Carbs Are Fuel. Buy Good Fuel.

Carbohydrates are not the enemy. Bad carbohydrates are.

It took me a while to learn the difference. For about five years, I ran my diet close to zero carbs, and it did exactly what I asked of it. I got lean and even managed to get my body fat percentage to about 11 per cent. No bread, no rice, no potatoes. Protein, fat, green vegetables (usually broccoli, kale and cavolo nero when I can get it). Repeat.

It worked. I won’t pretend otherwise.

Then the goal changed, and the diet had to change with it.

From lean to long-lived

Getting lean is a body composition goal. Living longer (and staying useful while I do it) is a different job, and it needs a different toolbox.

The number that changed my thinking was VO2max: the maximum amount of oxygen your body can take in and use during hard exercise. It is about as close as we have to a single measure of the engine you’re running on. And the data linking it to how long you live are brutal.

In 2018, a team at the Cleveland Clinic followed 122,007 patients who had done a treadmill fitness test, for a median of 8.4 years. People in the elite fitness bracket had roughly 80 per cent lower risk of death than those in the lowest bracket. Being in the bottom group carried a bigger mortality risk than smoking, diabetes or coronary artery disease. And there was no ceiling. Elite performers still beat the merely “high” fit group. More fitness, more benefit, all the way up.

(It’s an observational study, so it shows association, not proof of cause. But the effect is huge, it’s consistent with decades of other data, and I’m not waiting for a perfect trial before I act on it.)

So I rebuilt my training around cardio, following Iñigo San Millán’s polarised model. Most of the week is Zone 2: easy, conversational running on Monday, Friday and a long run on Sunday. That builds the base. Once a week, on Wednesday, I go hard: Zone 5 intervals. Sometimes a fartlek session replaces one of the Zone 2 runs. What I don’t do is spend time grinding away in the grey middle of Zone 3, which is too hard to build the base and too easy to raise the ceiling.

The hard day is where the ceiling goes up. In a classic 2007 study, 40 moderately trained men did eight weeks of training with the total workload matched across groups. Long, slow running and threshold running didn’t move VO2max significantly. Four-minute intervals at 90–95 per cent of max heart rate improved it by 7.2 per cent.

Here’s the catch though. High intensity runs on carbohydrate and I found out the hard way when my body just basically refused to do more work as it was running on fumes.

Fat is a fantastic fuel for easy work. It’s a lousy fuel when you’re going flat out. San Millán’s own research with George Brooks measured this directly: as intensity climbs, everyone shifts from burning mostly fat to burning mostly carbohydrate. Professional cyclists held on to fat until around 75 per cent of their maximum power. Moderately active people crossed over at around 55 per cent. By the time you’re doing VO2max intervals, you’re running almost entirely on glycogen, the carbohydrate stored in your muscles and liver.

Show up with an empty tank and the intervals go flat. You can’t hit the pace. You can’t hold the effort. The session that was supposed to raise the ceiling becomes another mediocre Zone 3 slog.

The best evidence on this comes from Louise Burke’s work at the Australian Institute of Sport. She put elite race walkers on either a high-carb diet or a ketogenic, low-carb high-fat diet during three weeks of hard training. The low-carb group more than doubled their fat burning. Sounds great. It wasn’t. Their oxygen cost at race pace went up (they got less economical) and they got none of the performance benefit from the training block. The high-carb group raced 10km 6.6 per cent faster. The low-carb group didn’t improve at all. Burke’s team repeated the study in 2020 with 26 walkers. Same result: high-carb 4.8 per cent faster, low-carb 2.3 per cent slower.

I’m not an elite runner or athlete and neither are most of the people reading this. The physiology is the same though. Hard efforts burn carbohydrate, and if you want quality hard efforts, you have to put some carbohydrate in the tank.

So the carbs came back. Some, not lots. Enough to fuel the work.

Choose your carbs like you’d choose your tools

Once you accept that you need some carbohydrate, the question becomes which carbohydrate. That’s where most people go wrong. They hear “carbs are OK now” and go straight back to white bread and pastries.

Don’t.

Fat is a fantastic fuel for easy work. It’s a lousy fuel when you’re going flat out.

My base is vegetables. Lots of them. They bring carbohydrate, but they also bring fibre, and fibre is the part that does the heavy lifting for long-term health. The biggest review on this, published in The Lancet in 2019, pooled nearly 250 studies and trials. People eating the most fibre had 15–30 per cent lower rates of death from all causes and from cardiovascular disease, plus less coronary heart disease, stroke, type 2 diabetes and bowel cancer, compared with people eating the least. The sweetspot was 25–29 grams a day. Most people fall well short.

