A few months ago, a client came to me frustrated. Running had helped her lose weight before, about ten years earlier, and it had worked well. So when the weight came back, she went back to what she knew. Same approach: run more, eat less, stay in a deficit.
But this time her body was not playing along. She was running more distance than she ever had before, eating carefully, and the scale was moving, but she was completely depleted. Exhausted in a way that felt different from normal tiredness. Her runs were getting harder instead of easier. Something was wrong, and she knew it.
What worked at 25 does not always work at 35 or 40. The body changes. Recovery capacity shifts. Hormonal function is more sensitive to energy availability. And when you add longer distances to the equation without adjusting how you fuel and how you run, you can end up working against yourself even when you are doing everything you think is right.
This is one of the most common patterns I see in women who return to running after a gap. It is also one of the most misunderstood.
What Was Actually Happening
When I looked at her full picture, training volume, calorie intake, protein, carbohydrate timing, the problem was clear. She was under-fueling relative to her output. Not dramatically, not in a way that looked alarming on a calorie tracking app, but enough that her body was starting to draw from reserves it needed for recovery, hormonal function, and performance adaptation.
Sports nutrition research refers to this as low energy availability: the gap between how many calories you consume and how many your body needs to support both basic physiological function and the energy cost of training. When that gap gets too large, the body prioritizes survival. Energy goes toward keeping your organs running. Everything else, including recovery, bone density, hormonal regulation, and performance, gets deprioritized.
This pattern has a name: Relative Energy Deficiency in Sport, or RED-S. While it is more frequently identified in women, it affects athletes of all sexes and is recognized by the International Olympic Committee as a serious performance and health risk. It can show up as persistent fatigue, declining performance, mood changes, disrupted sleep, and in more serious cases, stress fractures and hormonal disruption. My client was not at the severe end of that spectrum, but she was heading in a direction that would have gotten worse, not better, if nothing changed.
The weight loss she was seeing was real, but it was coming at a cost she could not see on the scale. Her body was losing weight by compromising performance, recovery, and hormonal function. That is not a sustainable trade. You can lose weight and train well at the same time, but not by ignoring what your body needs to actually do the work.
Why the Solution Worked: The Science
The fix was not just about eating more. It required addressing both how she was fueling and how she was running. Those two things were working against each other, and changing one without the other would not have solved the problem.
The protein fix
She was not eating enough to support muscle retention alongside her training volume. Protein is not just about building muscle. During periods of low energy availability, the body will break down muscle tissue for fuel, a process called gluconeogenesis. By increasing her protein intake and distributing it more evenly across meals, we gave her body a reason not to cannibalize the tissue she was working hard to maintain. Research consistently shows that spreading protein intake across three to four meals produces better muscle protein synthesis outcomes than consuming the same total in one or two sittings.
Fixing Her Training Intensity
One of the first things I changed was not her diet. It was her running. She was running her long runs too hard, which sounds counterintuitive as a problem, but it matters enormously from a physiological standpoint.
Your body uses different fuel sources depending on exercise intensity. At low to moderate intensities, roughly zone 2 on a five-zone heart rate scale, your aerobic system is dominant and fat is the primary fuel source. Your body has virtually unlimited fat stores to draw from, even in lean athletes, which is why easy long runs can be sustained for hours without bonking. At higher intensities, zones 3 and above, the body shifts toward carbohydrate as its primary fuel because fat oxidation cannot keep up with the energy demand. Glycogen, the stored form of carbohydrate in your muscles and liver, gets burned much faster.
She was running her long runs in zone 3 and above without realizing it. That meant she was burning through glycogen on runs that should have been building aerobic base and fat oxidation capacity. She was exhausting herself on runs that were meant to be easy, and because she was not fueling to match that glycogen cost, she was starting each new session already depleted.
We introduced proper zone training. Long runs slowed down significantly, staying in zone 2 where fat is the primary fuel and aerobic capacity actually develops. This also meant her post-run recovery nutrition became even more important because after a true long run in the right zone, the body has done significant work at the cellular level and needs to replenish and repair properly.
