Sports nutrition isn’t complicated. It’s just general nutrition with a little thought given to fueling the work you do in training.

But when you learn about nutrition from a torrent of misinformation via social media, you have no chance of sorting sound information from good or bad advice. I’m not offering much advice in this article. Go to SportsRD.org for verifiable, reputable, reliable good advice. What I’m offering today is a brief explanation of carbohydrate and fat metabolism. I offer this particular summary as a way of reinforcing this single point about sports nutrition: eat like an adult all day, every day (“meat for strength; veggies for health” says Pavel and “grains for brains” say I) then eat a little extra sugar around training.

There are lots of pre-med and exercise physiology videos about metabolism on the internet. We’re not going that deep. I just want you to think about how your body works so you can filter all the dumb information filling up your feed.

Your Body Is A Savvy Miser

When functioning normally, your body uses every unit of every thing you put into it. Food and water and sunshine enter the body. They get deployed to lots of places to be used and re-used and converted and stored and consumed in thousands of different ways. Then, the completely neutral elements of what you ingest are used to carry the toxic stuff out of your system. Water used for urine and bulk matter used for feces are methods of transport so that bad stuff can be taken out of your system. Menstruating athletes also have period blood to carry out unused eggs and a little bit of reproductive miscellany roughly every month.

So understand that everything retained in your body has a minimum of one purpose and often has several. Now start thinking about the food you eat. Every single part within every single swallow is going to be used. This article is about how sugar and fat are used. But they are not used in isolation, there is way more to this story than what I’m summarizing, and you shouldn’t trust any dietary fad because they always grossly oversimplify (just like I’m doing!).

Back to your body: the purpose of nutrition is to provide resources for your cells which your body cannot produce on its own. Nutrition provides minerals, micronutrients, and macronutrients that your body breaks down, distributes, then utilizes or stores or excretes. Every task your body completes requires ATP as an energy source. Your entire metabolism is a complicated, integrated system with the sole mission of maintaining enough ATP to keep you alive.

How Your Body Makes ATP

This isn’t a physiology lesson for professionals, so we’re not getting into metabolic cycles. This is a practical summary for athletes. Your body makes ATP from protein (slow & inefficient method), from carbs (very fast & inefficient method), or from fat (slow & very efficient method). ATP decays quickly so your body doesn’t store it. Think of this somewhat like raising chickens versus storing all the eggs you need to feed a village for a week. It is less complex and less expensive to keep chickens healthy and happy for long time periods so they keep laying eggs than it would be to have a massive warehouse of eggs to maintain.

Making ATP from protein is so slow and so inefficient that your body prefers not to do it. But it can and does – especially if you don’t eat enough. Undereating leads to low energy availability. Low energy availability triggers starvation responses. Starvation responses strip amino acids from muscle in order to convert them to ATP so your brain can run. This is bad. Undereating destroys your muscles, down-regulates your brain functions, and can damage your nervous system. Don’t worry about making ATP from protein as a sports nutrition tactic…because if you reach this point, you aren’t healthy enough to be an athlete anymore anyway.

Making ATP from carbohydrate is so fast that your body absolutely LOVES to do it. Carbs burn bright, metabolically speaking, but it takes a lot of them to produce the energy you need. Also, they don’t quite burn clean, so they produce a lot of waste products which need to be dealt with. With hours of moderate training or minutes of intense training or a blend of both as an athlete, then you would need outrageous quantities of carbohydrates to provide all the ATP your body needs to do cool sport stuff and keep your brain and hormones running. If you were to become awesome at only running on carbs by providing lots of them and training hard every day, expect your entire metabolism to morph to depend on that steady flow of carbohydrate and steady muscular activity to manage it, because you would be building up physiological debt.

