Feeling tired on a diet? How calorie restriction can affect your energy
Written byEllie Hughes
Reviewed byMillicent Chapman
Feeling constantly tired isn’t something you should ignore – it can be a sign your body needs more fuel. So, we talked to an expert to understand why you might feel tired on a diet and how to fix it
Summary
1What happens when you restrict calories?
When you cut calories, you're reducing your body’s energy intake. It's all about finding the right balance and ensuring you have sufficient fuel.
2How nutrients can affect your energy levels
Our bodies need both macronutrients and micronutrients. The body’s main efficient source of energy is glucose (a type of simple sugar).
3How can you maintain energy levels on a diet?
If you’re adjusting your eating habits, aim for balanced meals that contain complex carbohydrates, protein, healthy fats, fibre, fruit and veg.
Trying to lose a bit of weight? It’s not a one-size-fits all but most of us start by cutting down how much we’re eating. But it can be frustrating if you’re doing this, only to feel more tired than normal. So, what’s going on?
“Many different factors can impact energy levels, including sleep and stress,” explains Millicent Chapman, registered associate nutritionist at Holland & Barrett. “Often people may associate being on a diet with reducing their food intake, therefore reducing their intake of energy.”
But is this association correct? We asked Millicent to explain what’s actually going on in your body when you cut back on calories and how you can avoid feeling tired on a diet.
How your body converts food into energy
“Energy metabolism is the name for how your body breaks down food and turns it into usable energy,” says Millicent. “During digestion, food is broken down and processed by your cells to release energy.1 A calorie is a unit used to measure energy. In simple terms, it tells you how much energy food contains.”
“Your body needs energy for basic functions like breathing and keeping your heart beating, as well as for movement and physical activity,” explains Millicent.
“Carbohydrates are the body’s main and quickest source of energy. They can be stored in the body (mainly as glycogen) and later converted into energy when needed, helping power the brain, muscles and other tissues.”
What happens when you restrict calories?
“When you cut calories, you’re reducing your body’s energy intake,” says Millicent. “For some people, creating a small calorie deficit can support weight loss over time (which is using more calories than are being consumed), which requires the body to use pre-existing stored energy to fuel daily functions.”
But it’s a case of getting the right balance and it’s also important to remember that actively reducing calories isn’t necessary, or appropriate, for everyone.
“When the deficit is too extreme, this can influence symptoms such as tiredness, fatigue and lower concentration levels, as the body doesn’t have sufficient fuel to support normal bodily processes,” she explains.
How nutrients can affect your energy levels
What are macronutrients?
When it comes to our nutrient considerations, our bodies need both macronutrients and micronutrients. Macronutrients are nutrients required in large amounts (grams) that provide energy.2 “The macronutrients that the body relies on to support energy are carbohydrates, fats and proteins, with carbohydrates being the most efficient energy source for the body,” says Millicent.
“The body’s main efficient source of energy is glucose (a type of simple sugar), which is consumed through carbohydrates. Complex carbohydrates (for example, wholegrains, legumes and starchy vegetables like sweet potatoes) digest slowly, giving steady energy without sharp spikes or crashes in blood sugar.” All of this can also support your BMR (basal metabolic rate).
What are micronutrients?
Micronutrients are nutrients needed in small amounts (milligrams/micrograms) to support vital body processes, metabolism and immunity.2 They don’t provide energy themselves but can help your body produce it.
“Some key micronutrients that support the body to unlock energy from food are B vitamins, iron, and magnesium,” explains Millicent.2
- B vitamins: “These can help the body to unlock energy from food, reducing tiredness and fatigue.3 They assist in converting food into usable energy in our cells, known as ATP (adenosine triphosphate),” says Millicent
- Iron: “This plays an important role in your cells, especially in mitochondria, where energy (ATP) is produced from food. It also helps carry oxygen in the blood. If you don’t get enough iron, you may feel tired, while getting enough helps your body function properly."4
- Magnesium: “Magnesium helps your body produce energy by supporting many enzyme reactions.5 It’s especially important for making and using ATP and helps break down food into usable energy in your cells.”
How can you maintain energy levels on a diet?
If you’re feeling tired on a diet, it may be time to reassess what you’re eating so you can maintain your energy levels.
