Burgeoning rates of obesity and chronic disease here in the UK have caused our Government to consider and then implement a number of initiatives designed, we are led to believe, to assist us in our attaining and maintaining of a healthy weight. It is considering, for instance, putting a tax on fatty foods. The idea of a ‘fat tax’ is to discourage the eating of fatty foods that will then lead to people being less fat. The idea is elegant, but it’s also based on potentially faulty premises.
The first, and our Government is apparently aware of this, is whether making foods more expensive will actually lead to meaningful reductions in the purchasing and ingestion of these foods. I am not aware of any evidence that has looked at this specifically. However, even if we have not good evidence in this area, I wouldn’t think that this would stop our Government bringing such a tax into being. The UK Government, like many governments around the world, does not appear to be particularly inclined to base food policy on science, after all.
By way of example, I’d like to explore the other potentially faulty premise behind our Government’s latest wheeze – the idea that fat is fattening.
Because fat is called, well, fat, there is a natural tendency is to assume that the one sure way to get fat is to eat some. Digestion and metabolism of fat is quite a chemically complex process, and we would be hasty to assume that eating fat is somehow akin to injecting it directly into our fat cells. Apart from its name, the other thing that is used to incriminate fat is the fact that, per gram, it contains about twice as many calories as either carbohydrate or protein.
However, this again ignores the fact that different types of food can have different effects on things like appetite, metabolism and hormone levels that mean that eating something calorific is not necessarily fattening.
Here are just a few reasons why eating fat may not be necessarily fattening:
1. Not all calories have the same propensity to cause weight gain
Imagine your body is a lit barbeque, and you put two fuels on it: charcoal briquettes and petrol. Do they burn at the same rate? Of course not. Could food have a similar effect in the body? In a previous post (here) I wrote about a study in mice that found that even for a set number of calories, a high-fat diet had distinct advantages in terms of weight management.
Studies in humans have, generally speaking, not found a so-called ‘metabolic advantage’, but this may be because the studies used to assess this have failed to show one (due, for instance, because the study did not go on for long enough, or people ate other foods in addition to their prescribed diet).
2. Fat might help to sate the appetite, causing people to eat less overall
Fat stimulates the secretion of the hormone cholecystokinin, which slows the rate at which the stomach empties itself, and can prolong feelings of fullness. Studies have found that, for a given number of calories, fat can be more effective at sating the appetite than carbohydrate.
3. Not all fat that is eaten is necessarily absorbed from the gut anyway
Studies show that when fatty food is eaten, not all of it gets absorbed into the body. Some ends up going straight down the toilet.
4. Fat that is eaten doesn’t necessarily end up getting stuck in the fat cells
For fat to get fixed in the fat cells, three molecules of fat (called fatty acids) are combined with a molecule of something called glycerol, which is derived from sugar. In other words, the more sugar one eats (in the form of sugar but also starch), the greater the tendency will be for fat to get ‘fixed’ in the fat cells.
At least some of these ideas here are quite theoretical, and will never really inform us regarding the likely ‘benefits’ of a fat tax. What we require here are ‘intervention’ studies. Essentially, what happens when people eat less fat. If fat is fattening, then this approach should be effective for the purposes of weight loss, right?
When low-fat diets have been tested in research, they are usually of the ‘calorie-controlled’ variety. This means that in addition to restricting fat, individuals are required to eat less (in terms of calories too). Some studies have pitted low-fat calorie-controlled diets against those that are calorie-controlled but not necessarily low in fat. If fat is truly the demon in obesity, we would expect to see two things:
1. Low-fat diets out-perform diets in which fat is not restricted as much
2. Low fat diets lead to weight loss in the long term
The available evidence in the area was reviewed by an international group of researchers known as the Cochrane Collaboration in 2002 . The researchers were particularly interested in the ability of participants to sustain weight loss over a relatively long period of time.
Here’s what they found:
Higher-fat diets tended to lead to enhanced weight loss over time compared to those lower in fat.
On average, 18 months on a low-fat regime led to a net gain in weight (while those on a higher fat diet lost some weight on average).
This study was withdrawn in 2008, on the basis that it was out of date, and that the authors had no intention of updating it. Back in 2002 the evidence clearly showed that low-fat diets are not effective for the purposes of weight loss, and no significant evidence has come to light since then that would cause us to draw any other conclusion.
It seems to me that if the Government is considering taxing fatty foods, it should be pretty sure that fat is fattening and that eating less fat really does help combat obesity. The reality is, though, that the evidence does not support either of these premises.
There is however mounting evidence that carbohydrates that disrupt blood sugar levels and insulin (including grains) are a potent factor in the burgeoning rates of obesity seen here in the UK and other countries. In my blog on Wednesday I referred to the research presented in my latest book which reveals that, overall, low-carb eating is much more effective than low-fat dieting in terms of weight management. This superior effectiveness comes even though low-carb diets in the research are usually not restricted in terms of calories (though low-fat diets usually are).
In my opinion foods to be most wary of and consider taxing include those carbohydrate-based ones that are highly disruptive to blood sugar and insulin including bread, potato, rice, pasta and breakfast cereals – that’s right, some of the very foods that our Government have been encouraging us to fill up on for decades now. For Governments to act in accordance with the science regarding obesity would mean tacitly admitting that the high-carb diet they’ve been pushing for the last 40-odd years is a big part of the problem.
1. Pirozzo S, et al. Advice on low-fat diets for obesity. Cochrane Database Syst Rev. 2002; (2): CD003640