Making ATP from fat is so efficient that your body is hyper-specialized to do it. Fats burn clean, metabolically speaking, and they have an extremely high density of energy per unit mass. Fat takes a while to process within cells, so metabolizing fat limits your force production, since sustained forceful muscular contraction happens fast and consumes a lot of energy. Low-intensity activity can be sustained with energy from fat almost indefinitely. The real trouble is that a fat-only approach would limit your brain function (which exclusively extracts ATP energy from glucose [happily] or from ketones [necessarily]) and make you sluggish. You cannot train hard or frequently on fats alone.

Sugar and Fat as Chickens, Not Eggs

Said far, far too simply, your body stores the used-for-energy version of fat in deposits around your body (subcutaneous, visceral, and hepatic) and stores the used-for-energy version of sugar in your muscles. At any body fat percentage over 6%, humans carry enough fat in their fat deposits to pad their joints, fuel daily movements like unloaded walking, and sustain hormone production for up to a few weeks. Your body is extremely stingy with that fat, however, so only releases then metabolizes it in case of perceived starvation. This stored energy is like storing eggs in a warehouse, always ready to use.

Since humans living comfortable lives eat every day, our bodies assume a certain amount of energy will come in to be used to sustain function. Part of “sustaining function” is keeping a small reserve of ready-to-use energy. Any energy taken in beyond the need for actual activity and filling that reserve is also stored. Eating enough every day is like keeping chickens, always ready to produce new eggs.

This is where sports nutrition meets metabolism. Your body seeks to adapt to your activity level and the types of activity you do. When you start up a new training activity, you get sore. This is because your body has not distributed resources and built up capacity in the areas most punished by your activity. If you do that training activity regularly, lots of rebuilding and recovery are still needed if you want to improve, but you generally do not get sore from it anymore. The affected areas are reinforced with denser bone, thicker muscle, and blood supply to both.

The more you train, either the more you must eat or the larger your energy reserve has to be (therefore the more you must eat anyway, to keep that reserve full). As your muscles grow larger, they become capable of retaining more used-for-energy form of glucose. That form is glycogen. (Small amounts of glycogen are also stored in your liver, but that’s your brain’s special stash of sugars to use while you sleep, so we aren’t concerned with it for now.) As you use your muscles, they consume their stored glycogen. When you run out of glycogen, your muscles cannot contract as forcefully, so your output goes down. As an athlete, this feels like running slower, losing power, and losing strength.

The way to mitigate all this loss of muscular contractile force (literally how hard and fast your muscles can squeeze) is to supply glucose to replace what your muscles consume as they contract. Here we go, here’s the good stuff about sports nutrition at last.

What It Means To #fuelthework

You train. Your muscles run out of glycogen by converting it to glucose then burning it.* You eat carbs and protein. Your muscles fill up on glucose by converting it to glycogen.** You sleep. Your muscles use amino acids from protein to rebuild. You wake up stronger, so you train again, this time harder. On and on with this cycle.

But you can nearly deplete your entire muscular reserve of glycogen with barely 120 seconds of intense activity. Even in a hyper-minimalist training philosophy like mine, this barely gets you through a single workout. Then, your glycogen needs are greater than the amount your muscles store; glucose is needed for other tasks in your body. So you probably don’t eat enough carbohydrate to get all the glucose needed to supply your body with energy for ATP and enough to restore muscle glycogen.

This is where fat comes in. Triglycerides stored in your liver and in adipose tissue (fat deposits everywhere else in your body except your liver) are the used-for-energy form of fat. Each of your cells has a big furnace which, when provided with sufficient oxygen, can burn fat to produce outrageous amounts of ATP. Most of your body’s energy needs are handled via these fat+oxygen furnaces. Regular fat consumption keeps this reserve of fat-for-energy stocked up. When you huff and puff after a hard effort, that’s your body pulling in all the oxygen it can. The oxygen you inhale is used to make ATP from burning fat; the carbon dioxide you exhale is used to carry away waste products created from burning glucose.

