Reasons Why You Struggle to Put on Weight

7 min read

There is so much information out there for people who struggle to lose weight. I’m not saying it’s all great information. But there is a lot. If you’re someone who struggles to put on weight, and wants to put on weight, well, you’re on your own. And if you're female everyone hates you because you could eat donuts for breakfast and not put on weight.

When I was personal trainer, most of my clients wanted to lose weight, but a small percentage, that were mostly men, wanted help building muscle. This hasn’t changed since qualifying as a nutritionist. When I mean small percentage, I mean REALLY small, because most men wanted to be trained by the male personal trainer with the biggest muscles, because women don’t know anything about weight lifting.

They really need a sarcasm font.

Bottom line if you want to put on weight, you need to be in an energy surplus. I’ve covered the principle of energy balance in an article of weight loss. Just like some overweight people struggled to lose weight, some people struggle to put on weight.

There are many reasons why people struggle to put on weight and it’s unsurprising to find that these reasons are mostly the opposite to reasons why people struggle to lose weight. What I found was that common mistakes were being made in the quest for putting on weight. And these apply to both men and women. You might be different sexes, but both male and female bodies will respond to whatever energy balance they’re in.

Reasons You Are Struggling to Put On Weight

You aren’t eating enough for your activity levels.

You need to eat more. This may seem obvious, but it’s no more obvious than saying to an overweight person that they just need to eat less. I’ve explained in another article why this doesn’t work. There are powerful psychological and hormonal factors that regulate our eating behaviour. If people aren’t hungry due to psychological reasons or signalling hormones telling them they are full, people won’t be keen on forcing food down their throats. Maybe someone is busy and struggles to eat regular nutritious meals. Whatever the reason, if you are trying to alter your bodyweight, people are prone to getting it wrong when it comes to reporting their daily intake. Studies have consistently found that underreporting and overreporting is correlated with BMI, therefore if you are underweight, you are more likely to overreport what you eating, and if you are overweight you are more likely to underreport what you are eating. It may be worthwhile getting a nutritionist to analyse your diet for you.

You’re a picky eater

Depending on how picky you are with your food, you may need to see a dietitian or maybe a therapist if you have a strong aversion to vegetables. You don’t want to just eat high energy dense processed foods to compensate. But if you are only slightly picky, again it might be worthwhile speaking with a nutritionist to see how you can add more variety into your diet.


The same way some people are genetically predisposed for being overweight, some are for being skinny. However, this doesn’t mean you roll over, give up and accept your fate. Environment and lifestyle habits are still a huge factor, and it doesn’t mean it’s impossible for someone with ‘skinny genes’ to put on weight, it just means you are high risk for being underweight as an adult, and it isn’t as easy for you as everyone else to put on weight. Most genetics studies have investigated obesity genes, but Cambridge University recently performed the first study of its kind, looking at both underweight and overweight genes, and found that known genes associated with high risk for obesity, were present in high risk for thinness individuals, just with different gene variants.


Appetite suppressors that aren’t commercial fat burners (e.g. metformin), drugs that alter your sense of taste or smell, drugs that lead to dry mouth, nausea or difficulty swallowing are just some examples of drug types that could affect your ability to put on weight indirectly.

Medical conditions

There are too many to mention, but what I will say is that if you have a gastrointestinal disorder that is causing you to poorly absorb nutrients, then there is no point in any well-meaning health professional advising you to eat more until the root of the problem has been treated. You will need specialist treatment and specialist nutritional advice from a dietitian. Weight loss can also occur in cases of hyperthyroidism, diabetes type 1 and 2, and cancer.

Mental illness and stress

These can affect everyone differently. If I’m tired and busy stressed, I tend to eat for 3 people, and all the wrong foods. When I’m emotionally stressed I could quite easily not eat for a day or two. Everyone is different. If you are prone to losing your appetite when stressed or when trying to get well from your mental illness, just try and eat. Even if it’s a pack of super noodles. It’s still energy. It’s still self-care.

You’re a fidget!

You may have high levels of non-exercise activity thermogenesis. What that means is you move a lot in your everyday life. Fidgeting has been found to cause energy expenditure of between 100 – 800kcals! It’s not just fidgeting though. Maybe you walk a lot in your job, you run around after your kids all day, you look after your garden – it can be anything that causes you to move.

My advice

If there’s an underlying medical reason why you are struggling to gain weight, you need specialist help first. Then you can work on weight gain.

For everyone else, track your calories and gradually increase your daily intake, even if it’s just by 100kcals a day. If you really aren’t enjoying forcing food down, there’s no harm in trying a meal replacement drink like Complan. These can be a great way of increasing calories without making you want to throw it all back up again. Do whatever works best for you. If you are looking to build muscle and you struggle to put on weight, then this is the one time I recommend a protein shake. When I was bulking for my last bodybuilding competition, I was losing weight on 3000kcals a day, and trying to eat so much food was making me feel sick. Protein shakes can be a massive help for athletes.

Final thought.

The same way you don’t go up to someone who is carrying excess weight and say, ‘God, aren’t you fat?’ please don’t go up to someone who is obviously underweight and say, ‘God, aren’t you skinny?’ I can see why you may think it’s a compliment, but it isn’t always. I can’t speak for everyone of course. But I used to be in skinny minny club. I was a lanky, gangly, and awkward female, and drawing attention to the fact my body was different to everyone else made me feel uncomfortable and insecure. I felt like I should apologise for being born that way. Years of bullying, and other issues from my childhood gave me huge hang-ups about my body that persisted into adulthood and didn’t really stop until I started bodybuilding in my 30s ironically. You just don’t know what is going on in someone’s life to cause them to be underweight. So, if all women could stop hating on other women, this world would be a far more beautiful place. We must still deal with judgements, unwanted comments from men about our bodies, with some men still trying to control what we do with our bodies, so why do it to each other? Solidarity sisters.

Underweight men have hang-ups too. We just don’t hear them talk about it as much. Men are just as affected by the media portraying the ideal male body image as tall, muscly and handsome, as women are by gorgeous skinny models in magazines. The only difference is, that if you are female and naturally skinny, you should be grateful. This mentality is fucked up.