, because fat oxidation cannot keep pace with demand.

The key takeaways from a sports nutrition perspective: at low intensities, athletes can rely more on fat oxidation and preserve glycogen for later; at high intensities, carbohydrate availability becomes the main performance-limiting factor; and when glycogen stores run low, fatigue sets in, because fat oxidation alone cannot sustain high power output.

To my surprise, research shows that in many cases the most under-consumed macronutrient among athletes, even professionals, is not protein but carbohydrate, which is critical for high-intensity performance.

Fighters, long-distance runners, and triathletes are particularly vulnerable to RED-S (Relative Energy Deficiency in Sport), a condition of low energy availability with serious consequences for hormone balance, immune function, and bone health [4]. Among female endurance runners aged 20 to 30, 64% were found to be at significantly increased risk [5], and broader surveys estimate prevalence between 22 and 58% [6].

As a result, competition fuelling strategies often prioritise high-glycaemic-index carbohydrates while reducing protein and fat to avoid fullness, and limiting fibre to minimise gastrointestinal upset. Physiologically this makes sense. But from a preventive and long-term health perspective, it flips everything we know about healthful eating on its head.

Choosing fuel with intention

Commonly used race fuels, energy drinks, sweets, bars, and synthetic gels, are effective from a macronutrient perspective because they by-pass some of the body’s natural limits on carbohydrate use during exercise. Blood is diverted away from the digestive tract during exercise, slowing carbohydrate transport across the gut wall, which is why gut training is essential for endurance athletes. And different sugars use different intestinal transporters, so combining them, such as maltodextrin and fructose, increases total carbohydrate absorption and oxidation.

These principles are essential to understand for optimising race performance. But my experience has taught me that the human body is far more complex than a simple bomb calorimeter. The matrix in which nutrients arrive, the fibres, polyphenols, additives, or lack thereof, matters profoundly.

Risking an autoimmune relapse simply is not an option for me, so fuelling on sweets or sugary bars was a firm no. Synthetic gels also did not sit right. This approach is very normalised; my social media feed is full of marathon influencers eating white bread and chocolate spread at 6am followed by two synthetic gels before 9am, and yes, it works for performance. But I cannot help asking, at what cost to gut health, brain function, or long-term metabolic resilience?

The performance nutrition paradox: energy targets versus food quality

Conventional endurance fuelling prioritises rapidly oxidised carbohydrate, typically 60 to 90 g per hour with mixed sources, and in some protocols up to 120 g per hour. This strategy works on race day. But it says little about long-term health, the gut, or what happens when an ultra-sweet diet continues beyond competition.

The real question is not carbs or no carbs, but the quality of those carbs, and the additives and processing that often accompany them.

I have organised my concerns into four points.

1. High sugar intake, even without excess calories

Athletes may avoid weight gain because of high training loads, but the metabolic effects of sugar extend beyond calories. A cross-sectional study in semi-professional footballers found a strong association between simple sugar intake and inflammation [7]. High fructose intake in controlled trials increases liver fat, triglycerides, and insulin resistance, sometimes even without weight gain [8, 9, 10].

Sugars may also raise pro-inflammatory cytokines and impair gut barrier integrity [11], driving systemic inflammation, which increases susceptibility to injury. Copious added sugars displace nutrient-dense foods, drain the vitamin and mineral stores needed to metabolise sugar, and impair mitochondrial efficiency [12]. Over time, high sugar diets have been linked with poorer cognitive function, mood swings, and reduced concentration [13].

So what? Even in lean, high-performing athletes, a persistently high-sugar diet can quietly set the stage for insulin resistance, liver dysfunction, and chronic low-grade inflammation, while increasing injury risk and impairing brain function.

2. Ultra-processed foods, fibre and the athlete gut microbiome

Many training fuels are ultra-processed: high in sugar, additives, and stabilisers, but low in fibre. Large cohort studies show that every 10% increase in ultra-processed food intake is linked to a 12% higher risk of type 2 diabetes, alongside higher risks of breast, colorectal, and oesophageal cancers [14, 15].