“If you’re adjusting eating habits, we’d recommend aiming for a balanced meal plate at each of the three main meals,” says Millicent. “You want to ensure your main meals contain sufficient complex carbohydrates (the body’s main source of energy), protein, healthy fats, fibre and a variety of fruit and vegetables.”
Soluble fibre (found in oats, beans and fruits) and protein are important for several reasons. “They can slightly slow down digestion, promote appetite regulation and help keep you fuller for longer (creating that feeling of satiety), which can be useful for weight management related goals,” says Millicent. Good nutrient dense foods to eat to ensure you’re getting enough energy when you’re aiming to reduce calories include:
- bananas
- tuna and salmon
- sweet potatoes eggs almonds Greek yoghurt oranges
Looking for inspiration? Some of our favourite high-protein recipes include chickpea pasta with red lentil pesto and chocolate protein baked oats, while for a fibre boost, we love dishes like ginger tadka dal and chocolate orange mousse. Following a diet shouldn’t mean going hungry or giving up the foods you enjoy – it's simply about finding smarter, more nourishing ways to adapt them.
Dieting and tiredness FAQs
In some cases, your body may adapt to eating less by becoming more energy-efficient – this is sometimes called metabolic adaptation. However, the extent of this varies from person to person. “Your body lowers how many calories it burns to save energy, which can slow weight loss and make it easier to regain weight later,” says Millicent.
Eating less doesn’t necessarily mean you’re eating a healthy diet or enough. Low energy intake, skipping meals or not getting the right balance of carbohydrates, protein and fats can all lead to fatigue. Make sure you’re eating plenty of complex carbohydrates such as bananas, sweet potatoes and brown rice. If you still feel tired for a few weeks and this doesn’t seem to help, it may be worth getting your blood tested to make sure you’ve got enough of the key vitamins in your body. Plus, doing this can help rule out anything else.
“We would always recommend a food first approach where possible,” says Millicent. “But if someone on a calorie restricted diet is lacking certain nutrients, such as vitamin B, iron or magnesium, then supplementing with those nutrients could be beneficial to prevent deficiencies, if their healthcare provider also agrees as suitable.”
The final say
Feeling tired while changing the way you eat is often a sign that your body needs a little more support, not less. “One of the most common causes for feeling tired can be that a diet lacks the essential nutrients the body needs to function properly,” says Millicent. But with the right balance, it’s entirely possible to feel energised while working towards your goals.
Focusing on regular, balanced meals, enough overall energy and nutrient-dense foods can make a real difference, not just to how you feel day to day, but to how sustainable your approach is long term. Ultimately, supporting your health isn’t about eating as little as possible – it’s about giving your body what it needs to feel and function at its best. Just make sure to consult a registered nutritionist or dietician before starting any weight loss plan. For some extra support, take a look at 10 delicious foods and drinks to add to your mornings or our top products to support energy.
Disclaimer – This article provides informational advice and is not a substitute for medical care. Working with experts for accuracy, we take great care to ensure the information is up-to-date and relevant. However, you should always consult your GP or healthcare professional before using supplements or alternative products, particularly if you have medical conditions or are under supervision.
1. Sánchez López de Nava A, Raja A. Physiology, Metabolism [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2022. Available from: https://www.ncbi.nlm.nih.gov/books/NBK546690/
2. Espinosa-Salas S, Gonzalez-Arias M. Nutrition: Macronutrient Intake, Imbalances, and Interventions [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK594226/
3. Lee MC, Hsu YJ, Shen S, Ho CS, Huang C. A functional evaluation of anti-fatigue and exercise performance improvement following vitamin B complex supplementation in healthy humans, a randomized double-blind trial. International Journal of Medical Sciences [Internet]. 2023 Jan 1;20(10):1272–81. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10542023/]
4. National Institutes of Health. Iron: Fact Sheet for Health Professionals [Internet]. National Institutes of Health. 2024. Available from: https://ods.od.nih.gov/factsheets/Iron-HealthProfessional
5. Fatima G, Dzupina A, Alhmadi HB, Magomedova A, Siddiqui Z, Mehdi A, et al. Magnesium Matters: A Comprehensive Review of Its Vital Role in Health and Diseases. Cureus [Internet]. 2024 Oct 13;16(10). Available from: https://www.cureus.com/articles/300811-magnesium-matters-a-comprehensive-review-of-its-vital-role-in-health-and-diseases#
