I am a 45 year old man, what is the minimum I need to do to maintain my health and body weight?

This is a very very very simple question. Eat healthy (if you ever have to question wheter a food is healthy, it is not) and exercise 5 days a week for 30–45 minutes….I swear to god is it as simple as that.

The hard part is actually eating healthy and exercising regularly, and most importantly, do not lie to yourself and say you were eating healthy and exercising when you really were not.

How tall are you? You can look at this site for an idea of what the ideal body weight is for a man of your height. It even tells you how to figure out what frame size you have (which helps determine your ideal weight range).

Once you know your ideal body weight, you can figure out your basal metabolic rate (BMR). Your BMR It is the amount of calories per day your body burns, regardless of exercise. It changes with age, weight, height, gender, diet and exercise habits. Finding this out can tell you the number of calories you would need to consume per day just to maintain your current weight. If you wanted to lose weight, you would aim to eat fewer calories (or burn more calories with exercise). If you needed to gain weight, you would aim to have more calories per day. This site has a calculator for figuring out your BMR and your daily calorie needs. Knowing your BMR and daily calorie needs (which you can base on your activity level) will give you an idea of the minimum you need to maintain a certain weight since you burn a certain number of calories every day without doing any exercise at all.

Once you know how many calories you need each day to maintain your current weight (or to gain/lose weight as necessary), you can figure out what to eat. Eating foods that are nutrient dense are always a good idea. For example, you get more nutrients from an apple than you would a snickers bar (at fewer calories at that). So snacking on an apple would be better for you than snacking on a snickers. Even if the apple and snickers bar had the same amount of calories, the apple offers other nutrients that the snickers bar wouldn’t so it’s a good idea to pick the snack that offers the most nutrients.

As far as maintaining or improving your overall health, it may be helpful to have a complete physical done by your doctor if you haven’t had one lately. Be sure they do lab work to get an idea of how you are doing overall. Then you can discuss with your doctor if they think you need to make any changes to be healthier overall. Depending on your current health status, you may not have to make many changes if you are pretty healthy overall, but if you aren’t pretty healthy overall, you may have to make some life changes in your food choices and the amount of exercise/physical activity you get each day.

On eating healthy from Michael Pollan:

Pollan says everything he’s learned about food and health can be summed up in seven words: “Eat food, not too much, mostly plants.”

Probably the first two words are most important. “Eat food” means to eat real food—vegetables, fruits, whole grains, and, yes, fish and meat—and to avoid what Pollan calls “edible food-like substances.”

Here’s how:

Don’t eat anything your great grandmother wouldn’t recognize as food. “When you pick up that box of portable yogurt tubes, or eat something with 15 ingredients you can’t pronounce, ask yourself, “What are those things doing there?” Pollan says.
Don’t eat anything with more than five ingredients, or ingredients you can’t pronounce.
Stay out of the middle of the supermarket; shop on the perimeter of the store. Real food tends to be on the outer edge of the store near the loading docks, where it can be replaced with fresh foods when it goes bad.
Don’t eat anything that won’t eventually rot. “There are exceptions—honey—but as a rule, things like Twinkies that never go bad aren’t food,” Pollan says.
It is not just what you eat but how you eat. “Always leave the table a little hungry,” Pollan says. “Many cultures have rules that you stop eating before you are full. In Japan, they say eat until you are four-fifths full. Islamic culture has a similar rule, and in German culture they say, ‘Tie off the sack before it’s full.’”
Families traditionally ate together, around a table and not a TV, at regular meal times. It’s a good tradition. Enjoy meals with the people you love. “Remember when eating between meals felt wrong?” Pollan asks.
Don’t buy food where you buy your gasoline. In the U.S., 20% of food is eaten in the car.

Eat less calories than you burn. That simple.
12 calories per pound per day is a good place to start and see where that gets you.

I am 67 and in good health. I have been blessed, but I also exercise 30 minutes each day and take a multiple vitamin. I started at your age to pay attention to my body. Its paid off for me and it will for you.

I suggest you don’t eat anything white in the carb category, eat a higher ration of protein to complex carbs-plant based carbs, not potatoes and rice.
Do not drink soda, fruit juices, have caffeine with cinnamon and two glasses of red wine a night.
And if you want to exercise, lift weights, do the two or three exercises with the least amount of reps, 6–8 with the most weight that you physically can endure just short of throwing up, once or twice a week.
Walking in the morning will raise the metabolism for the rest of the day.
Eat sun drenched foods, flax seed, almonds, grass fed beef, free roaming chicken and eggs are good, and never forget your spinach and sauerkraut.