Sugar, Fat or Ketones: Which Gives Your Cells More Energy?

Your body can make energy from sugar, fat, ketones and even alcohol, but not all of these routes are equal. Some are fast and wasteful, some are slow and enormously efficient, and some leave a mess behind that your body then has to clean up. Which route your cells lean on, and how well your mitochondria handle the traffic, goes a long way to explaining why you might feel energetic after one meal and flat after another, or why fatigue and muscle pain can settle in when your energy system is under strain.

This is the more detailed companion to what mitochondrial dysfunction means for you. If you want the big picture first, start there.

The short version

• Your cells run on ATP, a molecule that works like energy currency.
• Breaking down one sugar molecule without the mitochondria gives you just 2 ATP. Sending it all the way through the mitochondria gives up to about 33.¹
• Fat gives far more energy than sugar, but it can only be burned inside the mitochondria and it needs oxygen.
• Food energy is carried to the mitochondria by a delivery molecule called NADH. Too much NADH arriving at once jams the system and creates free radicals.
• A second molecule, NADPH, does a different job: it powers repair, antioxidant defence and building things like cholesterol. Your cells have to share it between those jobs.

Do mitochondria produce ATP?

Yes. Your mitochondria make most of your body’s ATP, but not all of it. Energy production happens in three stages, and only the first one takes place outside the mitochondria:

1. Glycolysis happens in the main body of the cell. It splits sugar in half, making a small amount of ATP quickly and without oxygen.
2. The Krebs cycle (also called the citric acid cycle) happens inside the mitochondria. It strips the remaining energy out of sugar, fat and protein and loads it onto NADH, giving off carbon dioxide as it goes.
3. The electron transport chain sits in the inner wall of the mitochondria. It takes the energy carried by NADH and, using oxygen, turns it into large amounts of ATP. This final step is called oxidative phosphorylation, and it’s where the oxygen you breathe ends up.

A helpful way to picture it: NADH is the delivery van. Glycolysis and the Krebs cycle load the vans, and the electron transport chain unloads them and turns their cargo into ATP. Most of the vans are actually loaded in the Krebs cycle, not in glycolysis.

Comparing the energy pathways

Fuel and route	Where it happens	Needs oxygen?	Energy yield	What it’s good for
Sugar through glycolysis only	Main body of the cell	No	2 ATP per glucose	Speed, emergencies, low oxygen
Sugar fully burned	Cell, then mitochondria	Yes	Up to about 33 ATP per glucose¹	Everyday steady energy
Fat (beta-oxidation)	Mitochondria	Yes	Several times more ATP per molecule than glucose, and more than twice the energy per gram	Long, steady effort, fasting, rest
Ketones	Made in the liver, burned in other organs’ mitochondria	Yes	Efficient, and quick to enter the mitochondria	Fuel for brain, heart and muscle when sugar is low
Alcohol	Mainly the liver	Yes	Delivers a flood of NADH that can jam the system	Nothing your cells need

Glycolysis: the fast, emergency route

Glycolysis is quick and doesn’t need oxygen, which makes it the body’s emergency generator. It produces 2 ATP and 2 NADH for every glucose molecule it splits. When oxygen is plentiful and the mitochondria are working, the two halves of the glucose (called pyruvate) move into the mitochondria to be fully burned. When they aren’t, pyruvate is turned into lactate instead, which frees up NAD+ so glycolysis can keep running.

That lactate route is what happens in a sprint, when muscles need energy faster than oxygen can arrive. It’s also what happens when the mitochondria are struggling or deliberately holding back. A build-up of lactate is one reason strained energy production can show up as muscle aches without exercise.

Some cells lean on glycolysis by choice. Red blood cells have no mitochondria at all, so it’s their only option. Immune cells switch heavily towards glycolysis when they’re fighting an infection, because it supplies energy and building blocks quickly.² Cancer cells famously do the same even when oxygen is available, which is known as the Warburg effect.³ In every case, the cell is trading efficiency for speed.

The Krebs cycle and the electron transport chain

Once fuel reaches the mitochondria, it is broken down into a small molecule called acetyl-CoA, which enters the Krebs cycle. Sugar, fat and most amino acids all funnel into the cycle this way. As acetyl-CoA is taken apart, its energy is loaded onto NADH (and a related carrier, FADH2), and carbon dioxide is released.