Your Body After Hard Training

When your training is over, your body wants all the resources as soon as possible. See, training is stress. Stress mobilizes “go fast and hit hard” resources in the body so your muscles can contract powerfully and your brain can detect little details that might keep you alive. When stress is over, cortisol (erroneously called “the stress hormone” because it correlates to stress, but properly called “the recovery hormone” because it comes out to play after stress) mobilizes “recover and rebuild” resources. Protein is eaten so its amino acids can be deployed to reconstruct damaged tissues. Fat is eaten so its triglycerides can be stored for steady-use short-term and long-term energy supplies. Carbohydrate is eaten so muscles can gobble up the glucose to be ready for the next bout of stress.

Once again, if you remember to eat like an adult and you eat a little extra sugar around training, you can feel great most days and feel ready to train again sooner than if you don’t. Here’s where the “little extra sugar” comes in. Your body only stores enough energy to keep you alive at your baseline activity level. If you’re training smart, then each time you train, you’re challenging your body in a larger way than the previous time. You will consume more energy than it has prepared to provide. And if you’re training hard (by the Fast Kids Don’t Train Slow definition, meaning with high power output), you’ll consume all of your muscles’ stored glycogen early in the session. So you need to #fuelthework – you need to get some glucose in circulation right when your muscles need it so you can train at a higher level for longer!

This is incredibly simple to do.

How To #fuelthework

1. Eat some simple sugars between 10 and 45 minutes before you train.
   (Why the huge range? Carbs dissolved into liquid enter your blood stream from your small intestine dramatically faster than carbs chewed and processed. Also, foods chewed and processed need time to “settle” so you don’t get nauseous while training.)
2. Drink some simple sugars during your longest, hardest training sessions.***
   (Anyone doing 200yd shuttles after 45 minutes of other speed & power training? Anyone have their first team practice of the season this weekend? You need the sugar drinks.)
3. Eat a real meal as soon as you can after training.
   (If you’re going to be stuck without a real meal for a while, grab that smoothie or protein shake – a quick hit of sugar and amino acids fires up your “recover and rebuild” machinery so you recover faster.)

Other than those three special situations, just eat like an adult the rest of your day. “Meat for strength; veggies for health” combined with “grains for brains.”

(By the way, I do NOT write for endurance athletes. There are thousands of articles and books for endurance athletes with outrageously specific recommendations for fueling your work. Power athletes don’t need that degree of rigor. Amateur power athletes really, really don’t need it. So if you want more details, go read a good book. Several are listed at the end of this article.)

NERD ALERT

Here’s a little taste of the real physiology behind this article. Still simplifying here, but much closer to the technical explanations. Skip to the next heading if you don’t care.

(*) When your body burns converts glycogen to glucose then metabolizes that glucose to produce ATP, it produces two important waste products: glycol (which binds to fatty acids; a good thing) and pyruvate (which forms acid; a not-so-good thing). Glycol is needed to form triglycerides, so your liver makes use of it to store fat as energy. Pyruvate can be removed from muscle cells via the Krebs cycle (the name of the process where your cell furnaces burn up fat with oxygen) but it takes a long time. If you rack up too much pyruvate in an environment with lots of phosphate hanging around, such as in a muscle cell after many powerful contractions, you get lactate (of lactic acid fame). Then you stop running fast and jumping high, because lactic acid overloads your system until it is cleared. How is it cleared, you ask? Also via the Krebs cycle. We slow down because blood and intracellular pH goes wonky; we can go fast again once it’s back to normal. This is why long rest breaks are prescribed between peak power outputs like sprinting for speed development or maximal lifts for peak force development.

(**) Your brain gets first dibs on glucose. That means your liver fills up on blood sugar first, then stores it as hepatic glycogen. Your muscles are next in line for circulating sugars, storing them as muscle glycogen via a special one-way door. Finally, all your body’s other cells get a grab at blood sugar, but there may not be much left. Other cells can take glucose loans from your liver, starving your brain of what it needs. This is why you can think you ate right to #fuelthework yet still be dizzy or have brain fog after you train – you didn’t provide enough glucose for all the other cells of your body, so they borrow from your brain!

