Legumes, legumes and more legumes. You shouldn't need excessive amounts of sugar unless you're working out to exhaustion regularly, and it certainly won't make you bulkier; you need protein primarily for that. Be thankful you don't have a sweet tooth. It plays havoc with your gut and hormones. If you're looking for dense calories, my tip is lots of beans, seeds, nuts, peas, quinoa and lentils. They all have a great mix of carbs, protein, fibre and micronutrients. Lots of chilli's, pastas, porridge, stirfrys and stews. For fruits, look towards more sour fruits - berries, cherries and grapefruit. If you still struggle after that, try a vegan mass gainer powder (no lactose). Everyday, drink one minimum. That's about 30g of protein and 40g of carbs, 400kcal, in about 500ml worth of volume. Initially you'll probably struggle with stomach volume. Eat until you feel near to bursting, do that as much as you can, and that will stop being a problem within a week or two. I went from 63kg at 6'3" (due to 3 months in a hospital bed with a busted spine, and no appetite from the opiates) to just shy of 95kg within a year, only working out 2 times a week doing the same 5 exercise - deadlifts, squats, bench-press, bent-over rows and overhead press. Any questions feel free to ask!

I was underweight for a while, which turned out to be T1 diabetes. After treatment, I was still skinny and tried to eat enough to gain weight and it didn't work. I have a certain build and that's it. Training/exercise made me stronger but not much bigger.

Now, I'm starting to get older and I am putting on fat I'd rather not have.

At no point do I think sugar would have helped me at all (except to treat low blood sugar); quite the contrary. Fat has a lot more calories, and it sounds more likely that it would be healthy for someone. Is there a reason you are seeking out sugar rather than fat?

Fat is certainly a better option in general, but I'm already eating plenty of it - or at least as much as I can reasonably get without dairy. As for sugar, I pretty much only get it from what little fruit I consume, eggs, nuts and a tiny bit of chocolate. I don't think getting a bit more calories from sugar would hurt me given how little of it I currently consume.

This sounds so like me in my twenties. I am forever destined to have the build of a long distance runner. Shame I hate running. Acceptance, sometime in my late twenties, was the only real fix, somewhat helped by lifting at the gym. Some highlights for your amusement anyway... :)

Regular gym with trainer was the only thing that ever reliably increased my weight. I tried for years to find a food based approach - nope. With fitness+lifting, cycling, swimming and 4x gym weekly, strength increased fine, actual weight gain was still very slow. So I was super well defined, strong but still looked too scrawny. By the time I'd gained 28lbs, my waist had actually lost 2". What the actual fuck? I had nothing there to lose. I could match lifts with guys who looked far bigger, which I know irritated one or two. :D

Calories alone simply didn't work. I could push to eat 5k+ calories a day and gain nothing that month. Almost no one believed this of course, until they spent enough time with me to see what I regularly, and easily ate daily. I once caught a BBC weight loss documentary series that had everyone in a house like Big Brother, which also included a couple of calorie-proof people like this, along with regular guys and some who always struggled with obesity. They carefully monitored in, out, blood and metabolism, and tried to force feed the guys who couldn't to gain. They too concluded some were somehow immune beyond a certain, maintenance intake. I forget what explanation was offered, or whether that explanation was merely guesswork.

Still, for routes to sugar and calories, my own one real sugar weakness is cake. Mainly the more complex, older recipes - the rich Victorian cake or a brandy laden fruit cake in preference to newer, lighter sugar-only crap. Or a nice sharp apple or fruit pie (home made then, shop bought is pure sugar pretending to be apple) still has plenty of sugar. Lots and lots of snacking in between meals. I used to search out the full fat of everything. Premium brands of dark chocolate lose the ugly sweetness of regular, good 70 and 80% have a nice bitter edge, and some of the posher brands of milk chocolate are higher cocoa, lower sweetness. There's plenty of chocolate and sugar laden bars that have some bitter mint or something strong enough to take the sweetness away - always the expensive premium or continental though! Nuts, pies and pasties, especially pork pies are very energy dense, so plenty of those too.

Yet despite all this and pushing intake to the most I could cope with, I eventually realised that this was all completely pointless. The only way to win was not to play. I could still gain just as quickly (i.e. almost imperceptibly, and painfully slowly) via the gym, especially when I got a spotter/trainer (this was the only real secret actually - a trainer and/or training buddy to push me to breaking every single time), eating only what my body said it needed/wanted. If you go with gym buddy rather than trainer, make sure they want to push hard enough - I found almost none did. The second I drop to an 'approved' regular guy 2k a day calories, weight falls off like an amputation until it's back at the genetically mandated, underweight normal. So I've had to keep working to maintain above that. I'm sick to death of gyms these days, but I haven't yet quite lost the habit.

Ageing never brought the much threatened easy weight gain, neither did quitting smoking. My 50s finally gave me the ability to gain a little fat (not much - even that still seems severely limited to only 2") around the waist, but it still falls off easily, just a little slower, and I am still built like a marathon runner. My general fitness is way better than it would have been though.

Despite the long ramblings, I'm not sure there's too much useful actionable advice in there.

Fats, protein, and exercise. You won’t get fat, but you’ll put on muscle which will help move toward the appearance you’re looking for.

Without weight lifting or forcing yourself to eat way more than you should do you retain more fat, you may just have a slim build.