Why does eating a bowl of fries feel like a complete meal?
Our bodies associate salty, savoury flavours with foods that are high in protein – and that is what we're actually craving, says Australian scientist Stephen Simpson.
Most other organisms instinctively know how to eat a balanced diet, so why aren't humans better at eating what's good for our bodies?
With David Raubenheimer, Simpson has been studying animal appetites for over 30 years.
They explore the answer to this question in the book Eat Like the Animals: What Nature Teaches Us About the Science of Healthy Eating.
Although humans have exquisitely evolved appetite systems, we've also built ourselves a world that makes it really hard for us to achieve a balanced diet, Simpson says.
The global obesity epidemic of the last five decades is fundamentally underpinned by people not eating enough protein, he says.
Processed foods – which contain large quantities of fats and carbohydrates – have swamped our food environment.
"If you dilute protein in your food supply by adding extra calories in the form of either fats or carbs then your body will keep you eating until you get the right amount of protein.'
"We're eating more fat and carbs … but underlying this has been this drive provided by the body's need to get more protein."
In addition, rising CO2 levels in the atmosphere have altered the composition of the plants we eat to the detriment of our protein intake.
"As CO2 levels have gone up, because of photosynthesis, plants have created more carbohydrates – particularly starches and sugars in their tissues – and that has diluted protein in staple foods such as rice, wheat and barley and others… It would suggest that is contributing to global overconsumption of calories."
Although Simpson warns against eating too little protein, he's not a fan of too much either and doesn't recommend the paleo diet.
"If you eat too much protein – particularly we found if you couple it with too little carbohydrates – that combination will help you lose weight … but it comes with a cost and that cost is that you're driving the pathways that are pro-aging and can lead to late-life health issues."