Why do you build less muscle when you do a lot of endurance sport?
Research shows that you can build muscle mass just fine on a combined schedule. If you also want to maximise explosive power, it helps to train strength before endurance and to spread your sessions out.
The shortest answer: with an average training schedule, you probably do not build less muscle because of endurance sport. Many large analyses combining dozens of individual studies show that combined strength and endurance training produces almost as much muscle mass as strength training alone. The 'interference effect' that sports coaches have been discussing for decades turns out to be statistically undetectable when it comes to muscle mass.
That does not mean there is no friction at all. Explosive power, such as jumping ability and sprinting ability, does develop less well when you combine endurance and strength training, especially if you do both in the same session without a break in between. The type of endurance sport also matters: running causes more interference than cycling, probably because your legs are already heavily loaded during running. The more frequent and longer your endurance training is, the greater its dampening effect on strength and explosiveness.
There is also a striking difference between men and women. In men, combined training measurably reduces the development of leg strength; in women, that effect has not been found. For muscle mass, upper-body strength, and endurance capacity, this difference was not observed.
In practice, the biggest difference comes from the order and planning of your sessions. If you do both types of training in one session, do your strength training first. That produces almost seven percent more improvement in leg strength than the other way around. Even better is to plan endurance and strength training on separate days or with enough hours in between. Sprint interval training, short intensive sprints alternated with rest, seems to clash less with strength training as a form of endurance sport, although this has only been studied on a small scale.
This answer is based on five analyses that combine dozens of individual studies and on two separate searches of the scientific literature. The findings on muscle mass and maximum strength are drawn from the largest and most consistent portion of the evidence, and that evidence is strong. For explosive power, the specific influence of running, the sex difference in leg strength, and the ordering recommendation, the evidence is reasonable but less extensive. The finding on sprint interval training rests on just nine small studies with a total of 177 participants, which is too few for firm conclusions.
Do you need more protein if you eat little meat?
Eating little meat does not automatically mean taking in less protein, but the quality of plant protein is lower. Pay attention to your sources, combine them wisely, and from age 65 onwards it is definitely worth giving protein quality some extra thought.
Do you get enough essential amino acids on a purely plant-based diet?
A plant-based diet delivers all essential amino acids, as long as you eat enough total protein and keep your meals varied. For active people, a minimum of 1.2 grams of protein per kilogram of body weight per day is a sound starting point.