(***) Just the act of contracting opens the special one-way door on muscles to pull in glucose from the blood. Then, incredibly, a special hormone called insulin-like growth factor (IGF-1) is released concurrently to growth hormone when GH is in the presence of cortisol. IGF-1 can hold open the muscular door for glucose even in the absence of insulin. So your muscles gobble up glucose both during hard training and for a while after. They do so without waiting for insulin to ration out glucose to cells. This is why you need more food in general and far more carbs as an athlete. You train hard frequently, so your muscles get greedy about sugar, so much so that other cells don’t get what they need. That’s a bit of a bummer. But hey, this is also where the value of exercise comes in for Type I diabetics (pancreas doesn’t make insulin) and Type II diabetics (cells are insulin-resistant): hard exercise helps your body reduce its circulating blood sugar without requiring insulin!

One Last Note On “Eat Like An Adult”

Even though I have tried to avoid personal trainer-like nutrition advice, let me remind you: a sirloin cut from the butcher is better than Oscar Meyer deli ham; peppers from your farmer’s market are better than canned peas and multivitamins; rice you boil is better than boxed pasta.

Food processing and transport reduce nutrient density of your food. Commercially packaged food almost always has sugar and chemicals added to make it taste better (since it has fewer nutrients, it has less flavor). But I’m not going to fight you on your nutritional choices. You can build a very strong body with Spam, V-8 juice, and white bread. That body won’t be as hormonally resilient as it could be and you are compromising other areas of your health, but you would have, strictly speaking, satisfied the sports nutrition requirements.

Ultimately, sports nutrition isn’t complicated.

Citations

Look, this is not a comprehensive citations list. I have studied physiology, nutrition, and sports nutrition for over a decade, so know some things from earlier research that I’ll never find again. But these references will get you started if you want to go deep.

Beware: nutrition research is an overwhelming mess of speculation, poorly-structured research, and blatant conflicts of interest masquerading as impartial investigations. Be skeptical, be critical, and be discerning.

How to eat like an adult:

1. F*ck Calories free book by Krista Scott-Dixon
2. Fat Loss Happens On Monday book by Josh Hillis and Dan John
3. The Swing book by Tracy Reifkind (out of print; buy it used online)

How to #fuelthework:

1. SportsRD.org downloadable resources [link]
2. ROAR, 2ed book by Stacy Sims. Penguin Random House (2024).
3. The Athlete’s Plate by Adam Kelinson. VeloPress (2009).
4. Infographic from Gatorade Sports Science Institute [link]
   [Davis, JK et al. “In-season nutrition strategies and recovery modalities to enhance recovery for basketball players: a narrative review” (2021). Sports Med 2022 May;52(5):971-993. doi: 10.1007/s40279-021-01606-7.]

Books & Research Articles

1. Hardy, Chris and Marty Gallagher. Strong Medicine (2015). Dragon Door Publications.
2. Haff, Greg (editor). Essentials Of Strength Training and Conditioning. NSCA: Human Kinetics. [pick an edition, 1994, 2000, 2008, 2016, it’s all the same]
3. Fox, Stuart I. Human Physiology. McGraw-Hill. [same as above, just pick an edition]
4. Fuchs, Cas et al. “Carbohydrate intake of 10 g/kg body mass rapidly replenishes liver, but not muscle glycogen contents, during 12 h of post-exercise recovery in well-trained cyclists” (2025). J Physiol https://doi.org/10.1113/JP289115. [link]
5. Murray and Rosenblume. “Fundamentals of glycogen metabolism for coaches and athletes” (2018). Nutrition Reviews 76(4) pp243-259. [link]
6. Richter, Erik and Mark Hargreaves. “Exercise, GLUT4, and skeletal muscle glucose uptake” (2013). Physiol Rev 2013 Jul;93(3):993-1017. doi: 10.1152/physrev.00038.2012. [link]
7. Slimani, L et al. “Exercise restores skeletal muscle glucose delivery but not insulin-mediated glucose transport and phosphorylation in obese subjects” (2006). J Clin Endocrinol Metab 10.1210/jc.2006-0269. [link]