Carbohydrates still matter across all training, but the demand is highest during intensity work like tempo runs and intervals, not long easy efforts. We adjusted her carbohydrate strategy accordingly.
Matching Intake to Output
A moderate calorie deficit that is appropriate on a rest day is not appropriate on a day you are running 18 kilometers. Applying a flat daily calorie number regardless of training load ignores the basic physiology of energy expenditure. We adjusted her intake to reflect what she was actually asking her body to do on different days.
Fueling by Run Type: A Practical Breakdown
Not all runs have the same demands, and fueling should reflect that. Now that we understand the zone and fuel source relationship, the nutrition strategy for each run type becomes more logical.
- Easy runs and long runs in zone 2: These runs are primarily fat-fueled. You do not need to carb-load before them or consume gels during them unless you are out for more than two hours. What matters most is your baseline daily nutrition and what you eat after. Post-run recovery nutrition is critical even after easy long runs because of the cellular repair and adaptation happening at the muscle level, even when the effort feels manageable.
- Tempo runs and intervals: This is where carbohydrate demand is highest. You are working at intensities where glycogen is the dominant fuel. A proper carbohydrate-based meal two to three hours before a tempo session matters. Keeping carbs low on these days will blunt the quality of the effort and slow recovery afterward.
- Long runs over 90 minutes at any intensity: Even if you keep pace easy, duration eventually becomes a factor. After 90 minutes your glycogen stores can start to deplete even at aerobic paces, which is why taking in carbohydrates during very long efforts, around 30 to 60 grams per hour, becomes relevant as you build toward marathon distances. Post-run nutrition on these days should be treated as a priority, not an afterthought.
- Race pace work: Treat these like tempo runs in terms of pre-run fueling. High intensity means high glycogen demand.
Why Recovery Nutrition Matters
The work you do during a run is not where adaptation happens. Adaptation happens after, during recovery, when your body repairs damaged muscle fibers, replenishes glycogen stores, and makes structural changes in response to the training stimulus. If you do not eat after a run, or you eat too little too late, you interrupt that entire process.
Within the first 30 to 60 minutes after a hard or long run, your muscles are in a state of heightened insulin sensitivity. This is the window when GLUT4 transporters are most active, meaning your muscle cells absorb glucose more efficiently than at any other time. Carbohydrates consumed in this window go straight toward glycogen replenishment rather than being processed through normal metabolic pathways. Miss the window and you can still refuel, but the efficiency drops significantly.
Protein in the same window provides the amino acids needed for muscle protein synthesis, the process of repairing the micro-tears that running creates in muscle fiber. Without adequate protein post-run, repair is slower, soreness lasts longer, and adaptation is blunted. The research-supported target is 20 to 40 grams of high-quality protein alongside carbohydrates in a roughly 3 to 1 or 4 to 1 carb-to-protein ratio.
Practically: chocolate milk, Greek yogurt with fruit, rice and chicken, or a smoothie with protein. Not complicated. Just consistent.
For a deeper breakdown of post-workout nutrition including timing, food options, and the science behind the recovery window, read: Fuelling for Recovery: What to Eat After a Workout.
What Happened After
My client now trains at marathon-level mileage. The fatigue is gone. Her performance has continued to improve. And the weight loss she originally came in wanting is happening, sustainably, without her body fighting back.
That last part is worth sitting with. When you eat enough to actually support your training, your body stops holding on to everything out of a survival response. Under-eating is often what stalls fat loss in active women, not what accelerates it. Adequate fueling and body composition goals are not mutually exclusive. They are actually more compatible than most people realize.
What This Means for You
If you are running long distances, in a calorie deficit, and noticing fatigue, declining performance, constant hunger after long runs, or a feeling that your body is not responding the way it should, the answer is almost certainly not to eat less.
The question is not how to burn more than you eat. The question is whether you are eating enough to support what you are asking your body to do.
If you are training for a race or building mileage seriously and want to stop guessing, book a free 20-minute discovery call and we will figure out what is actually going on.
