We asked a personal trainer how he would spend $100 on supplements. Here's what he bought.

• A personal trainer shared his top picks to get the most value from buying $100 worth of supplements.
• Products like creatine, protein powder, and fiber are the most versatile for overall benefits, he said.
• He shared what to look for and what to avoid when in the supplement aisle for the best results.

Personal trainer Ben Foster used to describe himself as a "supplement minimalist."

He'd stick to a few tried-and-true basics of supplements for fitness and general health, without venturing very far into the wider world of powders, pills, and shakes.

But research increasingly suggests that supplements can offer promising benefits. They're particularly effective if you fine-tune them to your unique needs and goals, said Foster, founder and head coach of the People's Athletic Club, which offers virtual and NYC-based training.

He told Business Insider that if he were spending $100 on supplements, he'd buy three staples to improve his nutrition and performance.

Creatine: for muscle and brain health

Foster said one of his top picks is a fitness supplement that's recently become a massive trend, thanks to multiple benefits backed by science.

"Creatine supplementation has just blown up in interest over the last two years," he said. "If it were discovered today, it would be a banned substance because it's just really, really effective."

Creatine isn't a steroid. It's a combination of amino acids that our bodies naturally produce to provide energy to muscle tissue. Supplementing with creatine helps ensure your muscles have enough fuel when you hit the gym.

Foster said he notices a difference when he's taking creatine regularly because he can manage another rep or two in a workout before getting tired.

"The telltale sign is you feel like you have a little extra oomph," he said.

There's also emerging evidence that creatine is a brain-boosting supplement. Researchers think it works by providing energy to the brain to help mitigate stressors like those from aging and sleep deprivation, but studies are still underway to learn more.

Protein: for muscle and metabolism

Next, Foster recommends pairing creatine with a reliable protein powder.

While it's possible to get enough protein through diet alone, a good protein powder can make it more convenient and easier on your digestive system.

With a huge variety of brands available, it's important to read labels carefully. "Name recognition alone is not enough to go off," he said.

Steer clear of "proprietary blends" that don't disclose the ingredients you're getting, or in what amounts.

Aim for recognizable protein sources like whey, brown rice, egg white, or pea, and start by trying a few options to find what works best for your digestion.

However, even the best supplement can't replace overall healthy habits.

"Of course, none of this matters if you don't have those big rocks in place of general nutrition, sleep, stress," Foster said.

Fiber: for healthy digestion

Rounding out Foster's list is fiber, a nutrient that feeds beneficial bacteria in the digestive system.

You can get fiber in foods like beans, whole grains, berries, seeds, and some veggies, but most of us don't eat enough.

Getting enough fiber can support fitness goals, such as building muscle and burning fat, by keeping your gut and metabolism healthy. Fiber is also crucial for long-term health because it's linked to a lower risk of serious illnesses like heart disease and colorectal cancer.

Foster recommends starting slow with fiber supplements to give your body time to adjust and minimize potential gastrointestinal side effects.

Honorable mentions

A few other supplements Foster mentioned for specific health goals include:

• Omega-3s — fatty acids that help protect the heart and brain and reduce inflammation.
• Vitamin D — an essential nutrient for a healthy immune system, you might be missing if you don't get a lot of sunshine.
• Bicarb — an expensive version of baking soda that can help competitive athletes stave off fatigue during intense efforts.

Your response to supplements can vary, especially when it comes to specific vitamins or minerals you might be lacking.

"What may be the right answer is to spend that $100 on a full blood panel," Foster said. "If you're deficient in something, that's going to be the best bang for your buck."