The vegetables also keep the glycaemic index (GI) of my meals low. GI measures how fast a food raises your blood sugar. Low GI means steadier energy and no crash an hour later. Worth knowing though: the same Lancet review found the evidence for fibre much stronger than the evidence for GI on its own. Fibre is the main tool. GI is a useful secondary gauge.

On top of the vegetables, I add the occasional portion of three things:

Sweet potato. Brilliant, but only if you cook it right. This is the one most people get wrong. A 2011 study tested sweet potato cooked different ways. Boiled, it scored a GI of 41–50: low. Roasted, 79–93. Baked, 82–94. Same potato, and the oven takes it from low GI to higher than a lot of white bread. Boil it.

Brown rice. Better than white, mostly because it keeps the bran, so you get more fibre and more minerals. Don’t assume it’s automatically low GI, though. In the University of Sydney GI database, brown rice has tested anywhere from 48 to 75 depending on the variety and how it’s cooked. Keep the portion modest and eat it with vegetables and protein, which slow the whole meal down.

Flatbread. Occasionally. Pick wholemeal over white when you have the choice. And treat it as a side, not the main event. I’m lucky because most of the year, I’m based in Riyadh (Saudi Arabia) so flatbread or khubz al-jamri (flatbread baked in charcoal oven) is easily accessible.

The principle is simple. Vegetables are the foundation. The starchy carbs are the top-up, sized to the training I’m actually doing.

Strictly no empty carbs

This is where I refuse to budge.

No sugary drinks. No sweets. No pastries. No fruit juice pretending to be healthy. None of it.

“Empty” is the right word. Sugar gives you calories and nothing else: no fibre, no micronutrients, nothing that keeps you full. It spikes your blood sugar, drops it again, and leaves you hungrier than before.

The World Health Organization recommends keeping “free sugars” (added sugar, plus the sugar in honey, syrups and fruit juice) below 10 per cent of your daily calories, and ideally below 5 per cent. On a 2,000-calorie day, 5 per cent is about 25 grams. A single 330ml can of regular cola has around 35 grams. One can, and you’ve blown the whole day’s budget before you’ve eaten anything.

The drinks are the worst offenders. You don’t feel full after a sugary drink, so those calories get stacked on top of everything else you eat. They sneak in all day long: the fizzy drink with lunch, the sweetened coffee in the afternoon, the juice with breakfast. Cut them out completely. Not “cut down”. Cut out.

If I’m eating carbohydrate, it has to earn its place. Vegetables earn it with fibre. Sweet potato, brown rice and the occasional flatbread earn it by fuelling my hard sessions. Sugar earns nothing.

The blueprint

Here’s where I’ve landed, and I suspect it’s where a lot of people chasing longevity will end up:

• Low carb is a good tool for getting lean. Use it if that’s your goal.
• Once the goal becomes fitness and longevity, the hard sessions need fuel. High-intensity work runs on carbohydrate. Starve it and you undercut the very training that builds VO2max.
• Build every meal on vegetables. Fibre is the most underrated tool in the box but just remember that not all vegetables are equal.
• Add starchy carbs in moderation, and choose them carefully. Boiled not baked. Brown over white. Portions sized to the work.
• No empty carbs. Ever. No sugary food, no sugary drinks.

None of this is complicated. The hard part is sticking to it week after week when there’s a can of something sweet within arm’s reach.

Carbs are fuel. Buy good fuel. Burn what you buy.

Sources

1. Mandsager K, et al. “Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing.” JAMA Network Open, 2018;1(6):e183605.
2. Helgerud J, et al. “Aerobic high-intensity intervals improve VO2max more than moderate training.” Medicine & Science in Sports & Exercise, 2007;39(4):665–671.
3. San Millán I, Brooks GA. “Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals.” Sports Medicine, 2018;48(2):467–479.
4. Burke LM, et al. “Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers.” Journal of Physiology, 2017;595(9):2785–2807.
5. Burke LM, et al. “Crisis of confidence averted: impairment of exercise economy and performance in elite race walkers by ketogenic LCHF diet is reproducible.” PLOS ONE, 2020;15(6):e0234027.
6. Reynolds A, et al. “Carbohydrate quality and human health: a series of systematic reviews and meta-analyses.” The Lancet, 2019;393(10170):434–445.
7. Bahado-Singh PS, et al. “Relationship between processing method and the glycemic indices of ten sweet potato cultivars commonly consumed in Jamaica.” Journal of Nutrition and Metabolism, 2011;2011:584832.
8. University of Sydney Glycemic Index Research Service database, rice values. Summary.
9. World Health Organization. Guideline: Sugars intake for adults and children. Geneva: WHO, 2015.