Word of mouth and unprofessional advice from friends, family members, colleagues, others in the gym etc in fitness and nutrition can lead to many dangers stemming from misinformation, myths and incorrect advice. This can lead to injury and health complications from improper form to incorrect dietary advice. It is always best to seek professional advice regarding fitness and nutrition.

Dangers of unprofessional dietary and supplement advice

When taking supplements and changing diets, it is very important to consult a professional. You should have a blood test to check your own vitamin levels and if you have any deficiencies as taking a supplement you don’t need can be as dangerous as having a deficiency and can cause further health complications. The professional will also be able to advise on supplements based on your personal activity level and the best supplements to help replenish and rehydrate after exercise. Taking supplements and vitamins whilst on medication can cause further health complications or at least reduce the efficacy of your medications as interference with medications is very common. Only a registered health professional will be able to let you know if there are any problems with your medication and planned vitamin/supplement use.
When changing your diet it is very important to speak to a nutrition specialist as by taking on certain diets you will be removing or reducing certain food groups and, in turn, the nutrients they provide. It is very important to be aware of any nutrients you need to supplement to stay healthy and safe. For example, following the much publicised carnivore diet cuts out so many food groups that is is missing many vital nutrients. It is important to have regular health check ups whilst on a diet like this to check for any deficiencies that may have occurred. Vegetarian and vegan diets can also be dangerous when you take into account your age, bone density, muscle mass and any deficiencies.
If a friend/relative/colleague advises on protein intake it is highly likely that their advice will not take into account your activity level or bodyweight which are both important to protein intake. You should always seek advice from a nutrition specialist regarding diet and protein intake for fitness. 

Dangers of unprofessional fitness advice

Crossfit and advanced exercise dangers – taking part in crossfit type activity or advanced exercise without proper experience or strength can be dangerous. For example doing Olympic movements common at crossfit such as clean and jerk and snatch without first being experienced in compound movements such as deadlifts, shoulder press, kettlebell swings etc can put you at much higher risk of injury. You will also miss out on the full benefit or adaptation of these movements due to inexperience and incorrect form.

if a friend/relative/colleague tells you about something that worked for them or something they saw in a video or an article always research from trusted, professional sources to check the information is valid and if it will work for you. It is always best to consult a professional before undertaking new exercises. 

If a friend/relative/colleague advises on cardio but has not used a professional trainer themselves they will not understand how to programme the cardio or advise what type of cardio to do. They will also not be taking into account any joint/muscle issues you may have, what you have tried before, what you are using the cardio to achieve and as such will not have very effective advice. They will likely also not understand the effect that long term high intensity cardio can have on diastolic output – too much blood being forced through an untrained cardiovascular system - which can cause serious heart problems.