Metabolic series, part one. How the body actually handles the food we argue about.
For the last few years sugar has been discussed as though it were one thing with one fixed value, and fructose in particular has become shorthand for whatever a given diet has decided to oppose, whether that is the syrup in a soft drink or the sugar in a piece of fruit. The worry it generates pays almost no attention to dose, so the same word ends up covering a large sweetened drink taken on a sedentary afternoon and a banana eaten after a long ride, as if the body could not tell them apart. It can, and that difference is the entire subject. The question worth asking is not whether sugar is good or bad in general, but what happens to this carbohydrate, in this body, at this moment, because that is the question the physiology is actually built to answer.
This is an educational and strategic perspective, not personal medical advice.
Carbohydrate is not a moral category
Many dietary traditions quietly assume that the quality of a carbohydrate lives in the food, so that a food is either the kind you should eat or the kind you should avoid, and that judgement follows it around no matter who is eating it. Mediterranean and Nordic patterns both have solid population-level evidence, yet neither was designed to describe what a particular carbohydrate does inside one person on one day. That answer sits below the food, in the metabolism receiving it, and the same molecule entering two different metabolic states does not travel the same route.
The more accurate view is that carbohydrate metabolism depends heavily on context, which means the variables that matter are not printed on the label. They are the state of the liver, the demand the muscles are placing on the system, and the balance between energy taken in and energy spent, none of which a dietary rule can see from the outside.
The hepatic fork
Glucose and fructose share a chemical formula and are still handled quite differently once absorbed. Glucose is distributed widely, taken up across muscle, brain and fat tissue according to insulin signalling and energy demand, and under exercise or depletion it is steered toward refilling muscle glycogen, with the liver taking up only around twenty to thirty per cent of an oral glucose load and the rest moving on to the tissues that need it (Wolfe, 1998). Glucose is demand-responsive, and the body regulates tightly how much is burned and how much is stored.
Fructose is handled largely by the liver and, at lower doses, by the small intestine, which clears a good share of a modest fructose load and turns much of it into glucose and organic acids before it reaches the portal blood, partly shielding the liver at ordinary amounts (Jang and colleagues, 2018). What does reach the liver enters through fructokinase rather than through the tightly controlled step that governs glucose, so the usual rate-limiting brake is bypassed and the liver cannot throttle fructose as precisely as it throttles glucose (Tappy and Lê, 2010). From there the fructose is directed toward glycogen, toward oxidation, or toward the conversion of carbohydrate into fat, and which route dominates is decided by how full the liver's glycogen already is and whether the body is in surplus.
So the point is not that fructose is inherently worse than glucose, but that it is less tightly regulated and more hepatic, which makes its fate unusually sensitive to the state of the liver it lands in.
When making fat becomes the problem
The pathway that converts carbohydrate into fatty acids, de novo lipogenesis, is often treated as inherently harmful, which it is not. It is a normal route that becomes a problem mainly when it runs chronically against a background of caloric surplus. In people eating isocaloric diets the fat the liver makes from fructose stays comparatively low, and it climbs substantially during sustained fructose overfeeding combined with excess energy, which is the specific condition where the metabolic risk shows up (Stanhope and colleagues, 2009; Faeh and colleagues, 2005). Eating fructose and overfeeding it in surplus are not the same event.
The conditions that push this pathway upward tend to arrive together rather than alone:
liver glycogen that stays chronically full, leaving little room to store incoming carbohydrate
a sustained caloric surplus, with energy arriving faster than it is spent
a high fructose intake reaching an already saturated liver
low physical activity, which reduces the glycogen turnover that would create demand for that carbohydrate
Under these conditions the liver exports the resulting fatty acids, which is one of the mechanisms behind elevated triglycerides and fatty liver risk. The reasonable response is not to eliminate fructose while leaving total intake and energy balance untouched, since that treats a symptom. The real targets are the chronic surplus and the chronically full glycogen stores, which are properties of the system rather than of any single food.
The variable the debate leaves out
Liver glycogen status is the variable most of the public argument omits, and together with total energy balance it is one of the main things deciding where an incoming carbohydrate goes. The liver holds roughly eighty to a hundred and twenty grams of glycogen, drawn down by fasting, exercise and ordinary demand, and refilled by dietary carbohydrate, with the depletion rate varying considerably by how active a person is (Achten and Jeukendrup, 2004). The store is a buffer, and whether it sits near empty or near full changes what the next meal does.
Picture two people eating the same amount of fructose. One trains regularly and cycles their hepatic glycogen through repeatedly, so incoming fructose tends toward refilling depleted stores rather than making fat, and the food that worries the literature behaves, in that context, as restoration. The other is sedentary, keeps their stores replete, tips more easily into surplus, and gives the excess fructose a greater chance of feeding lipogenesis. The carbohydrate is identical. The system state is not, and the system state decides the outcome, which is exactly why a blanket rule about the food gives such different results from one person to the next.
The same fuel, read through context
During a full-distance Ironman I took in roughly a hundred grams of carbohydrate an hour across the whole effort, much of it glucose and fructose together, and in that setting the fructose was not meaningfully feeding any fat-making pathway, because demand was continuous, liver glycogen was being spent faster than it could be refilled, and oxidation was near its ceiling. A combined glucose and fructose feed is used precisely because fructose is absorbed through a separate intestinal route from glucose, letting total absorption rise beyond what glucose alone allows, so oxidation rates approaching ninety grams an hour become possible with the two together (Currell and Jeukendrup, 2008; Jeukendrup, 2014). The same quantity of the same carbohydrate eaten on a still afternoon, with the liver already full and nothing pulling on it, follows a different path entirely. The molecule does not change; the context does.
Three questions instead of a rule
The practical replacement for the good-or-bad rule is a short set of questions asked in order, each reading the context before deciding anything.
What is the liver's glycogen status right now? This depends on recent training, on how long since the last meal, and on whether the day has been active or sedentary, and it determines whether an incoming carbohydrate tops up depleted stores or adds to full ones.
What type of carbohydrate is it, and does it match the oxidation demand? A hard effort near threshold draws mainly on glucose and rewards a blend, a moderate endurance effort leans more on fat and cares more about quantity than type, and a rested state has only baseline demand, so anything beyond it accumulates.
What is the timing relative to that demand? Carbohydrate taken before or during effort is priming and fuelling stores that are being spent, whereas the same carbohydrate taken into a resting body with replete stores can exceed capacity if total intake is already high.
None of this asks anyone to avoid fruit, adopt a ketogenic protocol, or declare foods off-limits. It asks for an honest reading of the metabolic context, which is more demanding than a rule but also more accurate, because it accounts for the training, the glycogen turnover and the energy balance a population-average rule was never able to see.
Where Atlas Cove fits
This is close to the reasoning behind Atlas Cove. The week is built so that a person's metabolic context becomes legible, measured at the start and re-measured before they leave, so that decisions about food and fuel follow from that reading rather than from a rule inherited second-hand, and so that the habit of asking what an input will actually do, given the current state of the system, survives the return to ordinary life. Fructose arriving into a depleted liver after a long effort is not doing the same thing as fructose flowing into a replete liver after a desk-bound day, even though it is the same molecule, and the aim is simply to make the state of the body clear enough that the sensible choice becomes the obvious one.
Tom Wuerden · Co-Founder
Engineer turned Ironman