Nutrition – Part 2 - Carbohydrates: Your Body's Essential Fuel Source

If there's one macronutrient that gets more confusing and contradictory advice than any other, it's carbohydrates.

Low-carb diets promise rapid weight loss. Keto enthusiasts swear by cutting them out almost entirely. Meanwhile, endurance athletes can't imagine training without them. So what's the truth?

At Run Fit Fordingbridge, I believe in cutting through the noise and looking at what the science tells us—especially the research specific to women. Let's talk about carbohydrates: what they are, why your body needs them, and how to make smart choices that support your health and fitness goals.

What are carbohydrates?

Carbohydrates are one of the three macronutrients your body needs (along with protein and fat). They're your body's primary and preferred source of energy, particularly for your brain and during physical activity.

When you eat carbohydrates, your body breaks them down into glucose (blood sugar), which provides immediate energy or gets stored in your muscles and liver as glycogen for later use. Your brain alone uses about 120 grams of glucose per day—that's roughly 400-500 calories just to keep your mind functioning properly (Mergenthaler et al., 2013).

For active women, carbohydrates are especially important. They're necessary to fuel high intensity efforts, and they improve exercise performance for any efforts lasting longer than 45 minutes (Sims, 2024a).

Potatoes - Carbohydrates

Why women need carbohydrates

This is crucial to understand: women perform best in a fuelled state (Sims, 2024a). That means eating carbohydrates. Research shows that restricting carbohydrates can lead to low energy availability (LEA), with about 55 percent of individuals who train every day being in low energy availability, with females especially at risk (Sims, 2024b). If not addressed, this can lead to serious health consequences including dangerously low bone mineral density.

When women don't consume enough carbohydrates while training regularly, your body perceives this as a threat. It can disrupt your hormones, impact your menstrual cycle, compromise your immune system, and actually signal your body to store more fat rather than lose it.

Not all carbohydrates are created equal

This is where things get important. Not all carbohydrates affect your body in the same way. Understanding the difference helps you make better choices without cutting out entire food groups.

Complex Carbohydrates are found in wholegrains, vegetables, legumes, and fruits. These foods contain fibre, vitamins, minerals, and beneficial plant compounds. They're digested more slowly, providing steady energy and helping you feel fuller for longer.

Refined Carbohydrates have been processed to remove the fibre and nutrients. Think white bread, white rice, pastries, and many packaged snacks. These are digested quickly, causing rapid spikes and crashes in blood sugar levels.

The difference matters. Research consistently shows that diets rich in wholegrain carbohydrates support better health outcomes, while diets high in refined carbohydrates are associated with increased risk of type 2 diabetes, heart disease, and weight gain (Reynolds et al., 2019).

Here's what's important to understand: refined carbohydrates cause rapid spikes in your blood sugar, leading to energy crashes, whereas wholegrains release glucose slowly and steadily, maintaining stable energy levels throughout the day. This is why eating balanced meals with wholegrains, protein, healthy fats, and fibre helps prevent those afternoon slumps we talked about in the last blog post.

The role of fibre

Fibre is the part of plant foods that your body can't digest, and it's one of the most important reasons to choose whole food carbohydrates over refined ones.

Fibre does several crucial things:

• Slows down digestion, helping you feel fuller for longer

• Supports healthy gut bacteria

• Helps regulate blood sugar levels

• Supports digestive health and regular bowel movements

• May reduce cholesterol levels

The UK recommendation is to consume 30 grams of fibre per day, yet most women consume only around 17 grams (British Nutrition Foundation, 2023). Choosing wholegrains, vegetables, fruits, legumes, nuts, and seeds naturally increases your fibre intake.

Vegetables also have carbohydrates

Carbohydrates and exercise

If you exercise regularly, carbohydrates become even more important. Here's why:

They fuel your workouts. Your muscles use glycogen (stored carbohydrate) as their primary fuel source during exercise, especially during higher intensity activities like HIIT classes, indoor cycling, or running.

They support recovery. After exercise, women should aim for approximately 3 grams of carbohydrates per kilogram of body weight within 2 hours of finishing their workout to help replenish glycogen stores (Huberman, 2024).

They prevent excessive fatigue. When you consistently under-consume carbohydrates while training regularly, you risk chronically depleted glycogen stores. This can lead to persistent fatigue, poor performance, increased injury risk, and difficulty recovering between sessions.

They support your immune system. Research shows that adequate carbohydrate intake during and after exercise helps maintain immune function, while training in a carbohydrate-depleted state may suppress immunity (Bermon et al., 2017).

