MCT oil is billed as the wonder food supplement that can boost brainpower and give us endless energy. Fitness fans swear by it to improve physical performance, limit carb cravings and burn fat. It's easy to digest, takes a liquid, capsule or powder form - and can even be added to your morning "bulletproof" coffee for serious fans (coffee plus butter plus MCT oil).
But while there is certainly a growing hunger for it - with the global market for MCT oil valued at $1.2bn in 2020 - is this odourless oil really as effective as manufacturers claim?
"MCT stands for medium chain triglycerides. It's a type of fat, and it's found predominantly in coconut oil and palm oil," says Dr Jenna Macciochi, honorary senior lecturer in immunology at the University of Sussex.
The molecular structure of MCTs is shorter than other fats, which makes it easier for the body to digest and turn into fuel, explains Dr Macciochi, who is also head of innovation at Ancient + Brave.
She adds, "Unlike other fats, which have longer chains, MCTs go straight to the liver and can be used as an instant source of energy, bypassing the way we would normally get energy from glucose, which comes from carbohydrates.
"They can be turned into ketones, a different kind of fuel produced when we break down fat. Ketones can cross from the blood to the brain, so it's an alternative source of energy for the brain, which usually uses glucose for fuel."
There are different forms of MCT oil: typically, you may see products with C8, C10 or C12 written on packaging, referring to the length of the chain. "C8 and C10 seem to be the most efficient and readily used by the body," she says.
There are a few ways in which MCTs may help with weight management. One is that they may help us feel fuller for longer. Dr Macciochi says, "MCTs act on special cells on the gut lining, to promote release of hormones which signal fullness in the brain. That can help with that overall calorie intake across the day."
It may also promote thermogenesis - the burning of calories at rest - and to support fat oxidation - the burning up of fat cells for energy - and can help support those on a ketogenic (high fat, low carb) diet.
A 2015 meta-analysis of studies suggested replacing other saturated fats with MCT oils could help us lose weight.
There are positive signs MCTs may benefit our brains. Dr Macciochi says, "Studies show that MCTs can enhance cognitive performance and mental clarity. So we know the brain can use it as a fuel, and it may have these benefits."
A 2022 study found that Alzheimer's patients using MCTs saw an improvement or stabilisation of cognitive function: the condition impairs the brain's ability to use glucose, and ketones may compensate for this. Meanwhile, a study suggested MCTs may boost memory in older adults without dementia, while another found younger healthy adults saw cognitive improvements after consuming 12-18g of MCTs daily.
However, Dr Sinopoulou suggests benefits are unlikely unless you also lower your carb intake, saying, "As for the cognitive help that it might give, I think it would have to be one of your fat sources alongside a low carb diet."
Because MCTs can be easily used by the body, they may increase our energy levels in the short term. A 2009 study found that, when compared to LCTs (such as olive oil), consumption of MCTs could enable people to perform HIIT (high intensity interval training) for longer. However, another study in 2022 found there was little evidence consumption could lead to improved athletic endurance.
A 2016 study found piglets fed on an MCT-rich diet had better gastro-intestinal health. Dr Vassiliki Sinopoulou says, "MCTs have been compared to short chain fatty acids which get used as prebiotics - a food source for the microbes in your gut. That helps boost your flora and help your gut balance - and MCT oil is supposed to have a similar effect."
MCT oil is sold in a variety of different forms - either as an oil or a powder, or in capsule form, and can be taken at any time of the day. Recently, it has become more popular in a so-called "bulletproof coffee", which is pure coffee mixed with unsalted (preferably organic grass-fed) butter and MCT oil.
It can also be added to smoothies, soups and shakes, or added to no-bake cakes and protein bars. You can mix the oil into tea, your morning porridge or yogurt or drizzle onto salads.
Dr Sinopoulou adds, "What time of day you take it depends on why you are taking it. If you want to use it as a source of energy, then take it before you exercise."
MCT oil is a saturated fat and even though the body metabolises it in a different way, eating too much could still lead to weight gain.
However, Dr Sinopoulou says, "I wouldn't expect it to have a worse effect than eating lots of saturated fats like butter or coconut oil. But if you are taking high doses, it can cause problems in the liver and other digestive problems."
Several study participants have reported stomach troubles and diarrhoea at higher doses. Dr Macchioci explains, "For certain people, it can be harsh on the gut. Because MCTs are rapidly absorbed, they can have an osmotic effect. They can draw water into the lumen of the gut [the space inside the intestinal walls], and if you have it in coffee on its own without food, that effect can be magnified [causing a laxative effect].
"If you're having it with food, it buffers that. If you're not familiar with MCT, go with a lower dose and build up slowly."
While consuming MCTs may help with weight management, it's unlikely to target belly fat specifically. Dr Macciochi says, "The results you get depend on your overall diet and lifestyle. Taking MCT alone is not going to burn fat from your body, or particularly target areas that you would like it to. It can assist weight management, but as part of a holistic plan."
Our experts both agree more research is needed, as studies have produced mixed results. However, a 2021 study concluded that MCT oil could be incorporated into a weight loss program without fear of adversely affecting metabolic risk factors.
Dr Sinopoulou adds, "The theory is that as MCTs get used immediately and don't get stored in your adipose [fatty] tissue, they don't really contribute to obesity or weight gain, or influence your cholesterol and your triglyceride levels [the amount of fat in your blood from foods containing triglycerides]. It might even lower them."
"Do you have a specific problem you're interested in using this for?" says Dr Sinopoulou. "If you want to lower your weight, talk with an expert to guide you on the correct dosage and how it should be used."
Dr Macciochi adds: "Its application depends on why you want to use it. For example, If you want to exercise straight away, but without having to eat a meal before you go, then add it to coffee to get the combination of caffeine, which has a sports performance benefit, and that source of fuel from the MCT."
Dr Sinopoulou explains, "In the short term I would say the risk of taking it is fairly neutral. In the long-term, we don't know. But I wouldn't say to anyone don't try it - there are no red flags like that.
"Side effects might occur in cases where people are using it without any guidance and they're consuming so much of it because they're thinking, 'Oh, it's good for me, so I'll have as much as possible.'"