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Why you NEED to eat before you exercise

Fasted cardio: a fancy term for doing aerobic exercise on an empty stomach. It’s hailed by some as a great tool for boosting weight loss, the theory being it promotes the use of fat for fuel compared to exercising after you’ve had something to eat. Unfortunately, for women especially, the effects of exercising first thing before you’ve taken on any fuel really aren’t that clear cut. It’s actually not something I advocate to my clients at all, and would argue that fasted exercise may well hinder your efforts more than help them.


What does the research say?


If we take a look at research done on the topic, as is too often the case, women are very underrepresented making it tricky to draw definitive conclusions. BUT there have been a few small studies that can offer some insight. A couple studies have shown fasting to actually have a negative effect on training outcomes (like exercise performance and fat loss) while others show no difference between fasted exercise and exercising in a fed state. Looking specifically at the correlation between fasted cardio and fat loss - one study was carried out and found no difference at all between a sample of women completing 4 weeks of fasted cardio vs. a sample of women completing 4 weeks of fed state cardio. So, it would seem that if you’re looking to fasted exercise as a means to improve your body composition long term, it probably isn’t the answer.


Hormones gone haywire


It’s really helpful, particularly given the limited research, to take a look at what we know about female physiology to gain a greater understanding of how exercising on an empty stomach could be negatively impacting us and our goals. The fact is, doing too much intense exercise in a fasted state can wreak absolute havoc on our hormones. When we wake up in the morning our levels of cortisol (the stress hormone) are already elevated, having essentially “fasted” overnight whilst we’ve been sleeping. Exercise is, albeit good for us, a further form of stress on our bodies. If we head straight into our workout without fueling first, our cortisol levels increase further, reducing our ability to relax and potentially increasing levels anxiety. Fasted exercise can also disrupt the release of estrogen and progesterone, whilst depressing thyroid activity - which is key for regulating the menstrual cycle. Exercising without fueling appropriately can therefore leave you facing disrupted cycles, more anxiety, lower performance and due to hormones going haywire, even weight gain in the long term.


Women are also more sensitive to something called Low Energy Availability (LEA); this occurs when the amount of energy we take in from food isn’t enough to sustain the amount of energy we’re expending for the physiological functions we need to maintain optimal health. Signs and symptoms include

  • fatigue that isn’t relieved by resting

  • frequent colds or illness

  • IBS symptoms and general gut issues

  • missing or irregular periods

In the long run, LEA can obviously have very detrimental effects on not only our exercise performance, but our health in general.


Why fed is best


As far as I’m concerned, exercising in a fed state boasts far more benefits than a fasted one. Besides the obvious - you get to eat something tasty - here are a few of the advantages of training after taking on some fuel as opposed to training on an empty stomach

  • Better energy and performance

  • Improved glycemic (blood sugar levels) control throughout the day, preventing blood sugar dips and the desire to get snacky later in the day

  • Better regulation of reproductive hormones

  • Improved leptin and ghrelin control (the hormones responsible for regulating your appetite) throughout the day - essential for maintaining a healthy weight

What do I need to eat pre-workout?


I know some people choose to train on an empty stomach because eating before exercise makes them feel - well - ick. So let’s look at how much food you actually need to take on board to benefit. I promise it’s not a full buffet.

If you’re planning on exercising for less than 90 minutes then 15g protein before weight training is sufficient, or 15g protein plus 30g carbs before cardio. For some ideas of things that meet that quota that are easy to consume first thing

15g protein:

  • 150g greek yoghurt (low fat ideally just before training)

  • A scoop of whey protein or vegan protein

30g carb:

  • A banana

  • Thick slice of toast

  • Large glass of fruit juice

The importance of appropriate fuelling for women isn’t exclusive to what you’re eating preworkout. To feel your best, be your healthiest and get the best results from exercise long term, it’s really important you’re not undereating or avoiding major food groups (like carbohydrates or fats!). Check out my blog on postworkout nutrition and the foundations of nutrition to support you in eating in the best way possible for your body.



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