For women training for longer events—like half marathons or ultra-marathons—carbohydrates become absolutely essential. Your body simply cannot sustain prolonged endurance activity without adequate carbohydrate availability.

How much carbohydrate do you need?

Research suggests that active women should aim for 40-45% of daily calories from whole food carbohydrates (Huberman, 2024). The right amount for you depends on how you feel, your energy levels, your performance, and your individual goals.

Making better carbohydrate choices

Choose these more often:

• Wholegrains: brown rice, quinoa, oats, wholegrain bread and pasta

• Vegetables: all types, prepared in various ways

• Legumes: lentils, chickpeas, beans

• Fruits: whole fruits rather than juices

• Potatoes: with skins on where possible

Limit these:

• White bread, white rice, white pasta

• Highly processed snack foods

• Cakes, biscuits, pastries

• Breakfast cereals high in added sugar

Notice this isn't about never having refined carbohydrates. It's about building your eating habits primarily around whole food options and treating refined options as occasional choices.

Complex carbohydrates

Start small

Just like we discussed in the last blog post, making changes doesn't have to be overwhelming. Here are two simple swaps you can start with:

Swap white for wholegrain. Next time you buy bread, pasta, or rice, choose the wholegrain version. You'll get more fibre, more nutrients, and more sustained energy. If you're not used to wholegrains, you can start by mixing half white and half wholegrain until you adjust to the taste and texture.

Boost your meals with extra vegetables. Instead of a large bowl of pasta with sauce, use a smaller portion of pasta and bulk it up with roasted vegetables. Instead of toast for breakfast, have one slice of toast with scrambled eggs and sautéed mushrooms and tomatoes.

Small changes like these, repeated consistently, make a significant difference over time without requiring you to overhaul everything at once.

The bottom line

Carbohydrates aren't the enemy. They're an essential macronutrient that provides energy, supports your active lifestyle, and when chosen wisely, delivers important nutrients your body needs.

The key is focusing on quality—choosing whole food sources most of the time, understanding how different carbohydrates affect your energy and how you feel, and consuming an appropriate amount for your activity level.

Next time, we'll explore protein—why it matters, how much you need, and the best sources to support your health and fitness goals.

If you'd like personalised guidance on nutrition to support your fitness goals, book a "Let's Chat - How Can I Help?" session through the online booking system. We can discuss your goals and create a path forward that works for your lifestyle.

References

Bermon, S., Castell, L.M., Calder, P.C., Bishop, N.C., Blomstrand, E., Mooren, F.C., Krüger, K., Kavazis, A.N., Quindry, J.C., Senchina, D.S., Nieman, D.C., Gleeson, M., Pyne, D.B., Kitic, C.M., Close, G.L., Braakhuis, A.J., Rogero, M.M., Pelly, F.E. & Burke, L.M. (2017) 'Consensus Statement: Immunonutrition and Exercise', Exercise Immunology Review, 23, pp. 8-50.

British Nutrition Foundation (2023) Nutrition Requirements. Available at: https://www.nutrition.org.uk/putting-it-into-practice/food-based-dietary-guidelines/dietary-reference-values/

Huberman, A. (2024) 'Dr. Stacy Sims: Female-Specific Exercise & Nutrition for Health, Performance & Longevity', Huberman Lab Podcast, 22 July. Available at: https://www.hubermanlab.com

Mergenthaler, P., Lindauer, U., Dienel, G.A. & Meisel, A. (2013) 'Sugar for the brain: the role of glucose in physiological and pathological brain function', Trends in Neurosciences, 36(10), pp. 587-597. https://doi.org/10.1016/j.tins.2013.07.001

Reynolds, A., Mann, J., Cummings, J., Winter, N., Mete, E. & Te Morenga, L. (2019) 'Carbohydrate quality and human health: a series of systematic reviews and meta-analyses', The Lancet, 393(10170), pp. 434-445. https://doi.org/10.1016/S0140-6736(18)31809-9

Sims, S.T. (2024a) 'Female Athletes Need Carbohydrates', Dr Stacy Sims Newsletter, 12 October. Available at: https://www.drstacysims.com/newsletters/articles/posts/Female_Athletes_Need_Carbohydrates

Sims, S.T. (2024b) 'Restricting Carbs Hurts Performance and Health', Dr Stacy Sims Newsletter, 14 October. Available at: https://www.drstacysims.com/newsletters/articles/posts/Restricting_Carbs_Hurts_Performance_and_Health