The NADH then hands its electrons to the electron transport chain. As the electrons pass along the chain, the energy released is used to make ATP, and at the very end the electrons are handed to oxygen to make water. This is why you need to breathe: oxygen is the final catch at the end of the chain.

The chain isn’t perfectly sealed. A small number of electrons leak out along the way and form free radicals. In small amounts, some of these, particularly hydrogen peroxide, act as useful signals that tell the cell to switch on its antioxidant and repair systems.⁴ The trouble starts when more NADH is pushed into the chain than it can handle. Electrons back up, more of them leak, and free radical production outpaces your antioxidant defences.

NAD+ and NADH: the balance that runs everything

NAD+ is the empty delivery van, made from vitamin B3. NADH is the loaded van. Your cells need a healthy supply of empty vans to keep collecting energy from food, so in a healthy cell NAD+ comfortably outnumbers NADH.

NAD+ does more than carry energy. It’s also used up by enzymes called sirtuins, which help regulate metabolism, inflammation and how cells cope with stress, and by enzymes that repair DNA. When NAD+ is plentiful relative to NADH, as happens with fasting and calorie restriction, these enzymes become more active. That’s one reason NAD+ is so closely tied to research on ageing.⁵

Things that push NADH up (too many loaded vans, not enough empty ones):

• Overeating, especially large amounts of sugar and refined carbohydrate at once
• Alcohol, which loads NADH very quickly⁶
• Low oxygen, so the electron transport chain can’t unload the vans
• A slowed or blocked electron transport chain

Things that lower NAD+ overall:

• Ageing, partly because an enzyme called CD38 that breaks NAD+ down becomes more active⁵
• Ongoing inflammation and infection
• A diet short on vitamin B3 and the protein it can be made from

Eating less often, leaving gaps between meals and moving your body all give the system time to unload the backlog and rebuild the supply of empty vans.

Burning fat: slow but powerful

Fat is the most energy-dense fuel you have. Fatty acids are broken down two carbons at a time, in a process called beta-oxidation, into acetyl-CoA that feeds the Krebs cycle. Every round also loads NADH and FADH2, which is why fat yields so much more ATP than sugar. Fats are grouped by length: short chain (up to 6 carbons), medium chain (8 to 12) and long chain (longer than 12).

The main bottleneck is getting fat into the mitochondria. Long-chain fats need to be attached to carnitine to cross the mitochondrial wall, using an enzyme called CPT1. That step controls how fast fat can be burned.⁷ Medium-chain fats can cross more easily, which is why they’re used as a quicker fat-based fuel.

Fat burning also depends on the mitochondria themselves: B2 and B3 for the delivery carriers, carnitine for transport, enough oxygen, and thyroid hormone, which sets how many mitochondria your cells keep and how hard they run. If any of these are short, fat burning slows down.

Unsaturated fats (the ones with double bonds, found in olive oil, nuts, seeds and fish) need extra enzymes to reposition those bonds before they can be broken down. Some of these steps use NADPH, so burning polyunsaturated fat draws on the same supply your antioxidant system needs.⁸

Odd-chain fats are a small group with an odd number of carbons, found mainly in dairy fat, and your body may make some itself. Their last piece becomes propionyl-CoA rather than acetyl-CoA, which is converted to succinyl-CoA and slots straight into the Krebs cycle partway round.⁹ That makes them unusual: unlike ordinary fats, part of an odd-chain fat can be turned into glucose if the body needs it.

Fat and sugar compete. When your cells are burning plenty of fat, they burn less sugar, and the reverse is also true.¹⁰ It’s a sensible system for switching between fuels. The problem arises when both arrive in large amounts at the same meal. Your cells can’t burn both at full speed, so the excess sugar stays in your blood longer and is pushed towards storage.

