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Losing weight quickly? An expert’s top nine watch-outs

Rapid weight loss can affect more than the number on the scales. From muscle loss and loose skin to digestive changes, tiredness and brain fog, nutritionist Jenna Hope explains what to look out for and the practical steps that can help.

By Mumsnet HQ | Last updated Aug 11, 2026

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This article provides general nutritional and wellbeing information and is not a substitute for medical advice. Supplements should complement, rather than replace, a varied and balanced diet.

Rapid weight loss can bring changes that are easy to miss when attention is fixed on the scales. Muscle, skin, hair, digestion, energy and mood can all be affected - and knowing what to look out for makes it easier to act early.

A reduced appetite can also make it harder to get enough protein, fibre, vitamins and minerals. When you’re eating less, what you eat matters more.

Nutritionist and Absolute Collagen ambassador Jenna Hope says: "Rapid weight loss can involve more than losing body fat. If you’re eating much less overall, it can become harder to get enough of the nutrients your body needs. This may contribute to symptoms such as tiredness, hair thinning, headaches, nausea, bloating and disrupted sleep.”

Here, Hope answers common questions about nine effects of rapid weight loss, with practical advice on what may help and when to seek medical support. 

Jump to the issue affecting you

1. Muscle loss can happen alongside fat loss

Weight lost during a calorie deficit is not necessarily all body fat; some lean muscle mass can be lost too. Protecting it matters for strength, metabolism and longer-term health.

“Why am I losing muscle as well as fat during my weight loss journey, and how can I protect it?”

Hope says: “During weight loss, the body breaks down fat for energy, but it can also use muscle tissue to release amino acids that help support energy production.”

“Is strength training important, even if you're eating less?” 

Resistance exercise doesn’t have to mean long or punishing gym sessions. Lifting weights, using resistance bands or doing bodyweight exercises at home can all help.

Hope says: "Strength training is important in order to protect muscle breakdown which occurs as a result of calorie restriction. Muscle plays an important role in metabolism and insulin sensitivity which is vital when it comes to maintaining weight loss, so regular training is important."

Start gradually and choose activities that feel manageable. If you have a medical condition, an injury or any concerns about exercising safely, speak to your GP or healthcare professional before starting a new exercise programme.

Where Absolute Collagen SCULPT PRO fits: SCULPT PRO’s evening formula includes creatine, alongside magnesium and vitamin D. It's designed to work alongside, not instead of, enough protein and strength-based exercise. In a four-week survey of 134 SCULPT PRO users, 77% said they felt they'd maintained muscle tone.

2. Loose skin and facial changes can become more noticeable

Skin does not always adapt at the same rate as the body beneath it. Age, genetics and the amount and speed of weight loss can all affect how it looks and feels.

“I’ve started to notice loose skin mainly around my bingo wings and tummy. What can I do about this?” 

Hope says: “Loose skin can occur after rapid or substantial weight loss. Regular strength training may help by building and preserving the muscle beneath the skin.

“Eating enough protein and maintaining a balanced diet also supports normal skin health, although the amount of loose skin will vary from person to person.”

Strength training can improve muscle tone, but it can't guarantee that loose skin will disappear. If you're left with a significant amount of excess skin that's affecting your comfort or quality of life, speak to your GP or another healthcare professional about your options.

“How do I avoid ‘Ozempic face’?” 

‘Ozempic face’ isn't a medical diagnosis. It's simply an informal term people use to describe a face that looks leaner after significant weight loss, with less fullness and more noticeable lines.

Hope says: “When you're eating a lot less and have a low nutrient intake, your skin can end up drier and less plump. A balanced diet, protein, enough water, and the right vitamins all matter here, especially vitamin C, which your body needs to make collagen."

What the research shows: In a 12-week clinical study of Absolute Collagen's original formula, the same collagen and vitamin C combination used in SCULPT PRO's morning formula delivered the following results: 83% of participants said their skin felt softer and smoother, 75% said their skin felt plumper, and 100% of participants showed a positive change in the appearance of their fine lines and wrinkles.


Support for the things the scales don’t mention 

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3. Hair shedding may appear later

Hair shedding can appear some time after rapid weight loss begins. Low nutrient intake is one possible factor, although illness, stress and hormonal changes can also play a part.

“Why is my hair shedding while I’m losing weight?”

According to Hope: “Hair thinning can occur as a result of nutritional deficiencies. When nutrient intake is low, the body prioritises the functions it needs most. Hair is one of the first things to be affected.”