Mechanistically, these effects may be driven via the gut microbiome, independently of calorie excess. Emulsifiers like carboxymethylcellulose and polysorbate 80 disrupt the gut microbiota, thin the protective mucus layer, and increase gastrointestinal symptoms [16]. An 11-day randomised trial confirmed that such additives altered the human microbiome and metabolome while worsening gut comfort [17].

Fibre is a critical missing piece. Only 4% of UK adults reach the 30 g per day target, and athletes often cut fibre before training to reduce gastrointestinal upset. Chronic low intake starves the beneficial gut microbes that produce short-chain fatty acids, essential for gut barrier health and immune regulation. Exercise benefits the microbiome [18], but poor food quality may blunt those gains.

So what? If athletes fuel mostly on ultra-processed foods and cut back on fibre, they risk undermining gut health, immunity, and even cancer protection.

3. Sweetness, brain chemistry and taste adaptation

Sugary, hyperpalatable foods hijack the brain’s reward centre. Ultra-processed foods are engineered to hit the bliss point of sugar, fat, and salt, triggering dopamine release and encouraging overconsumption [19]. Neuroimaging studies show sugar activates dopamine and opioid pathways similar to addictive substances, while repeated exposure reshapes brain reward systems, driving cravings and weakening satiety [20, 21].

This might not matter when energy expenditure is extreme, but it becomes problematic during injury, off-season, or retirement. Athletes accustomed to ultra-sweet fuelling may struggle to recalibrate their diets.

So what? Over time, reliance on ultra-sweet fuels could hardwire cravings and make post-career nutrition transitions harder, raising the risk of obesity, diabetes, and mood disorders once training volume drops.

4. Dental health, the overlooked performance factor

Teeth do not care about split times. Surveys of Olympic athletes, including those at London 2012, revealed high rates of caries (55%) and enamel erosion (45%), driven by frequent sugar exposure, acidic drinks, and dry mouth during training [22]. Oral pain and infection can compromise training quality, impair nutrition, and are now recognised as contributors to wider systemic issues, including autoimmunity [23].

So what? Ignoring oral health does not just risk cavities; it can reduce training quality, damage systemic health, and shorten a career through preventable issues.

Pulling it together

When you are aware of the science, it is easy to overthink every detail. I even found myself questioning whether to avoid apple cider vinegar before sessions, given evidence that it can slow carbohydrate digestion; great if you are trying to improve insulin sensitivity, but less helpful when you actually need rapid fuel delivery to muscles.

What matters most is the bigger picture. As someone who values long-term health above marginal gains, and who is a very amateur athlete, I am not willing to compromise my health for the sake of an extra few watts or seconds. The real challenge is finding fuel that meets energy demands in training and competition, while still protecting the gut, brain, and body for the long run.

My fuel plan

As my training volume ramped up, I made a conscious effort to increase my carbohydrate intake around the heaviest sessions. For me, that looked like white rice, gluten-free pasta, Medjool dates as a quick snack, or a slice of homemade gluten-free banana bread.

After sessions I would mix all the macronutrients: a scrambled oats bowl (banana, oats, egg, cooked in butter or coconut oil, topped with honey, chia jam, peanut butter, nuts, and seeds), supporting satiety, recovery, and the gut.

Then I discovered organic honey gels with electrolytes: gentle on my gut, naturally energising, and scientifically sound [24]. They became my go-to in training and on race day. Honey naturally provides both glucose and fructose, is rapidly absorbed, and brings added antioxidant and antimicrobial benefits. Studies show honey can match dextrose for performance outcomes [24].

Reflections post-race

The race honestly could not have gone any better. I probably ran a little low on energy towards the end, but the energy of the course carried me through. I crossed the line in 4 hours 49 minutes, finishing 14th out of 61 in the F25-29 category, just four places off qualifying for the World Champs, and I could not be happier. No gut issues, no cramping, on a hot day.

This experience reminded me just how extraordinary our bodies are. It proved that performance and long-term health do not have to be in conflict; they can align when fuel is chosen with intention and grounded in science.