For a long life, we know that kilojoule restriction is the single most effective dietary change you can make.
Now, a "game-changing" new study has found that a low-protein, high-carb diet (LPHC) is as effective for a long, healthy, albeit probably fatter, life.
Eighty years ago, a seminal paper was published showing that cutting the recommended daily intake of calories by 30 to 40 per cent resulted in prolonged life.
"We have known for many years that caloric restriction diets increase lifespan in all manner of organisms," said the lead author of the new study, Professor Stephen Simpson from the University of Sydney.
"However, except for the fanatical few, no one can maintain a 40 per cent caloric reduction in the long term, and doing so can risk loss of bone mass, libido and fertility."
Professor Simpson and his team decided to find out how LPHC – arguably a more sustainable method – compared with restricted calorie intake.
They placed mice on different macronutrient-based diets and followed them for eight weeks (the equivalent of about two human years) and found those on a LPHC or restricted-calorie (RC) diet lived up to 30 per cent longer and were healthier.
Interestingly, the LPHC mice craved more food (presumably because they lacked satiating protein) and so tended to gain more weight.
It might seem a contradiction that the mice eating the most had the same health and longevity outcomes as the thinnest mice eating the least, but Professor Simpson says they are simply two methods to stimulate the same metabolic pathways.
"The surprise was that we can activate these pathways without calorie restriction," he said.
The opposing metabolic pathways, linked to higher risk of premature ageing and cancer, but better for lean muscle growth and reproduction, are turned off with the LPHC and RC diets and turned on with a high-protein, low-carb (like Paleo) diet.
While, in the long-term, LPHC has been associated with increased body weight, adiposity and fatty liver, Professor Simpson said they found "no measurable change" in these areas in their study.
"The reason is they increased their energy output – their metabolism went up," he hypothesises, noting that these mice put on "a bit" of weight, but still lived the longest.
Not that it is an excuse to eat more sugar and high GI carbs, which will cause different problems, Professor Simpson says. Rather, a mediterranean-style diet of leafy vegetables and fruit, wholegrains and other low GI carbs is the way to go.
"Complex carbohydrates hang around longer in the digestive tract," Professor Simpson said. "By the time they get to the colon, there's plenty for the bacteria to eat."
Whether people should adopt a LPHC diet depends on their circumstances, age and aspirations, he said.
"If you're obese and want to lose weight, high protein, low carb is as good as it gets for helping you do it." But there is a caveat: "It should be short term."
"You can achieve different things with different diets, but not everything at once."
If your goal is to stay healthy into your later years, however, he believes this is the way to go.
"We have again shown that changing the macronutrient composition of a diet is vitally important, and in this case is a more feasible intervention than caloric restriction for managing human health," Professor Simpson said.