“My hair feels much thinner. Is it likely to grow back?”

Hope says: “Supporting your overall nutritional intake is important.” Hair loss after rapid weight loss is often temporary, but recovery depends on the underlying cause. 

Hair loss can have several causes, including illness, stress, hormonal changes and nutritional deficiencies. Speak to your GP if it's sudden, severe, patchy, persistent or accompanied by other symptoms.

Where Absolute Collagen SCULPT PRO fits: The SCULPT PRO evening formula contains zinc, iron, magnesium and B vitamins, which contribute to the normal maintenance of hair. Supplements should be used alongside a balanced diet.

4. Digestion can change as your appetite and diet change

Eating less, changing your diet or taking certain medicines can affect digestion. Nausea, constipation and feeling full quickly can then make balanced eating and hydration more difficult.

“I’m experiencing constipation and nausea - especially when I go up a dose. What’s the best way to manage this?”

Hope says: "Ensuring you're eating enough high fibre foods can help to manage constipation. Beans and pulses are rich sources of fibre and work really well when added to soups and salads. 

"Nausea is also a common side effect, however eating smaller meals more regularly and at a slower pace can help to reduce nausea."

If this happens after changing the dose of prescribed medication, contact your prescriber for advice rather than altering the dose yourself.

Seek medical help if you experience severe or persistent abdominal pain, repeated vomiting, signs of dehydration, blood in your vomit or stools, or constipation that doesn't improve.

“I read that digestive problems can be a long-term side effect of losing weight fast. Is this true?”

Hope says: "If nutrition is not well considered whilst on a rapid weight loss journey, impaired gut health can be longer-lasting. Supporting your gut through a varied diet, high fibre foods, hydration and live cultures can help to promote better gut health."

Increase fibre gradually and drink plenty of fluid to reduce the risk of bloating.

5. Tiredness and poor sleep can reinforce each other

Low energy can be linked to reduced food intake, but stress, disrupted sleep, nutritional deficiencies and hormonal changes may also contribute. The first step is to consider the wider picture rather than assuming weight loss is the only cause.

“How do I know if I’m going through perimenopause/menopause or if I’m just CRAZY tired?”

Hope says: “Whilst fatigue is a symptom of perimenopause, it’s unlikely to be the only symptom. 

“Other symptoms can include changes to the menstrual cycle, anxiety, impaired sleep, hot flushes, night sweats, difficulty losing weight/not losing weight as quickly, stress and more.
“It’s unlikely that just feeling very tired is indicative of perimenopause. If you’re unsure, always seek personalised advice from your GP.”

“How do I combat the 3am insomnia? It’s killing my energy levels throughout the day.”

“Insomnia is a challenging symptom and can be affected by a wide variety of factors,” explains Hope. “However, reducing the risk of nutritional deficiencies, ensuring you’re not going to bed hungry and having a healthy evening routine can help. 

“Try avoiding screens too close to going to bed, have a clear wind-down routine, avoid consuming caffeine after 12pm and try to stick to consistent sleep and wake times.”

“I don’t have the energy to go to the gym like I used to. How can I stay active when I’m running on empty?”

“You don’t have to be training to the same level you previously have. Try to slot your exercise into your daily routine at a time that suits you best. Switching a gym session for a home workout or evening Pilates can be just as effective,” advises Hope.

Some days, a walk or a short workout at home may feel more achievable than a trip to the gym. The aim is to find movement you can do consistently without pushing through pain, dizziness or significant fatigue.

Where Absolute Collagen SCULPT PRO fits: SCULPT PRO's morning formula contains B vitamins, which contribute to the reduction of tiredness and fatigue. In a four-week survey of 134 SCULPT PRO users, 69% of participants said they felt they had more energy.

6. Smaller meals need to work harder nutritionally

When appetite is low, there may be fewer opportunities to get the nutrients you need. The aim is not a perfect plate, but smaller meals that still include protein, fibre, healthy fats, vitamins and minerals.

“Which foods or supplements should I focus on while calorie counting?”

Hope says: “Build your meals around good sources of protein, such as meat, fish, eggs, dairy, beans and pulses, alongside fruit, vegetables and wholegrains. This will help you get a broader range of nutrients while supporting your energy levels. If your appetite is very low, a carefully chosen supplement can provide additional support."

