That's a decrease in glycogen (low blood sugar), and causes a decrease in performance until the athlete just can't go on anymore. Especially sports where a lot of energy or calories are burned.(Like road cycling, which I do, btw).
Also, I've read recent articles that show women burn energy differently than men. Women are more likely to need fuel before and during, whereas men can start on a empty stomach, and just fuel as they go. But, they typicall need more calories since they burn more.
When I ride at an average of 19 mph, I burn about 450 calories per half hour. (As determined by my weight and speed). Since I might do a metric century (100 km or around 63 miles) or a century (100 miles), my need for fuel is considerable. On the day of my last metric, I averaged 19.5 mph, completed it in 3 hours and 20 minutes, and ate in excess of 4500 calories that day. By the next day, I had lost 3 pounds. I also ate a lot on the following day, and continued to lose 2 pounds per day for the next 3 days, until I leveled out. I leveled out by eating well. (High calorie, high quality whole foods). I had burned too much energy to consider losing weight under those circumstances.
And that is the key, really. A high-performance athelete has a lot of control over their body weight, caloric intake and output, rate of metabolism. Losing weight under the wrong circumstances is not healthy. Moderation is better during weight loss. Not high-performance.
On a short ride (aroud 20-30 regular miles) I might drink 250-300 calories of sports drink, plus 120 calories per hour or so of food. (Sports drink is juice smoothie, protein powder, salt, and water. Solid food is high whole grain carb and protein energy bar and/or honey). Plus, I eat an additional 350 calories of high protein and whole grain carbohydrates within an hour of stopping, because your muscles absorb optimum amounts of protein during that time. This is considered a recovery meal. All of this is in addition to my regular calories per day. I generally eat 1500-1800 calories per day with no or little exercise (under 1.5 hours of exercise is a light day of exercise for me). When I go to the gym, I might add 200 calories or so, but not too much. Just depends. If I go to the gym for 4 hours, I eat more.
The reason I add salt to my home-made sports drink is to prevent hypotranemia, or an electrolyte imbalance. When you sweat a lot and hydrate without replacing salt, this can happen.
The term for not fueling properly in cycling, so that a rider can't go on, is 'bonking.' Seriously If you bonk, you didn't hydrate and fuel properly. You are also considered a newbie rider if you get into that kind of trouble. And rightly so.
I should add, I typically eat: whole grain carbohydrate/high protein Cliff Bars, Odwalla bars, Kashi Cereal, Oatmeal, juice smoothies, fruits like banannas, berries, citrus, apples, kiwi, veggies like avocados, greens, yellow veggies, legumes, brown rice, other whole grains like Teff, Amaranth, Buckwheat, raw nuts, eggs, milk, Greek Strained Yogurt, Mushrooms (very high in protein, btw), Bolthouse Farms protein and or smoothie drinks. Etc etc. I attempt to eat only whole grains. I never eat fried foods, though I use a lot of olive oil, olives are good too, come to think of it. Snacks are, uh, Cliff bars, Wasa Crackers, Edamame (soy) nuts, raw brazil nuts, fig bars, some cheese. I use real butter too. Small amounts though.
Absolutely no 'diet' foods. Diet foods are high in salt, sugar, or fat to compensate for what is lost. It's low quality food and I refuse to eat it.
Okay, so. Can you answer your own question now? What kind of condition is the athlete in? How long did they exercise and at what intensity? Did they hydrate properly and maintain electrolyte balance? Do they eat well in general? Or do they eat garbage? Are they trying to lose weight by extreme measures? (Leaving out entire food groups like carbs, extended periods of too few calories, wrong balance of salts, maybe too much caffiene, avoiding food before exercise to lose weight. Or, do they attempt too much exercise for achieved fitness-level? Didn't cover that, but self-explanatory, conditioning is necessary. Did they warm-up properly? That could contribute to the body shutting down if they jump in without warming up plus no food.