Why It’s Better to Over-Fuel Than Under-Fuel


by Megan Kuikman, RD

Most athletes are concerned about how nutrition can help them recover faster, perform better or reach an ideal body composition. However, adequate calorie intake should be a main focus for all athletes, as the negative consequences of under-fuelling can be more detrimental than over-fuelling.

Under-fuelling means not eating enough calories to cover the calories burned through exercise, plus the calories to simply get through the day. Energy from calories is required for the heart to beat, for the lungs to breathe and for the brain to think. Even if a person was to sleep all day, they would still require calories. If not enough calories are consumed, there are metabolic and hormonal changes that occur (in both females and males) that are detrimental to one’s health.

Stefania Crudeli/Unsplash

The consequences of not eating enough go beyond the risk of nutrient deficiencies such as iron deficiency or being chronically fatigued. For instance, under-fuelling can cause a negative change in lipid profile and endothelial dysfunction, which increases cardiovascular risk. That’s right, someone may be exercising to improve heart health, but if they’re under-fuelling, this could actually be increasing their cardiovascular risk. Here are some other hormonal and metabolic changes that can occur from under-fuelling. 

Reduced metabolic rate
While weight loss may come easily at first, many athletes become frustrated when their weight becomes stable despite eating very little and exercising a lot. Eating fewer calories than required does not magically result in weight loss. The body responds to calorie restrictions by decreasing resting metabolic rate and increasing body fat stores. The body is smart: if not properly fuelled, it is able to make changes that reduce the amount of calories required rather than increase weight loss. This reduced metabolic rate can result in symptoms such as constipation, feeling constantly cold, or having a lack of energy. 

Increased injury risk
Under-fuelling significantly increases risk of injury. The hormonal changes that can occur with inadequate calorie intake reduce bone density. This not only increases the risk of osteoporosis, but also puts athletes at a significantly higher risk of a stress fracture. Beyond bone health, exercising with low glycogen stores (which occurs when not enough carbohydrates are consumed) increases risk of injury. Finally, if an athlete doesn’t properly refuel after exercise, the recovery phase is compromised. To stay injury-free, eating enough is vital.

Decreased athletic performance
Not eating enough impairs athletic performance and will prevent athletes from reaching their full potential. Under-fuelling decreases muscle strength, lowers endurance and impairs judgement. To optimally perform, fuelling must also be optimal.

Recurring injuries, decreased performance or any of the other negative results mentioned above could signal a problem with under-fuelling. If a female athlete ceases to have a period (known as amenorrhea), it should be taken as a serious sign that not enough calories are being consumed, and it should be investigated further. Other signs of not eating enough include changes in mood, such as irritability and depression.

Athletes should focus specifically on eating enough around exercise. Fuel up before exercise and again immediately afterwards. Don’t try to “save” calories for later on. Under-fuelling will not only lead to sub-optimal performance but may also cause injury, which could mean no exercise at all.



Megan Kuikman is a Registered Dietitian and distance runner from Brantford, Ontario.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.