“I’m trying to prioritise eating the protein on my plate first when I’m not that hungry. Is that the right thing to do?”

“Yes, protein is vital for hormone production, muscle and bone health, cognitive function, energy and more,” shares Hope. “Prioritising protein is important but don’t forget to eat your fibre too. Beans and pulses are a great source of both!”

Try to include a source of protein with each meal, alongside vegetables, fruit, wholegrains and other fibre-rich foods whenever you can.

7. Brain fog can make everyday tasks harder

Poor concentration and forgetfulness can affect work, caring responsibilities and everyday tasks. Hydration and regular nourishment are sensible first checks, but persistent symptoms should not be dismissed.

“Wow! The brain fog is real. I’m finding it really hard to focus on tasks throughout the day. HELP please!”

Hope says: "Impaired cognitive function can occur when you're on a restricted diet as you're getting less energy and fewer nutrients. Staying hydrated is important, and try to limit high-sugar foods as these can contribute to energy spikes and subsequent crashes."

Regular meals, fluids and a balanced diet are likely to be more effective than relying on caffeine or sugary snacks. 

Speak to your GP or another healthcare professional if your brain fog doesn't improve, gets worse or starts affecting your day-to-day life.

8. Weight loss can affect how you feel as well as how you look

Physical progress does not guarantee an emotional transformation. Changes to your body, routine and eating habits may sit alongside stress, anxiety or low mood, even when you are pleased with your weight loss.

Can losing weight affect my mood or anxiety levels?

“For some people, eating fewer calories and getting fewer nutrients can contribute to lower moods and increased anxiety,” warns Hope. “Ensure you’re getting enough Vitamin D3 as low levels of Vitamin D have been associated with low mood. For those who are prone to anxiety, eating regularly, limiting caffeine and sugar consumption and prioritising sleep health can all be helpful.”

If you're thinking about taking a vitamin D supplement, check the label first and ask a pharmacist or healthcare professional if you're unsure what's right for you.

“I’m loving my new reflection in the mirror but I’ve not been feeling myself for a few months (since my weight loss journey started). When should I seek medical advice?”

Hope says: “If you feel your moods or ability to function on a daily basis is affecting your quality of life, it’s vitally important that you seek personalised medical advice.”

Don’t wait for your mental health to reach a crisis point before asking for support. Speak to your GP if low mood, anxiety or changes in your behaviour are continuing or affecting your relationships, work or daily life. Seek urgent help if you feel unable to keep yourself safe.

9. New vision, bone or joint symptoms need attention

Do not assume every new symptom is caused by weight loss. Changes to vision in particular deserve proper assessment, while painful or swollen joints and restricted movement should also be checked.

Can losing weight quickly affect your eyesight? I feel like my vision’s been quite blurry recently. Is that a coincidence?

“Some people on a rapid weight loss journey have reported poorer eye health,” shares Hope. “Nutritional deficiencies (particularly Vitamin A, omega-3 and zinc) and changes to blood sugar levels can affect vision. If you are concerned, please seek advice from an optician.”

Seek urgent medical advice if you experience sudden loss of vision, severe eye pain, flashing lights, a dark shadow across your vision, or any rapid and significant change in your sight. 

“I’m a lot more ‘clicky’ over recent months. Can losing weight affect your bones and joints?”

Hope says: “If you’ve lost a lot of weight and haven’t been on top of your nutrition, you may find that your bones and joints are weaker. Eating adequate amounts of protein and healthy fats is important for maintaining bones and joint health.”

Clicking joints isn't always a sign of a serious problem, but if they're painful, swollen, limiting your movement or accompanied by weakness, speak to your GP or another healthcare professional.

About the expert

Jenna Hope is a registered nutritionist and author specialising in practical, evidence-based approaches to nutrition. She works with individuals and brands across areas including skin health, gut wellbeing, hormonal health and energy. She also reviews research into collagen supplementation and its potential role alongside, rather than in place of, a varied and balanced diet.

About Absolute Collagen

Absolute Collagen is the original collagen specialist - and the UK’s No.1 expert backed collagen supplement. Each daily dose delivers a powerful 8,000mg dose of hydrolysed marine collagen peptides, clinically formulated for high absorption and visible results.


Food supplements should not be used as a substitute for a varied, balanced diet and healthy lifestyle. Always read the product information and check suitability before use. Consult a qualified healthcare professional before taking supplements if you are pregnant, breastfeeding, taking medication or have an existing health condition. 

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