Ketones: fat’s fast lane

When sugar is scarce, during fasting, very low-carbohydrate eating or long exercise, your liver turns fat into ketones and releases them into the blood. Other organs, especially the brain, heart and muscles, can then burn them. Ketones don’t need carnitine to get into the mitochondria, so they reach the Krebs cycle quickly. The liver makes ketones but can’t burn them itself, which keeps the supply available for everyone else.¹¹

Alcohol: a flood of NADH

Alcohol is processed mainly in the liver and loads NADH faster than almost anything else. That flood slows the Krebs cycle, pushes sugar towards lactate, blocks the liver from making fresh glucose and shifts it towards making fat.⁶ I’ve walked through it stage by stage in what happens to your cells when you drink alcohol.

NADPH: the repair and defence molecule

NADPH looks almost the same as NADH, but it does a completely different job. NADH carries energy to be burned. NADPH supplies the power to build and protect.¹²

• It recycles glutathione, your main antioxidant. Without NADPH, used glutathione can’t be returned to its active form.
• It’s used to build things, including cholesterol, fatty acids and the building blocks of DNA. Demand rises during growth, repair and infection.
• It arms your immune cells. White blood cells use NADPH to produce bursts of free radicals that kill invading microbes.

Most NADPH is made in a side branch of glucose metabolism called the pentose phosphate pathway. Another source comes from citrate leaving the mitochondria. Citrate carries acetyl-CoA out into the main body of the cell, where it’s split back up. The acetyl-CoA becomes the raw material for fat and cholesterol, and the leftover pieces can be used to make more NADPH.

The important point is that NADPH is a shared resource. The same supply has to cover antioxidant recycling, immune defence and building fat and cholesterol.

Where this connects to cholesterol and blood vessels

When the Krebs cycle is backed up, as it is with heavy drinking or regular overeating, acetyl-CoA can’t move into the cycle as freely. More of it leaves the mitochondria as citrate and becomes raw material for fat and cholesterol. That’s part of how alcohol and overeating feed into fatty liver and metabolic syndrome.⁶

Here is how I see it connecting to blood vessels, and this is my interpretation rather than something that has been directly tested. If a larger share of NADPH is being spent on building fat and cholesterol, less is left over for recycling glutathione. Blood vessel walls under oxidative stress are more easily damaged, and cholesterol is part of the repair material the body sends to that damage. So the same overload that pushes cholesterol production up may also weaken the antioxidant protection of the vessel wall. I’ve written more about this in the biochemistry behind atherosclerosis.

Why this matters if you’re tired or in pain

When your mitochondria are struggling or holding back, your cells fall back on glycolysis. You get less energy from the same food, more lactate, and more free radicals from a jammed electron transport chain. That combination can feel like heavy fatigue, aching muscles and poor recovery. It’s one of the threads running through why fibromyalgia pain can feel like burning.

Often the slowdown is protective. During an infection or when free radicals are already high, your mitochondria deliberately turn production down to avoid making more damage, which I’ve covered in mitochondria and the cell danger response. So the answer usually isn’t to force more fuel through the system. It’s to reduce what’s overloading it, give it gaps to clear the backlog, and supply what it needs to run.

Frequently asked questions

Do mitochondria produce ATP?
Yes. Mitochondria produce most of your body’s ATP through the Krebs cycle and the electron transport chain. A small amount is also made outside the mitochondria by glycolysis, which is the only source of ATP for red blood cells.

How much ATP does one glucose molecule make?
Glycolysis alone makes 2 ATP per glucose. When the glucose is fully burned through the mitochondria with oxygen, the total rises to around 30 to 33 ATP.

Which fuel gives the most energy?
Fat gives the most energy, with more than twice the energy per gram of sugar, but it can only be burned inside the mitochondria with oxygen. Sugar is faster to use, and glycolysis can run without oxygen at all.

What is the difference between NAD+ and NADH?
NAD+ is the empty energy carrier and NADH is the loaded one. NADH carries energy from food to the electron transport chain, where it’s unloaded to make ATP and turned back into NAD+.

What is the difference between NADH and NADPH?
NADH carries energy to be burned for ATP. NADPH supplies the power for building and protecting, including recycling glutathione, making cholesterol and fatty acids, and arming immune cells to kill microbes.

Can cells make energy without oxygen?
Yes, through glycolysis, but only a little: 2 ATP per glucose, with lactate as a by-product. Burning fat and fully burning sugar both need oxygen.