You may have heard about the experiments on animals that show long-term, severe reduction of calories slows aging and makes them live longer. It also reduces their risks for cancer and diabetes. It would be a cheap and effective way to ensure a long, healthy life if it applied to humans too. But, who is going to severely restrict calories for the rest of their lives? It’s not easy. However…
One theory for why caloric restriction (CR) slows aging is that it lowers levels of the protein IGF-1 (insulin-like growth factor-1) in animals and in some human fasting studies. IGF-1 is an anabolic hormone; it promotes growth. It’s been shown to promote growth of cancer cells too. People who are born without a lot of IGF-1 or functioning receptors for IGF-1 are almost completely free of cancer and other maladies. But they don’t grow well.
The best of both worlds seems to be… having adequate IGF-1 around when we’re young, and reducing it as we age. Indeed, the study I just blogged gave evidence for that: 50-to-65-year-olds with lower IGF-1 had less cancer, less diabetes, and lived longer.
Do we have to severely restrict our calories to lower IGF-1? No, we can restrict just our protein, especially animal protein:
Long-Term Effects Of Calorie Or Protein Restriction On Serum IGF-1 And IGFBP-3 Concentration In Humans, Aging Cell, October 2008
This study describes 4 experiments. Here’s a quick-and-dirty summary:
- 1st: Compared a CR group (16% protein) to an exercise group (16% protein) for 1 year – Result: Same IGF-1, even though the CR group ate less and weighed less.
- 2nd: Compared a CR group (6 years, ~1800 kcal/day, 24% protein) to a Western Diet group (16% protein) – Result: Same IGF-1 (You would have thought the CR group who ate less and weighed less would have had lower IGF-1, wasn’t the case.)
- 3rd: Compared a CR group (24% protein) to vegan group (10% protein, 0.76 g/kg body weight) – Result: Vegans had lower IGF-1 even though they ate more, weighed more, and had more body fat.
- 4th: Had a high-protein CR group (24% protein) lower their protein (from 1.67 g/kg to 0.95 g/kg) – Result: Eating less protein lowered IGF-1.
So, in this group of experiments, humans who restricted their calories didn’t have lower IGF-1:
The findings from these [first] two studies demonstrate that 1 year and 6 years of CR do not reduce total and free IGF-1 levels in humans.
But if they ate less protein, they did lower IGF-1:
This short-term isocaloric reduction of protein intake [4th study] resulted in a 25% reduction in serum IGF-1 concentration … suggesting that the high protein intake was preventing a reduction in IGF-1 levels in response to CR.
In conclusion, our findings demonstrate that, unlike in rodents, long-term severe CR does not reduce total and free IGF-1 levels in healthy humans if protein intake is high. In addition, our data suggest that chronic protein intake is more powerful than calorie intake in modulating circulating IGF-1 concentration in humans.
This is important because the median protein requirement of the healthy adult population is 0.65 g kg−1 per day and the reference daily intake (97.5th percentile) is 0.83 g kg−1 of body weight per day that is close to the protein intake of our vegan group in this study. In contrast, half of the US males are eating 40% or more protein (≥ 1.34 g kg−1 per day) than the reference daily intake, which is presently considered to be harmless and, according to public opinion and advocators of ‘low-carb’ diets, may even be beneficial.
More studies are necessary to understand the biological and clinical implications of a chronic high protein intake, especially in sedentary people with a positive family history for cancer.
In addition, more studies are needed to understand the effects of PR [protein restriction] and methionine restriction on metabolism, disease prevention and longevity in humans, because several studies in rodents have shown major beneficial effects.
Finally, these findings underscore the importance of dietary macronutrient intake in regulating metabolic events, and suggest that reduced protein intake may become an important component of anti-aging and anticancer dietary interventions, due to the importance of IGF-1 in the biology of aging and in the pathogenesis of many human tumors.
Did you see that bit of data where half of US males were eating 40% or more of their calories as protein? People in this country are convinced they need lots of protein. Advertisements ask, “Are you getting enough protein?” People ask vegetarians, “Where do you get your protein?” People who are still fairly young, say, up to 40 years old, can probably get away with eating more protein. Beyond that, they should consider that their extra growth hormone may be harming them, especially if there is a family history of cancer.