Like 33 million other Americans, I recently found myself sucked into an Instagram hole – the Tasty channel. I watched from an overhead camera at 5x speed as sectioned potatoes were dunked in oil and dipped in mayonnaise, marshmallow frosting was poured over chocolate cake, a mass of cheeseburgers smashed between two large pizzas. Yum.
Understanding human appetite is a difficult prospect. A complex interplay of biochemistry and neurocircuitry has to situate itself within a hostile cultural environment where millions of dollars are spent on maximising the deliciousness of french fries. The supersized role food plays in our culture and economy highlights the difficulty we’re confronting with the obesity epidemic.
Among this barrage of environmental threats, it’s kind of amazing that we actually have some pretty simple medical approaches for reducing appetite. For example, FDA approved appetite suppressants like Belviq and Lorcaserin target neurotransmitters like serotonin, dopamine, and epinephrine. There are effective surgeries that implant balloons in the stomach or bypass it altogether. But then, there is a tried and true approach that every attentive mother has been trying to push on us since we were kids – eating more dietary fibre.
Yes, it seems, eating more fibre can reduce appetite. As the story goes, dietary fibre isn’t broken down by human enzymes; and so we chew, we crush, we surround with intestinal acids and digestive enzymes, but dietary fibre remains resilient, mostly intact and unabsorbed. In the 1970’s, Dr Ken Heaton proposed that dietary fibre makes us feel full simply by occupying space in our gut.
Embedded in the wall of our intestines is a network of peripheral nerves. Some have receptors that are attuned to detect particular nutrients, some are simple machines capable of responding to intestinal stretching. As fibre sits in our gut, stretch receptors are activated, letting our brains know that they’ve had enough. Thank you.
But clearly, there’s a limit to fibre’s effectiveness. Most of the time, our pasta-infatuated modern brain responds to intestinal fullness by pretending it didn’t hear anything. And this is 2019; fibre may be useful, but it’s about time for some optimisation. You see, fibre is not a single thing, it’s an umbrella term used to describe all non-digestible carbohydrates. There are soluble fibres, fermentable fibres, viscous fibres, fibres that are made of digestion-resistant starch, fibres from grains and fibres from vegetables, and on the list goes.
These fibres all have slightly different impacts on the body, and therefore, you might expect, a different impact on appetite. Some seem to work poorly, some work pretty well, and some form the basis for multimillion dollar weight-loss products.
Fibre supplements and long-term weight loss
If proven to cause weight loss, fibre supplement brands like metamucil (psyllium) or skinny shirataki noodles (glucomannan) stand to make a good bit of money. Fortunately for these brands, the highly unregulated US supplement industry doesn’t get in the way of them marketing weight-loss claims without a firm government-sponsored evaluation. So which types of fibre are, and which aren’t supported by research?
- Psyllium is extracted from the Indian plantago seed and is processed into the appropriately named, thick, gluey, mucilage. Gross. Branded as metamucil (still gross), psyllium, as a type of dietary fibre, is associated with a range of health benefits, some of which are well-supported by scientific evidence. But when it comes to weight loss, a recent review of 22 randomised controlled trials showed that supplementing with metamucil wasn’t any better than a placebo.
- Glucomannan is derived from the Asian konjac plant, also known as the devil’s tongue. The konjac plant has a lot of culinary applications, including the somewhat strangely textured skinny shirataki noodles. In Europe, glucomannan is legally marketed as a weight-loss aid, but the data is ambiguous. In 2008, a review of 14 studies showed that glucomannan contributed to an average weight loss of 1.5lbs. However, a more recent review in 2015 looking at eight studies came to a different conclusion, they did not find evidence supporting the value of glucomannan.
- Guar gum is a powder extracted from the Pakistani guar bean. It’s use is widespread in processed foods ranging from ketchup to ice cream. As it relates to health, guar gum is another complex case. In 2001, a review of 20 studies found that guar gum, at the doses tested, did not have an impact on weight loss. At the same time, given a large enough dose of any fibre (or anything), people say they start to feel full. At times, this can be a big problem, as in 1992, 18 instances of medical complications were reported to the FDA. It seems that guar gum is so thick and sticky that it can plug up your oesophagus or small intestines, requiring at times risky surgical intervention.
- Chitosan is a material obtained from crustacean shells, and is used in a bunch of strange commercial applications including as a biopesticide, a wine preservative, a paint additive, an antibacterial agent, and, as a dietary supplement. In a review of seven studies, all but one showed that chitosan supplementation contributed to weight loss. A separate review of 14 studies found that chitosan reduced body-weight by an average of 2.2lbs. Unlike many other sorts of fibre, there seems to be little objection to the claim that chitosan supplementation promotes weight loss, albeit not a lot.
There are many types of fibres
There are lots of other types of fibres, for example beta glucan, pectin, inulin, and resistant starch, all of which have proven to reduce body weight in at least one study. However, it’s clear from the types of fibres that are well studied, there’s a lot of discrepancy between experiments, and a lot of remaining ambiguity. Some of this has to do with how much fibre people ate, how often they took the supplement, and what was used as a comparison condition. For these slightly more neglected fibres, the jury is still out.
But taking a step back, does the difference between fibre source really matter? To some sceptics, the differences between fibre source as it relates to appetite and weight loss isn’t important. All that matters, according to some researchers, is that fibre clogs things up. It’s possible that there aren’t meaningful differences between fibres, but it neglects a really interesting possibility, which is a complex interaction with gut microbes.
Different types of fibre with different chemical and physical properties will inevitably have a different impact on gut microbes. This could shape which organisms live in the gut (the prebiotic concept), which could then impact our health. Or, maybe, as these bacteria break down these different types of fibre in the colon, a unique blend of metabolites is created, which can then have a different impact on our bodies. It’s possible that fibre’s only real impact on appetite is through intestinal bulking, but it’s also possible that some fibres, say chitosan, are having a subtle and complex effect on the gut microbiome, which is impacting our appetite in complex and mysterious ways.
At the same time, sceptics may also say that the magnitude of weight loss induced by fibre is too minimal to have a substantial clinical impact. Losing two pounds over a couple of months won’t make a real dent in the obesity epidemic. Maybe so, but fibre’s not just about weight loss. Randomised controlled trials show that fibre has a beneficial impact on cardiovascular health, and the CDC recommends increasing fibre consumption as a means for decreasing cholesterol. A couple of pounds may not be a ton, but, our mums were right, fibre is good for our health, and when it comes to weight loss, every little bit could help.
Andrew Neff manages Neuroscience from Underground. He writes about health, well-being and neuroscience