Weight gain is a negative symptom stemming from a multitude of things, and every individual is different. There's age, depression, illness or disease, medication, & dysfunction of the thyroid gland to name a few.. and sometimes, there's just plain old bad dietary habits.
I've come across quite a few people who don't really know how to get their vegetables. I know.. seems crazy.. but it's true. First off, potatoes are carbohydrates and full of starch, and should not be considered a vegetable. They match more in nutritional value to rices, pastas and grains. They break down to sugars. Glucose that your body doesn't need becomes fat. Potatoes are also loaded with calories. Bottom line: Potatoes will make you fat. They cause more weight gain than red meat.
The fastest weight loss I've ever seen without nutritional deficiency comes from the 40-day liquid diet of blended or juiced vegetables (primarily greens) and fruits that people started trying after that documentary, "Fat, Sick, and Nearly Dead" got added to Netflix. Over the years, I've dated and/or lived with a couple of bodybuilders and a small number of guys heavily into weight training. These men were very serious about their vegetables and home made smoothies, and I learned a lot by watching them. I learned how people can lose weight and stay healthy, as opposed to trying fad diets, feeling weak, and eventually failing.
1. Green smoothies are fantastic sources of vitamins and minerals, and will help you remain healthy while reducing fat intake significantly. Try having smoothies for breakfast and lunch, and limiting a solid meal to once per day. (And make sure that solid meal is not something like heavy gravy over a pork roast with potatoes and bread.)
2. Reducing carbs significantly will decrease weight. Carbohydrates break down into glucose, which turns into fat and stores up in the body.
3. Metabolism is a complicated thing. Not getting into the intricate details of how it works, you can simply raise it and burn off fat by doing 30 minutes of cardio every day. A walk down the road or a ride on a bike is sufficient. Make sure you're breathing a little heavily and breaking a sweat, or you're not getting enough cardio to make a difference.
4. Take a breather from beer. Have a glass of red wine instead, and don't overdo it.
5. Eat more vegetables than meat. We have a bad habit in America of making the bulk of our meals a big slab of meat, surrounded by a few overcooked veggies. Reverse that, and don't cook the vegetables until they're limp and dead.
We get a ton of fat from eating heavy gravies, sauces, breads, pastas and too much dairy, and limiting our vegetable intake. Salads don't count for crap.. unless you do away with the poor excuse for one. That would be the iceberg lettuce thing with a few tommy tomatoes and cheese, drowned in Ranch. If you do salad, do it with spinach and put a ton of veggies or fruits on top of it. By eating vegetables, I mean cut up a ton of vegetables, barely cook them, season them with something that's not fattening and eat them. Eat them raw. Eat them all the time. You can eat the heck out of vegetables all day long and not worry too much about weight gain. (careful of avacados.. they're actually high in calories but still great for the brain) Ethnic cuisine can incorporate a ton of veggies.. there are Ethiopian dishes, a ton of Korean and Thai dinners and Sri Lankan meals that are loaded with good stuff and use hardly any fat at all. Just cut back on the rice and put more veggies on your plate.
Finally, there's one more thing that causes weight gain that not many people consider. Sleep deprivation. It works both ways. It causes gain in some people and loss in others. A lack of sleep and stress screws around with the thyroid as well as adrenal glands and starts all sorts of weight trouble. So learn how to stress less, and get a decent amount of sleep when you can.