Dr Zoë Harcombe is a researcher, blogger and public speaker. She has a PhD in public health nutrition – her thesis being a review and meta-analysis of the evidence supporting the introduction of dietary fat recommendations in 1977 and 1983. Dr Harcombe has also authored multiple books on the subjects of nutrition and health. This talk was given at the Public Health Collaboration conference 2018 at the Royal College of General Practitioners in London.

“Why do we say eat less, do more?” Dr Harcombe asks, referring to the general advice given to people wishing to lose weight and improve their health. She explains that this advice is built on the ideas that weight gain or loss is dependent solely on calories i, calories out (CICO), and losing a pound of fat requires one to burn 3500 calories. In turn, these ideas are built on the laws of thermodynamics.

As Dr Harcombe explains, the first law (conservation) does not make things as simple as ‘energy in = energy out’ for the body, as energy can still be ‘lost’. The second law (entropy) shows that a calorie is not a calorie (i.e. not all calories from different sources are treated the same). Furthermore, ‘energy in’ and ‘energy out’ are dependent variables, meaning when one is changed, so is the other. These points imply that the laws of thermodynamics cannot be applied to a complex system such as the body as simply as they often are.

Regarding the ‘3500 calories per pound of fat’ mantra, Dr Harcombe cites numerous papers that suggest one cannot simply lose weight in this way – the figures just don’t add up in practice. She notes that it is very hard to find where this theory comes from; her best guess is that it comes from a 1918 diet booklet. Most of the British health authorities that Dr Harcombe asked did not know where the figure came from, but two came back with a reference. However, this reference showed that just 12 people lost an average of 11 pounds on a 600 calorie per day deficit over a year, which is significantly lower than the 62.6 pounds they should have lost if the formula were correct. Equally, Dr Harcombe says, a person will not gain weight at the rate calculated by this formula when in a calorie surplus, the reason being that humans do not work like bomb calorimeters.

An important point to consider when thinking about weight loss is the idea of energy balance. Dr Harcombe explains that if caloric intake is reduced, then the number of calories burned also reduces to compensate, as the body does not want to continue burning lots of calories in periods of poor food availability. The body can and does adjust to reductions in caloric intake, as it funnels less energy into processes such as reproduction and thermoregulation.

Basal metabolic rate (BMR) is the amount of energy burned at rest and this accounts for most (45-70%) of a person’s total energy expenditure. This has been shown to decrease with calorie restriction dieting, meaning a calorie deficit has to be maintained long-term in order to not regain lost weight afterwards. Dr Harcombe opines that this ‘small print’ must be made known to patients before they embark on a calorie restriction diet. Furthermore, she explains that carbohydrates cannot be used for certain bodily processes that fat and protein can, meaning that they are stored if not used directly for energy. Physical activity level is another factor in total energy expenditure. Dr Harcombe notes that many people forget to account for how many calories they would have burned at rest, so the actual number of calories burned by the exercise is lower.

So if eating less and doing more doesn’t work so well for weight loss, what does? Dr Harcombe says it’s about stopping storing fat and starting un-storing fat. To achieve this, she explains, we need to switch off insulin, which works to store fat, and enable glucagon, which is essentially insulin’s antagonist. To reduce insulin, one can stop grazing (snacking), manage carbohydrate intake and use the ‘right fuel’ by avoiding highly processed and sugary foods. Meanwhile, to enable glucagon, one can limit alcohol consumption and increase movement.

To summarise, Dr Harcombe reiterates a few of her key points. Firstly, there are two laws of thermodynamics that can apply to the body, but there is no law that says calories in = calories out, and these laws regard energy, not weight. Secondly, the body is not a ‘cash machine’ for fat and readily adjusts its energy use when calories are restricted. Finally, weight control is about storing and un-storing fat, which is influenced heavily by the hormones insulin and glucagon.


If you would like to view Dr Harcombe’s Public Health Collaboration talk from the previous year, click here

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